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Lumpy skin disease, an African capripox virus disease of cattle
Br. vet . J (1991) . 147, 4 8 9
SPECIAL REVIEW SERIES
LUMPY SKIN DISEASE, AN AFRICAN CAPRIPOX VIRUS DISEASE OF CATTLE
F . G . DAVIES* Veterinary Research Laboratory, PO Kabete, Kenya
SUMMARY Lumpy skin disease is an infectious viral disease of cattle, which often occurs in epizootic form . The disease is characterized by the eruption of nodules in the skin, which may cover the whole of the animal's body . Systemic effects include pyrexia, anorexia, dysgalactia and pneumonia ; lesions are often found in the mouth and upper respiratory tract. The severity of the disease varies considerably between breeds and strains of cattle . Many cattle suffer severe emaciation and loss of production for several months . The skin lesions cause permanent damage to the hides . The mode of transmission of the disease has not been clearly established . Contact infections do not readily occur and the evidence from the epizootiology strongly suggests that insect vectors are involved . The disease has been confined to sub-Saharan Africa, until it recently appeared in epizootic form in Egypt and in Israel . Transmission occurs in a wide variety of biotypes, from semi-desert to temperate grasslands and irrigated land . It has the potential to extend its range further .
INTRODUCTION Lumpy skin disease (LSD) was first identified in Northern Rhodesia (Zambia) in 1929 (MacDonald, 1931) where it was first thought to be an allergic reaction to insect bites . The syndrome recurred irregularly there until 1943, when it spread to Botswana (Bechuanaland) (von Backstrom, 1945) and in 1945 to Zimbabwe (Southern Rhodesia) (Huston, 1945), and to South Africa (Thomas & Mare, 1945) . The transmissible nature of the disease became apparent to South African workers (Thomas & Mare, 1945 ; von Backstrom, 1945) during an epizootic which * Present address : G2 The Court, St . Mary's Place, Shrewsbury SYl 1 DY, U .K.
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affected most of the country . Some 8 million cattle were involved in this outbreak with enormous economic losses (Diesel, 1949) . In 1957, LSD was identified in East Africa, where it has recurred at intervals ever since (Burdin & Prydie, 1959) . Epizootics were reported from the Nile basin in the Sudan in the 1970s and during the period 1970-1985 LSD occurred in most Central and West African countries (Annual Reports FAO/OIE) . The disease is now enzootic throughout sub-Saharan Africa (including Madagascar) and recently appeared in Egypt in 1988 (Ali Moussa, personal communication, 1989) and in Israel in 1989 (Shimshony, 1989) . LSD extended from a focus in Ismailyia in Egypt and largely in the summer months of 1989, infected cattle in 22 of the 26 Governates of Egypt . Cases occurred from Aswan to the delta and also in the desert oasis well away from the Nile itself . The disease is now enzootic in Egypt . An extension has occurred eastwards into Israel, probably by the movement of insect vectors . Further spread westward may occur despite all the control strategies . The significance of an overlap in the distribution of LSD with Cochliomyia hominivorax in North Africa is great, for the multiple necrotic foci of the LSD lesions furnish ideal oviposition sites for the fly . Despite the success of the control measures, this remains a possibility . LSD has occurred in all the diverse ecological zones in Africa, from the high altitude temperate grasslands, the wet and dry bushed and wooded savannah to the very dry semi-desert . It has caused epizootics in irrigated areas of Africa and Egypt . Clearly, the potential exists for spread beyond its historical range, into North Africa, the Middle East and northern Mediterranean countries .
AETIOLOGY The initial observations suggested that the disease was an allergic response to insect bites (Morris, 1931) which was supported by the prevalence of the condition after the rains, when biting insect populations were at their highest . Experimental studies in South Africa (Thomas & Mare, 1945 ; von Backstrom 1945) showed that a transmissible agent was involved . The early attempts to identify this in tissue cultures were hampered by the simultaneous isolation of two different herpesviruses from LSD or similar lesions (Alexander et al., 1957) . A bovine herpesvirus 2 (Allerton or pseudo-lumpy skin disease virus) was isolated, which produces skin lesions which are very similar to LSD in the early stages (Alexander et al., 1957 ; Haig, 1957 ; Weiss, 1968 ; Davies et al., 1971) . A bovine herpesvirus 3 was also isolated from LSD tissue biopsy material but this caused no pathological changes in bovine skin when inoculated from infected tissue cultures (Alexander et al., 1957) . The third group of viruses isolated was of the pox type . They reproduced the disease when tissue culture harvests were reinoculated into susceptible cattle (Alexander et al., 1957) . Their morphology and physical characteristics were similar to those of the orthopoxviruses . Serological and immunological tests showed that they were capripoxviruses, very closely related to sheep and goat pox viruses (SGPV) (Capstick, 1961 ; Davies & Atema, 1981) . Most strains of these protect cattle against LSD when inoculated into cattle and LSD virus protects sheep and
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goats against capripox disease . Serological studies by immunofluorescence and virus-serum neutralization have not shown any differences between strains of LSD and of SGPV (Davies & Atema, 1981) . The strains of LSD thus far examined have all proved to be serologically identical (Davies, 1982) . Restriction enzyme studies suggest that LSD strains are identical and that their genome appears to be the same as that of the Kenya SGPV (KSGPV) (Black et al., 1986) . There are differences, however, between LSD and many of the SGPV from the Middle East and elsewhere (Gershon & Black, 1988 ; Kitching et al., 1989) .
CLINICAL SIGNS LSD is an acute infectious disease of cattle of all ages, well described by MacDonald (1931), von Backstrom (1945) and Ayre-Smith (1960) . There has recently been a clinical report of the disease in the Asian water buffalo (Bubalis bubalis), where five cases were observed in a group of 300 (Ali et al., 1990) . The incubation period after experimental inoculation is 4-12 days, usually about 7 days . The introduction of infected cattle is followed by disease in 2-5 weeks . There is a pyrexia of 40 .0-41 .5°C which may last 1-10 days . This is accompanied by lachrymation, increased nasal and pharyngeal secretions, anorexia, dysgalactia, some depression and a disinclination to move . Within 1-2 days, there is a cutaneous eruption of nodules, which may cover the whole of the body, while in other animals only a few may develop . The predilection sites are the head and neck, perineum, genitalia and udder, and the limbs (Figs 1-3) . The nodules are 5-50 mm or more in diameter, appearing as circumscribed areas of erect hair, firm and slightly raised from the surrounding normal skin . There is hyperaemia and drops of serum appear on the affected skin surface . The lesions are of a full skin thickness involving the epidermis, dermis and subcutis, which may be oedematous . The regional lymph nodes are enlarged and oedematous . Lesions develop on the muzzle, in the nares and oropharynx . They show a ringlike margin, where there has been separation from the surrounding healthy epithelium . Lesions may develop in the larynx and trachea, throughout the alimentary tract, and especially in the abomasum . They become necrotic and ulcerate . Mucopurulent nasal discharges, persistent dribbling of infected saliva, coughing and stertorious and often distressed respiration result . Conjunctivitis and keratitis commonly occur (Fig . 4) . Inflammatory and oedematous swellings of the limbs, brisket and genitalia may develop; the limbs can be three to four times their normal size (Fig . 5) . The lesions become necrotic and while some remain in situ, where they are recognizable for at least a year, others slough away to leave a full skin thickness hole, which usually becomes infected by suppurative bacteria . Larger areas of skin over a hard oedematous limb may become necrotic and slough away leaving a huge raw area ripe for bacterial infection and myasis . Herein lies the danger for infestation by Cochliomyia hominivorax. Lesions on the udder and teats may cause a mastitis and oedema with secondary infections and can result in sloughing of mammary tissue . Other nodular lesions develop in the subcutis, and may also be distributed throughout the connective tissue and muscles of the limbs and body .
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A limb swollen as a result of ISD infection.
Pneumonia is a common and often fatal complication of LSD. I,esions may be found throughout the upper respiratory tract and focal or larger areas of grey consolidation may develop in the lungs together with larger areas of bronchopneumania. Inhalation is a sequel of the necrotic lesions in the respiratory tract and it may prove f’ntal even months after the initial infection when a necrotic slough 1947). The pneumonias rarel, OCCL~ from an old tracheal lesion (de Boom, rrspond to antibiotic therapy. Xhortion frequently follows the acute infection in females, and infertility has been a problem in the months succeeding an outbreak. Bulls may have painful lesions of their genitalia, which can prevent them serving, and may remain infertile for 3-6 months after the disease. Females mav be anoestrous for se\~ral months. These effects are thought to he a consequence of the serious loss in COIIdition associated with LSD. Fetuses may be aborted, or the skin of calvrs t,orlI after an epi/ootic may be covered with lesions of ISD (Davies, unpublished data). Such intrauterine infections have also been described with swinepox ( BOI-st r/ (I/., 1990). Extensive skin lesions which produce a hidehourld condition, limb oedema and mouth and respirator-v lesions mav cause a prolonged anorexia and disinclination to I~OVC~. Rapid dete&ration in body condition follows and, LII~~CIrange COIIditions, the illability to move to water results in a higher mortality from dehvdration and predators. Some cattle become so emaciated that euthanasia is indicated. <:attle recovering from the disease may remain in extremely poor condition for 4-6 months (Diesel, 1949).
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PATHOLOGY The nodules in the . skin are a grey-white colour on section, and involve all layers of the skin and the subcutis . A reddish-yellow oedema may be present in the subcutaneous tissue in the region of multiple lesions . Nodules may be found in the subcutaneous tissue, muscle fascia and even in the muscle itself. The oropharyngeal lesions show the same features, but necrosis develops more rapidly than in the skin . The characteristics of the nodules in any part of the respiratory or alimentary tract are basically the same, as well as in the prepuce, vagina and wall of the uterus (Thomas & Mare, 1945 ; von Backstrom, 1945 ; Ayre-Smith, 1960) . Histological sections of early skin lesions show oedema, hyperplasia of the epidermis and infiltration with an epithelioid cell (the celles clavelleuse of Borrel), which is characteristic of lesions of LSD as well as of sheep and goat pox . Lymphocytes, plasma cells and fibroblasts appear in the later stages . Polymorphonuclear leucocytes and red cells are not seen in any numbers unless secondary infection and necrosis have developed . A vasculitis and perivasculitis with thrombosis of the blood vessels occur in the dermis and subcutis, and are the basic cause of the characteristic necrotic changes . There is a cuffing of the blood vessels with the white cells . Typical intracytoplasmic pox inclusion bodies may be seen in the epithelioid cells, in epithelial cells associated with hair follicles, in smooth muscle and skin glands . They are basically eosinophilic and a halo may surround them (probably an artefact) . The thrombosis, characteristic cellular infiltration and inclusion bodies are the important diagnostic features (Thomas & Mare, 1945 ; Burdin, 1959) . These basic histopathological changes are common to all LSD nodules wherever they occur in the body . The lungs will usually show a suppurative bronchopneumonia, with LSD nodules of varying sizes, grey-white in colour, which may be mistaken for secondary carcinomata .
DIAGNOSIS A presumptive diagnosis of LSD may be made on recognition of an epizootic disease of cattle, producing the characteristic skin nodules and systemic disease . Other domestic animals are not affected . It spreads in a totally random manner in cattle of all age groups, and no chemotherapeutic agent nor antibiotic has any effect upon the course of the disease . Restrictions on cattle movements do not prevent spread to neighbouring cattle . A laboratory diagnosis is desirable, especially in a situation where extension may have occurred from enzootic areas in Africa . This may be achieved by animal inoculation, histopathological examination of the lesions, virus isolation, or identification of the virus by electron microscopy or serology . Animal inoculation If quarantine facilities are available, the inoculation of calves with antibiotictreated suspensions of tissue from the skin lesions might be helpful . It would provide evidence for a viral aetiology and furnish excellent fresh material for virus isolation and other laboratory purposes . LSD virus will grow in embryonated hens
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eggs, but this system is insufficiently sensitive for primary virus isolation (van Rooyen et al., 1968) .
Histopathology Full skin thickness biopsies should be taken under local anaesthesia from recently formed skin nodules ; elliptical sections are recommended to include normal skin at the poles . Material should be taken from more than one animal, with at least two biopsies from each . Half of each biopsy should be placed in formol saline and half in a transport medium on ice for virological examination . Should a cryostat be available, the tissue may be taken immediately to prepare frozen sections . Histopathology provides a means of distinguishing the lesions from a variety of other conditions with which it may be confused (Burdin, 1959) . Those characteristics mentioned above serve to distinguish LSD from streptothricosis, globidiosis, anaphylaxis and bovine herpes type 2 infections . The latter shows only a superficial necrosis of the epidermis, which does not involve the dermis and subcutis to any great extent (Martin et al., 1966 ; Davies et al., 1971) . Anaphylaxis of the delayed type, following foot and mouth disease vaccination, can result in lesions grossly similar to LSD . These often become necrotic but show a quite different initial cellular reaction with more oedema, red cell infiltration and many eosinophils . Later there are many polymorphonuclear cells and often a superficial epidermal necrosis . Dermatophilus congolensis and globidiosis will be evident in smear preparations of the former and sections of the latter . The skin lesions of rinderpest are small and superficial and do not resemble those of LSD .
Virus Isolation Virus isolation may be attempted in primary or secondary cultures from lamb or calf tissues . None of the commonly used cell lines has proved sensitive enough for the primary isolation of the virus . Lamb testis is probably more sensitive than any other culture although lamb and calf kidney and calf testis are satisfactory (Alexander et al., 1957 ; Plowright & Witcomb, 1959) . Fetal lung, skin, muscle, endothelial cells and choroid plexus cells have all been successfully employed for this purpose . There are variations in the sensitivity of tissues from different breeds of sheep and cattle, wool sheep tissues have usually proved to be more sensitive than those from hair sheep and Bos taunts tissues more sensitive than those from Bos indicus. For example, a titre of 10"/0 .1 ml was obtained for a freeze dried stock of LSD in a testis culture from a wool sheep (Davies, unpublished data) . The same material titrated in testis cultures from a Kenyan Maasai sheep gave a titre of 10"/0 .1 ml . Such differences in sensitivity could greatly influence the success of virus isolation attempts . Biopsy tissue should be minced with scissors before grinding in a pestle in a medium containing antibiotics . Freezing and thawing of this suspension should be carried out two or three times in dry ice and alcohol or acetone, to release intracellular virus, the suspension clarified by centrifugation and inoculated onto suitable cultures . Cultures should be used with 75-80% cover; tube cultures with flying coverslips or slide cultures are recommended . These may be stained after 48-72 h by H&E or immunofluorescence or peroxidase methods . The poxvirus
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inclusions may be recognized in infected cultures and specifically identified as LSD by the immunological stains (Davies et al., 1971) . Cytopathological changes may become evident after 4-10 days in most cell cultures . These are focal initially, with groups of denser cells showing some rounding and stranding . These foci gradually enlarge to include much of the monolayer . Testis and kidney cultures usually show these changes earlier than others (Plowright & Witcomb, 1959) . The routine passage of cell cultures after 14 days or so is recommended in cases where little virus was present in the original inoculum and cytopathic changes were not obvious ; they will become apparent on passage .
Electron microscopy Tissue fragments from skin biopsies may be stained to show the brick shaped poxvirus particles . They are very readily seen in the early skin lesions . The point of a needle may be inserted into a tissue fragment from a fresh lesion, washed in a drop of distilled water and a 400 mesh carbon coated grid is then placed upon the drop, blotted and stained with phosphotungstic acid containing 0 .4% sucrose at pH 7 .0 for 1 min . The grid is blotted and dried before being examined at magnifications of 6000-18 000 . Several grids are prepared from each sample (Davies et al., 1971) . A recurring problem both in the isolation of LSD virus in tissue culture and the identification of the virus particles by electron microscopy has been the contamination of the material with herpesviruses . It is for this reason that biopsy material from two or three animals should be examined by both methods . Orphan herpesviruses of the bovine herpes group 3 (Gibbs & Rweyemamu, 1977) may overgrow cultures or be more readily seen on electron microscopy than the capripoxvirus particles ; they are unlikely to be present in material from all affected cattle . BHV 2 produces syncytia with intranuclear inclusions, while BHV-3 produces inclusions but not syncytia . Electron microscopy is a very rapid and valuable method to confirm LSD infections in cattle, with the reservation that the presence of many herpesvirus particles in a specimen does not negate the possibility of a positive diagnosis . Serological methods may be used retrospectively to confirm recent infections, but are not recommended for primary diagnosis . Fluorescent antibody and serumneutralization tests have been extensively used and have proved satisfactory (Weiss, 1968 ; Davies & Atema, 1978) .
EPIZOOTIOLOGY Source of virus Members of the family Poxviridae are resistant viruses . They may remain viable in scab or tissue fragments for very long periods of time . Virus persistence with long transmission intervals is thus possible . Transmission by contact with infected cattle does not readily occur in insect-proofed cattle pens . The sharing of water troughs was necessary to effect transmission between infected and uninfected cattle in adjacent pens (Weiss, 1968) . No transmission occurred if separate water troughs were provided . Virus is present in nasal, lachrymal and pharyngeal
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secretions, semen, milk, and blood (Thomas & Mare, 1945 ; Weiss, 1968) . Viraemia can usually be detected for only 2-4 days, the virus may be in saliva for up to 11 days and in semen for up to 22 days . All could play some role in the short interval transmission of LSD . In virtually all outbreaks, however, the virus appears to be propagated by a continuous cattle-arthropod vector-cattle cycle (MacDonald, 1931 ; Morris, 1931 ; Thomas & Mare, 1945 ; von Backstrom, 1945 ; Diesel, 1949 ; Haig, 1957 ; Burdin & Prydie, 1959 ; MacOwan, 1959) . Transmission There is little information on the transmission of LSD by insect vectors, Weiss (1968) described the isolation of the virus from Stomoxys calcitrans and from Biomyia fasciata, Stomoxys spp . have been shown to be capable of transmitting SGPV (Kitching & Mellor, 1986) . Field observations of biting flies associated with cattle during epizootics of LSD suggest that mosquitoes, tabanids, Culicoides, and Glossina spp . may play some role in the propagation of the virus by mechanical transmission . Any biting fly could transmit LSD following an interrupted feed . Infected secretions could be siphoned by flies such as Musca spp . and transferred to susceptible cattle . Further research on the transmission of LSD is required to clarify these points . Cold weather with frosts causes a sharp reduction in the transmission of LSD, with either very few or no new cases appearing . This has been related to the reduction in insect vector populations . An interseasonal quiescent period is followed by the re-emergence of disease in the areas where it was seen before the onset of colder weather . The virus is thought to persist by cycles of inapparent infections in cattle or in old lesions . Patterns of disease LSD may occur sporadically throughout an enzootic area over a period of several years or behave as an epizootic with fairly rapid spread of the disease throughout a country or region . Examples of the latter are the South African epizootic of 1945-1949 (Diesel, 1949) and the Egyptian summer epidemic of 1989 . Similar epizootics have occurred in Kenya, the Sudan and Ruanda . The epizootics in different countries have shown much variation in morbidity and mortality, and the rates at which they spread . Such differences are thought to reflect the efficiency and population densities of vector populations . The rapid extension of LSD throughout Egypt in the summer of 1989 would suggest that large numbers of potential insect vectors were present, and this was the case . Similar rapid spread has been seen in Africa, although slower rates of transmission have generally been found in sub-Saharan countries, with periods of increased activity after the seasonal rains . The morbidity rates vary enormously; rates of 80% or more have been seen in South Africa and elsewhere but 2-20% is more usual . Both high and low rates may be found at different farms during the same epizootic . The mortality rates are usually low, in the range 2-10%, but rates of 40-75% have been recorded . There are differences in the susceptibility of Bos taurus and Bos indicus cattle . Breeds such as the Jersey and Guernsey appear to be clinically more severely affected by LSD than others (Ayre-Smith, 1960) ; Friesian and Ayrshire cattle are also highly susceptible .
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The breeds of zebu type indigenous to Africa are generally less susceptible . They may develop extensive skin lesions, but have less severe clinical disease and lower mortality rates than cattle exotic to Africa . Occasional cases of LSD have appeared in cattle in Kenya, during inter-epizootic periods, when no clinical disease has been identified for periods of several years . These have often been in relatively small isolated herds with no history of movement nor contact with other cattle . The source of the virus in those outbreaks was not apparent. Investigations showed that many African buffalo (Syncerus caffer) in the adjacent forest areas, had serum antibody to Capripoxvirus, but not to cowpox virus (Davies, 1982) . The antibody may have followed an infection with LSD virus or another unknown Capripoxvirus strain . It was not possible specifically to identify antibody to LSD . A recent investigation of .a buffalo population from an area, where LSD had been prevalent, found no antibody to LSD (Hamblin et al., 1990) . Natural infections have not been observed in any other ruminant species during epizootics of LSD, with the possible exception of a recent report of five cases in the Asian water buffalo (Bubalis bubalis) in Egypt (Ali et al., 1990) . Clinical disease has not been seen in the African buffalo (Syncerus caffer) nor in by far the majority of the water buffalo (Bubalis bubalis), when these were present in considerable numbers and closely associated with cattle affected with LSD in the Egyptian epizootic . No sero-surveys have been carried out to determine whether the buffalo suffered inapparent infections with seroconversion . Experimental pathogenicity studies are necessary to clarify this point . Antibody to Capripoxvirus has been found in the African buffalo, but may not be the result of an LSD infection . Camels are insusceptible . Rodents and lagomorphs are refractory to experimental laboratory infection . Experimental infections with the development of LSD-like skin lesions have been produced in impala (Aepyceros melampus), Thomson's gazelle (Gazella thomsonii) and the giraffe (Giraffa camelopardalis) (Young et al., 1968 ; Davies, unpublished data) . The first observation of LSD in Kenya in 1957 was made at a farm where there was an outbreak of sheep and goat pox (Burdin & Prydie, 1959) . There was a suggestion that the LSD had arisen following a change in the host affinities of the KSGPV . The lesions produced by these viruses are very similar, they are serologically identical and recent DNA restriction enzyme analysis has shown no differences . Intradermal inoculation of LSD into sheep or KSGPV into cattle produces a similar skin nodule (Capstick, 1961) . This explanation cannot be discounted . No sheep and goat pox has occurred in South Africa, however, during the LSD epizootics of the last 50 years, despite the presence of 40 million sheep .
PREVENTION Introduction of LSD by cattle movements New foci of infection can be created by the movement of cattle, from an area where there has been recent LSD virus activity (Diesel, 1949 ; Haig, 1957) . The animals may be infected or have recovered from recent infections . Statutory control regulations have been drawn up by the Office International des Epizooties (OIE)
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and are designated to prevent such occurrences . It is strongly recommended that they be followed . Many new outbreaks of LSD in sub-Saharan Africa have been related to cattle movements both within a country and to other countries . The introduction of LSD to Egypt followed the importation of cattle from Somalia, where there had been outbreaks of LSD . Such animals may not have shown more than a few old skin lesions of LSD, which a cursory inspection might not detect . They could, however, be infective . Clearly it is unsafe to import animals from areas where there has been LSD activity within the previous 1-3 years . Introduction of LSD by vector movements Much of the epidemiological evidence from Africa suggests that the movement of insect vectors has been responsible for the extension of LSD from disease foci, and has occurred despite the existence of strict movement control regulations (Haig, 1957) . An analysis of all the possibilities supports a conclusion that the recent outbreak of LSD in Israel followed the aerial movement of infected insect vectors (Shimshony, 1989) . LSD virus transmission was occurring between 70 and 300 km away to the west in Egypt. The meteorological data suggest that winds existed at the critical period which could have carried infected insects from the Nile delta . A previous experience of aerial movement of vectors from the delta, which introduced Anopheles pharoensis infected with malaria, was described by GarretJones (1962) . The potential for the spread of LSD from enzootic areas by such means, creates a need for continuous surveillance in all potentially receptive countries, and the establishment of contingency plans to deal with such an emergency . Vaccination Two vaccines have been widely used to protect cattle against LSD, both of which have been successful . More recently two further strains of vaccine virus have been used . Cross-protection between SGPV and LSD was first described in Kenya (Capstick & Coackley, 1961a), and the strains of SGPV so far studied have proved to be serologically identical with LSD (Davies & Atema, 1981) . A Kenyan strain has been used to protect cattle against LSD (Capstick & Coackley, 1961a, b ; Davies & Mbugwa, 1985) . The virus was cultured in lamb testis or fetal muscle cells for 16 passages and used as a freeze dried live virus vaccine . It can be used for the prophylaxis of SGPV and of LSD . In South Africa, the Neethling strain was passaged 60 times in lamb kidney cells and then 20 times in embryonated eggs (Weiss, 1968) . This vaccine, produced in tissue culture, is a satisfactory immunogenic modified live virus vaccine . It produces a granulomatous lump at the site of inoculation in up to 50% of cattle, which makes it unpopular with some cattle owners . An immunizing dose of approximately 10 35 tissue culture infective dose 50s (TCID50) is used for both vaccines in the field, although good protection is usually obtained with 10 2 TCID 50s . The Kenya KSGPV vaccine is absolutely safe in Bos indicus cattle but may produce a regional lymph node enlargement and on occasion mild LSD skin lesions in Bos taurus cattle . This complication has not been reproduced under experimental conditions despite the inoculation of up to 1000 times the field dose, and the
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possibility remains that it may be due to needle transmission of virulent LSD in the field . The reactions are seen in approximately 0 .02% of cattle and justify the continued use of the vaccine in the face of epizootics . In Egypt in 1989, a Roumanian sheep pox vaccine strain produced in sheep was successfully used to immunize cattle . No complications were recorded following its use (Ali-Moussa and Barsoum, personal communication) . The immunity induced by the live vaccines is considered to last for at least 2 years (KSGPV) and 3 years (South African strain) ; they probably produce a lifelong immunity in common with other poxvirus vaccines (Weiss, 1968) . Passive immunity in calves born to cows which have recovered from the disease persists for up to 6 months . The immunity may be assessed by serological methods (Weiss, 1968 ; Davies & Atema, 1978) . An indirect fluorescent antibody and a virus serum neutralization test have both been extensively used . After vaccination, however, a proportion of cattle with no detectable serum antibody to LSD will be refractory to challenge . An hypersensitivity test was developed at Kabete to detect this cellular immunity to LSD and is most valuable in determining the results of field vaccination campaigns (Capstick & Coackley, 1962) . Serological testing will show a much lower proportion of positive reactions .
CONTROL AND ERADICATION LSD has not been eradicated from any country iii sub-Saharan Africa in which it has appeared. The rapid response of the Israeli authorities, both in the diagnosis of LSD and in slaughtering all the diseased and in-contact cattle, appeared to have eradicated it for the first time . Elsewhere, an enzootic situation has developed in all the affected countries, with periodic cycles of sporadic and epizootic virus activity. In enzootic countries LSD is classified as a group A infectious disease by the OIE, and it is recommended that all countries at risk follow OIE's recommendations . Most countries have their own infectious disease control regulations, restricting all animal movement within a certain radius of an infected focus of disease . Cattle in the controlled area should be vaccinated against LSD as soon as possible and kept under close surveillance for at least 6 months . These measures can exert some control over outbreaks, but their success or otherwise may depend upon the climatic conditions and prevalence of insect vectors . Extension can and does occur despite these measures . For this reason, a much wider vaccination cover should be attempted and should include larger areas sharing the same biotype as the disease focus . All vaccination campaigns should be followed by a repeated vaccination at 4-6 month intervals to include calves born subsequent to the previous visit . Calves over 10 days of age may be vaccinated, although some may be passively protected by maternal antibody at the time . An annual vaccination policy should be maintained for 2-3 years . A reduction in clinical cases of LSD will follow such policies .
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Wherever possible, limited surveys should be carried out to determine the effectiveness of the vaccination campaigns . More intensive efforts to eliminate remaining foci of infection by slaughter could reduce the disease to the point where it might be eliminated . In practice, the resources which are required to achieve this have not been available on the African continent, where there are many other demanding animal disease problems simultaneously competing for the resources . Control in uninfected countries Efforts should be made to ensure that LSD does not become established in any country where it appears for the first time . In countries beyond the usual range of LSD in Africa, all infected and in-contact animals at the initial focus of disease should be slaughtered and the carcases disposed of by burial or burning, as soon as possible after the disease has been identified . Whether any vaccination should take place around the infected focus is debatable . If cattle in the surrounding areas are left unvaccinated, any residual infection in vector populations will quickly become apparent and further slaughter will be necessary . Vaccination will probably ensure that no further clinical cases occur, but there is a risk that the virus may persist and the disease may reappear at a later period . Which policy is followed will depend upon the resources available, the cattle populations at risk and other climatic and ecological factors . The costs of further slaughter could be avoided, however, and may make the second approach the more attractive .
GLOBAL IMPORTANCE OF LSD LSD must rank with foot and mouth disease, as an economically important disease of cattle . The debility which follows LSD may persist for 4-6 months and where there has been a high morbidity, the results can be disastrous (Diesel, 1949) . The chronic effects (anorexia due to mouth lesions, pneumonia, lameness, infertility, mastitis, cellulitis, infected lesions, etc .) all cause considerable consequential economic losses in affected herds . The hide damage is permanent, for the holes in the hide persist and greatly reduce their value to the leather industry (Green, 1959) . The prospect of the coexistence of LSD with the screw-worm fly Cochliomyia hominivorax creates a further dimension to its importance . The cycles of sporadic disease and epizootics are a continual drain upon the resources available for animal disease control in African countries . There is potential for extension into North Africa, the eastern and northern Mediterranean area, and possibly Asia . An important observation of LSD epizootics in the past 50 years is that it has the ability to spread rapidly in a wide range of ecotypes both in Africa and elsewhere . The delta and irrigated lands associated with the Nile have provided ideal conditions for epizootics of LSD . Well-head irrigation systems create a similar habitat and the summer populations of biting flies associated with cattle throughout the Mediterranean region could well propagate LSD . No studies have been made to identify the insect vectors capable of transmitting LSD virus in different situations . Such information is crucial to the control of the disease should it extend its range . There is no knowledge of the factors which pre-
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dispose to epizootics of the disease in Africa . There may be a relationship with rainfall and the characteristics of the intertropical convergence zone, as has been suggested for other African insect borne epizootic diseases such as Rift Valley fever and ephemeral fever (Davies et al., 1986, 1990) . These factors can be monitored and with the help of remote sensing satellite scanning, could allow some predictions to be made of the possibility of LSD epizootics . Cost effective vaccination campaigns could then be mounted to prevent their occurrence .
ACKNOWLEDGEMENTS The author wishes to thank the Directors of the Egyptian and Israeli Veterinary Services for many of the photographs.
REFERENCES ALEXANDER, R .A ., PLOWRIGHT, W . & HAIG, D . A . (1957) . Bulletin of Epizootic Diseases of Africa 5, 489 . ALI, A. A ., ESMAT, M ., ATTIA, H ., SELIM, A. & ABDER HAMID, Y. M . (1990) . Veterinary Record 27, 549 . AYRE-SMITH, R. A. (1960) . Veterinary Record 72, 469 . BLACK, D . N ., HAMMOND, J . M . & KITCHING, P . (1986) . Virus Research 5, 277 . BORST, G . H . A ., KIMMAN, T . G ., GIELKINS, A . L . J . & VAN DER KAMP, J . S. (1990) . Veterinary Record 127, 61 . BURDIN, M . L . (1959) . Bulletin of Epizootic Diseases of Africa 7, 27 . BURDIN, M . L . & PRYDIE, J . (1959) . Bulletin of Epizootic Diseases of Africa 7, 21 . CAPSTICK, P . B . (1961) . Annual Report Kenya Veterinary Department, p . 45 . CAPSTICK, P . B . & COACKLEY, W. (1961 a) . Research in Veterinary Science 2, 362 . CAPSTICK, P . B . & COACKLEY, W. (1961b) . Research in Veterinary Science 2, 369 . CAPSTICK, P . B . & COACKLEY, W. (1962) . Research in Veterinary Science 3, 287 . DAVIES, F . G . (1982) . Journal of Hygiene, Cambridge 88, 95 . DAVIES, F . G . & ATEMA, C . (1978) . Journal of Comparative Pathology 88, 205 . DAVIES, F . G . & ATEMA, C . (1981) . Research in Veterinary Science 31, 253 . DAVIES, F . G . & MBUGWA, G . (1985) . Journal of Comparative Pathology 95, 565 . DAVIES, F . G ., KRAuss, H ., LUND, L . J . & TAYLOR, M . (1971) . Research in. Veterinary Science 12, 123 . DAVIES, F . G ., LINTHICUM, K. J . &JAMES, A. D . (1986) . Bulletin of the World Health Organisation 63,941 . DAVIES, F . G ., OCHIENG, P . & WALKER, A . R . (1990) . Veterinary Microbiology 22, 129 . DE Boom, H . P . A. (1947) . South African Science 1, 44 . DIESEL, A . M . (1949) . Proceedings of the 14th International Veterinary Congress, London 2, 492 . GARRETT JONES, C . (1962) . Bulletin of the World Health Organisation 27, 299 . GERSHON, P . D . & BLACK, D . N . (1988) . Virology 164, 341 . GIBBS, E . P . J . & RWEYEMAMU, M . M . (1977) . Veterinary Bulletin 47, 411 . GREEN, H . F . (1959) . Bulletin of Epizootic Diseases of Africa 7, 63 . HMG, D . A. (1957) . Bulletin ofEpizootic Diseases of Africa 5, 421 . HAMBLIN, C ., ANDERSON, E . C ., JAGO, M ., MLENGEYA, T . & HIRII, K. (1990) . Epidemiology and
Infection 105, 585 . HUSTON, P . D . (1945) . Annual Report of the Chief Veterinary Surgeon, Southern Rhodesia . KITCHING, P . R . & MELLOR, P . S . (1986) . Research in Veterinary Science 40, 255 . KITCHING, P . R., BHAT, P . H . & BLACK, D . N . (1989) . Epidemiology and Infection 102, 335 .
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MACDONALD, R . A . S . (1931) . Northern Rhodesia Department of Animal Health. Annual Report, 1930, 20 . MACOWAN, K. D . S . (1959) . Bulletin of Epizootic Diseases ofAfrica7, 7 . MARTIN, W. B ., HAY, D ., CRAWFORD, L . V., LE BOUVIER, G . L . & CRAWFORD, E . M . (1966) . Journal of General Microbiology 45, 325 . MORRIS, J . P . A . (1931) . Northern Rhodesia Department of Animal Health, Annual Report, 1930, 20 . PLOWRIGHT, W . & WITCOMI, M . A . (1959) . Journal of Pathology and Bacteriology 78, 397 . SNIMSFIONY, A . (1989) . Proceedings of the 93rd Annual Meeting of the United States Animal Health Association, p . 334 . THOMAS, A. D . & MARE, C . V . E . (1945) . Journal of the South African Veterinary Medical Association 16, 36 . VAN ROOYEN, P . J ., MLNZ, B . K. & WEISS, K . E . (1968) . OnderstepoortJournal of Veterinary Research 36,165 . VON BACKSTROM, U . (1945) . Journal of the South African Veterinary Medical Association 16, 29 . WEISS, K . E . (1968) . In Virology Monographs, Vol . 3, p . 111 . Vienna & New York : Springer-
Verlag . YOUNG, E ., BASSON, P . A. & WEISS, K. (1968) . Onderstepoort journal of the Veterinary Research 37, 79 . (Accepted /brpublication 28 February 1991)
SPECIAL REVIEW SERIES
LUMPY SKIN DISEASE, AN AFRICAN CAPRIPOX VIRUS DISEASE OF CATTLE
F . G . DAVIES* Veterinary Research Laboratory, PO Kabete, Kenya
SUMMARY Lumpy skin disease is an infectious viral disease of cattle, which often occurs in epizootic form . The disease is characterized by the eruption of nodules in the skin, which may cover the whole of the animal's body . Systemic effects include pyrexia, anorexia, dysgalactia and pneumonia ; lesions are often found in the mouth and upper respiratory tract. The severity of the disease varies considerably between breeds and strains of cattle . Many cattle suffer severe emaciation and loss of production for several months . The skin lesions cause permanent damage to the hides . The mode of transmission of the disease has not been clearly established . Contact infections do not readily occur and the evidence from the epizootiology strongly suggests that insect vectors are involved . The disease has been confined to sub-Saharan Africa, until it recently appeared in epizootic form in Egypt and in Israel . Transmission occurs in a wide variety of biotypes, from semi-desert to temperate grasslands and irrigated land . It has the potential to extend its range further .
INTRODUCTION Lumpy skin disease (LSD) was first identified in Northern Rhodesia (Zambia) in 1929 (MacDonald, 1931) where it was first thought to be an allergic reaction to insect bites . The syndrome recurred irregularly there until 1943, when it spread to Botswana (Bechuanaland) (von Backstrom, 1945) and in 1945 to Zimbabwe (Southern Rhodesia) (Huston, 1945), and to South Africa (Thomas & Mare, 1945) . The transmissible nature of the disease became apparent to South African workers (Thomas & Mare, 1945 ; von Backstrom, 1945) during an epizootic which * Present address : G2 The Court, St . Mary's Place, Shrewsbury SYl 1 DY, U .K.
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affected most of the country . Some 8 million cattle were involved in this outbreak with enormous economic losses (Diesel, 1949) . In 1957, LSD was identified in East Africa, where it has recurred at intervals ever since (Burdin & Prydie, 1959) . Epizootics were reported from the Nile basin in the Sudan in the 1970s and during the period 1970-1985 LSD occurred in most Central and West African countries (Annual Reports FAO/OIE) . The disease is now enzootic throughout sub-Saharan Africa (including Madagascar) and recently appeared in Egypt in 1988 (Ali Moussa, personal communication, 1989) and in Israel in 1989 (Shimshony, 1989) . LSD extended from a focus in Ismailyia in Egypt and largely in the summer months of 1989, infected cattle in 22 of the 26 Governates of Egypt . Cases occurred from Aswan to the delta and also in the desert oasis well away from the Nile itself . The disease is now enzootic in Egypt . An extension has occurred eastwards into Israel, probably by the movement of insect vectors . Further spread westward may occur despite all the control strategies . The significance of an overlap in the distribution of LSD with Cochliomyia hominivorax in North Africa is great, for the multiple necrotic foci of the LSD lesions furnish ideal oviposition sites for the fly . Despite the success of the control measures, this remains a possibility . LSD has occurred in all the diverse ecological zones in Africa, from the high altitude temperate grasslands, the wet and dry bushed and wooded savannah to the very dry semi-desert . It has caused epizootics in irrigated areas of Africa and Egypt . Clearly, the potential exists for spread beyond its historical range, into North Africa, the Middle East and northern Mediterranean countries .
AETIOLOGY The initial observations suggested that the disease was an allergic response to insect bites (Morris, 1931) which was supported by the prevalence of the condition after the rains, when biting insect populations were at their highest . Experimental studies in South Africa (Thomas & Mare, 1945 ; von Backstrom 1945) showed that a transmissible agent was involved . The early attempts to identify this in tissue cultures were hampered by the simultaneous isolation of two different herpesviruses from LSD or similar lesions (Alexander et al., 1957) . A bovine herpesvirus 2 (Allerton or pseudo-lumpy skin disease virus) was isolated, which produces skin lesions which are very similar to LSD in the early stages (Alexander et al., 1957 ; Haig, 1957 ; Weiss, 1968 ; Davies et al., 1971) . A bovine herpesvirus 3 was also isolated from LSD tissue biopsy material but this caused no pathological changes in bovine skin when inoculated from infected tissue cultures (Alexander et al., 1957) . The third group of viruses isolated was of the pox type . They reproduced the disease when tissue culture harvests were reinoculated into susceptible cattle (Alexander et al., 1957) . Their morphology and physical characteristics were similar to those of the orthopoxviruses . Serological and immunological tests showed that they were capripoxviruses, very closely related to sheep and goat pox viruses (SGPV) (Capstick, 1961 ; Davies & Atema, 1981) . Most strains of these protect cattle against LSD when inoculated into cattle and LSD virus protects sheep and
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goats against capripox disease . Serological studies by immunofluorescence and virus-serum neutralization have not shown any differences between strains of LSD and of SGPV (Davies & Atema, 1981) . The strains of LSD thus far examined have all proved to be serologically identical (Davies, 1982) . Restriction enzyme studies suggest that LSD strains are identical and that their genome appears to be the same as that of the Kenya SGPV (KSGPV) (Black et al., 1986) . There are differences, however, between LSD and many of the SGPV from the Middle East and elsewhere (Gershon & Black, 1988 ; Kitching et al., 1989) .
CLINICAL SIGNS LSD is an acute infectious disease of cattle of all ages, well described by MacDonald (1931), von Backstrom (1945) and Ayre-Smith (1960) . There has recently been a clinical report of the disease in the Asian water buffalo (Bubalis bubalis), where five cases were observed in a group of 300 (Ali et al., 1990) . The incubation period after experimental inoculation is 4-12 days, usually about 7 days . The introduction of infected cattle is followed by disease in 2-5 weeks . There is a pyrexia of 40 .0-41 .5°C which may last 1-10 days . This is accompanied by lachrymation, increased nasal and pharyngeal secretions, anorexia, dysgalactia, some depression and a disinclination to move . Within 1-2 days, there is a cutaneous eruption of nodules, which may cover the whole of the body, while in other animals only a few may develop . The predilection sites are the head and neck, perineum, genitalia and udder, and the limbs (Figs 1-3) . The nodules are 5-50 mm or more in diameter, appearing as circumscribed areas of erect hair, firm and slightly raised from the surrounding normal skin . There is hyperaemia and drops of serum appear on the affected skin surface . The lesions are of a full skin thickness involving the epidermis, dermis and subcutis, which may be oedematous . The regional lymph nodes are enlarged and oedematous . Lesions develop on the muzzle, in the nares and oropharynx . They show a ringlike margin, where there has been separation from the surrounding healthy epithelium . Lesions may develop in the larynx and trachea, throughout the alimentary tract, and especially in the abomasum . They become necrotic and ulcerate . Mucopurulent nasal discharges, persistent dribbling of infected saliva, coughing and stertorious and often distressed respiration result . Conjunctivitis and keratitis commonly occur (Fig . 4) . Inflammatory and oedematous swellings of the limbs, brisket and genitalia may develop; the limbs can be three to four times their normal size (Fig . 5) . The lesions become necrotic and while some remain in situ, where they are recognizable for at least a year, others slough away to leave a full skin thickness hole, which usually becomes infected by suppurative bacteria . Larger areas of skin over a hard oedematous limb may become necrotic and slough away leaving a huge raw area ripe for bacterial infection and myasis . Herein lies the danger for infestation by Cochliomyia hominivorax. Lesions on the udder and teats may cause a mastitis and oedema with secondary infections and can result in sloughing of mammary tissue . Other nodular lesions develop in the subcutis, and may also be distributed throughout the connective tissue and muscles of the limbs and body .
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DISEASE
A limb swollen as a result of ISD infection.
Pneumonia is a common and often fatal complication of LSD. I,esions may be found throughout the upper respiratory tract and focal or larger areas of grey consolidation may develop in the lungs together with larger areas of bronchopneumania. Inhalation is a sequel of the necrotic lesions in the respiratory tract and it may prove f’ntal even months after the initial infection when a necrotic slough 1947). The pneumonias rarel, OCCL~ from an old tracheal lesion (de Boom, rrspond to antibiotic therapy. Xhortion frequently follows the acute infection in females, and infertility has been a problem in the months succeeding an outbreak. Bulls may have painful lesions of their genitalia, which can prevent them serving, and may remain infertile for 3-6 months after the disease. Females mav be anoestrous for se\~ral months. These effects are thought to he a consequence of the serious loss in COIIdition associated with LSD. Fetuses may be aborted, or the skin of calvrs t,orlI after an epi/ootic may be covered with lesions of ISD (Davies, unpublished data). Such intrauterine infections have also been described with swinepox ( BOI-st r/ (I/., 1990). Extensive skin lesions which produce a hidehourld condition, limb oedema and mouth and respirator-v lesions mav cause a prolonged anorexia and disinclination to I~OVC~. Rapid dete&ration in body condition follows and, LII~~CIrange COIIditions, the illability to move to water results in a higher mortality from dehvdration and predators. Some cattle become so emaciated that euthanasia is indicated. <:attle recovering from the disease may remain in extremely poor condition for 4-6 months (Diesel, 1949).
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PATHOLOGY The nodules in the . skin are a grey-white colour on section, and involve all layers of the skin and the subcutis . A reddish-yellow oedema may be present in the subcutaneous tissue in the region of multiple lesions . Nodules may be found in the subcutaneous tissue, muscle fascia and even in the muscle itself. The oropharyngeal lesions show the same features, but necrosis develops more rapidly than in the skin . The characteristics of the nodules in any part of the respiratory or alimentary tract are basically the same, as well as in the prepuce, vagina and wall of the uterus (Thomas & Mare, 1945 ; von Backstrom, 1945 ; Ayre-Smith, 1960) . Histological sections of early skin lesions show oedema, hyperplasia of the epidermis and infiltration with an epithelioid cell (the celles clavelleuse of Borrel), which is characteristic of lesions of LSD as well as of sheep and goat pox . Lymphocytes, plasma cells and fibroblasts appear in the later stages . Polymorphonuclear leucocytes and red cells are not seen in any numbers unless secondary infection and necrosis have developed . A vasculitis and perivasculitis with thrombosis of the blood vessels occur in the dermis and subcutis, and are the basic cause of the characteristic necrotic changes . There is a cuffing of the blood vessels with the white cells . Typical intracytoplasmic pox inclusion bodies may be seen in the epithelioid cells, in epithelial cells associated with hair follicles, in smooth muscle and skin glands . They are basically eosinophilic and a halo may surround them (probably an artefact) . The thrombosis, characteristic cellular infiltration and inclusion bodies are the important diagnostic features (Thomas & Mare, 1945 ; Burdin, 1959) . These basic histopathological changes are common to all LSD nodules wherever they occur in the body . The lungs will usually show a suppurative bronchopneumonia, with LSD nodules of varying sizes, grey-white in colour, which may be mistaken for secondary carcinomata .
DIAGNOSIS A presumptive diagnosis of LSD may be made on recognition of an epizootic disease of cattle, producing the characteristic skin nodules and systemic disease . Other domestic animals are not affected . It spreads in a totally random manner in cattle of all age groups, and no chemotherapeutic agent nor antibiotic has any effect upon the course of the disease . Restrictions on cattle movements do not prevent spread to neighbouring cattle . A laboratory diagnosis is desirable, especially in a situation where extension may have occurred from enzootic areas in Africa . This may be achieved by animal inoculation, histopathological examination of the lesions, virus isolation, or identification of the virus by electron microscopy or serology . Animal inoculation If quarantine facilities are available, the inoculation of calves with antibiotictreated suspensions of tissue from the skin lesions might be helpful . It would provide evidence for a viral aetiology and furnish excellent fresh material for virus isolation and other laboratory purposes . LSD virus will grow in embryonated hens
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eggs, but this system is insufficiently sensitive for primary virus isolation (van Rooyen et al., 1968) .
Histopathology Full skin thickness biopsies should be taken under local anaesthesia from recently formed skin nodules ; elliptical sections are recommended to include normal skin at the poles . Material should be taken from more than one animal, with at least two biopsies from each . Half of each biopsy should be placed in formol saline and half in a transport medium on ice for virological examination . Should a cryostat be available, the tissue may be taken immediately to prepare frozen sections . Histopathology provides a means of distinguishing the lesions from a variety of other conditions with which it may be confused (Burdin, 1959) . Those characteristics mentioned above serve to distinguish LSD from streptothricosis, globidiosis, anaphylaxis and bovine herpes type 2 infections . The latter shows only a superficial necrosis of the epidermis, which does not involve the dermis and subcutis to any great extent (Martin et al., 1966 ; Davies et al., 1971) . Anaphylaxis of the delayed type, following foot and mouth disease vaccination, can result in lesions grossly similar to LSD . These often become necrotic but show a quite different initial cellular reaction with more oedema, red cell infiltration and many eosinophils . Later there are many polymorphonuclear cells and often a superficial epidermal necrosis . Dermatophilus congolensis and globidiosis will be evident in smear preparations of the former and sections of the latter . The skin lesions of rinderpest are small and superficial and do not resemble those of LSD .
Virus Isolation Virus isolation may be attempted in primary or secondary cultures from lamb or calf tissues . None of the commonly used cell lines has proved sensitive enough for the primary isolation of the virus . Lamb testis is probably more sensitive than any other culture although lamb and calf kidney and calf testis are satisfactory (Alexander et al., 1957 ; Plowright & Witcomb, 1959) . Fetal lung, skin, muscle, endothelial cells and choroid plexus cells have all been successfully employed for this purpose . There are variations in the sensitivity of tissues from different breeds of sheep and cattle, wool sheep tissues have usually proved to be more sensitive than those from hair sheep and Bos taunts tissues more sensitive than those from Bos indicus. For example, a titre of 10"/0 .1 ml was obtained for a freeze dried stock of LSD in a testis culture from a wool sheep (Davies, unpublished data) . The same material titrated in testis cultures from a Kenyan Maasai sheep gave a titre of 10"/0 .1 ml . Such differences in sensitivity could greatly influence the success of virus isolation attempts . Biopsy tissue should be minced with scissors before grinding in a pestle in a medium containing antibiotics . Freezing and thawing of this suspension should be carried out two or three times in dry ice and alcohol or acetone, to release intracellular virus, the suspension clarified by centrifugation and inoculated onto suitable cultures . Cultures should be used with 75-80% cover; tube cultures with flying coverslips or slide cultures are recommended . These may be stained after 48-72 h by H&E or immunofluorescence or peroxidase methods . The poxvirus
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inclusions may be recognized in infected cultures and specifically identified as LSD by the immunological stains (Davies et al., 1971) . Cytopathological changes may become evident after 4-10 days in most cell cultures . These are focal initially, with groups of denser cells showing some rounding and stranding . These foci gradually enlarge to include much of the monolayer . Testis and kidney cultures usually show these changes earlier than others (Plowright & Witcomb, 1959) . The routine passage of cell cultures after 14 days or so is recommended in cases where little virus was present in the original inoculum and cytopathic changes were not obvious ; they will become apparent on passage .
Electron microscopy Tissue fragments from skin biopsies may be stained to show the brick shaped poxvirus particles . They are very readily seen in the early skin lesions . The point of a needle may be inserted into a tissue fragment from a fresh lesion, washed in a drop of distilled water and a 400 mesh carbon coated grid is then placed upon the drop, blotted and stained with phosphotungstic acid containing 0 .4% sucrose at pH 7 .0 for 1 min . The grid is blotted and dried before being examined at magnifications of 6000-18 000 . Several grids are prepared from each sample (Davies et al., 1971) . A recurring problem both in the isolation of LSD virus in tissue culture and the identification of the virus particles by electron microscopy has been the contamination of the material with herpesviruses . It is for this reason that biopsy material from two or three animals should be examined by both methods . Orphan herpesviruses of the bovine herpes group 3 (Gibbs & Rweyemamu, 1977) may overgrow cultures or be more readily seen on electron microscopy than the capripoxvirus particles ; they are unlikely to be present in material from all affected cattle . BHV 2 produces syncytia with intranuclear inclusions, while BHV-3 produces inclusions but not syncytia . Electron microscopy is a very rapid and valuable method to confirm LSD infections in cattle, with the reservation that the presence of many herpesvirus particles in a specimen does not negate the possibility of a positive diagnosis . Serological methods may be used retrospectively to confirm recent infections, but are not recommended for primary diagnosis . Fluorescent antibody and serumneutralization tests have been extensively used and have proved satisfactory (Weiss, 1968 ; Davies & Atema, 1978) .
EPIZOOTIOLOGY Source of virus Members of the family Poxviridae are resistant viruses . They may remain viable in scab or tissue fragments for very long periods of time . Virus persistence with long transmission intervals is thus possible . Transmission by contact with infected cattle does not readily occur in insect-proofed cattle pens . The sharing of water troughs was necessary to effect transmission between infected and uninfected cattle in adjacent pens (Weiss, 1968) . No transmission occurred if separate water troughs were provided . Virus is present in nasal, lachrymal and pharyngeal
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secretions, semen, milk, and blood (Thomas & Mare, 1945 ; Weiss, 1968) . Viraemia can usually be detected for only 2-4 days, the virus may be in saliva for up to 11 days and in semen for up to 22 days . All could play some role in the short interval transmission of LSD . In virtually all outbreaks, however, the virus appears to be propagated by a continuous cattle-arthropod vector-cattle cycle (MacDonald, 1931 ; Morris, 1931 ; Thomas & Mare, 1945 ; von Backstrom, 1945 ; Diesel, 1949 ; Haig, 1957 ; Burdin & Prydie, 1959 ; MacOwan, 1959) . Transmission There is little information on the transmission of LSD by insect vectors, Weiss (1968) described the isolation of the virus from Stomoxys calcitrans and from Biomyia fasciata, Stomoxys spp . have been shown to be capable of transmitting SGPV (Kitching & Mellor, 1986) . Field observations of biting flies associated with cattle during epizootics of LSD suggest that mosquitoes, tabanids, Culicoides, and Glossina spp . may play some role in the propagation of the virus by mechanical transmission . Any biting fly could transmit LSD following an interrupted feed . Infected secretions could be siphoned by flies such as Musca spp . and transferred to susceptible cattle . Further research on the transmission of LSD is required to clarify these points . Cold weather with frosts causes a sharp reduction in the transmission of LSD, with either very few or no new cases appearing . This has been related to the reduction in insect vector populations . An interseasonal quiescent period is followed by the re-emergence of disease in the areas where it was seen before the onset of colder weather . The virus is thought to persist by cycles of inapparent infections in cattle or in old lesions . Patterns of disease LSD may occur sporadically throughout an enzootic area over a period of several years or behave as an epizootic with fairly rapid spread of the disease throughout a country or region . Examples of the latter are the South African epizootic of 1945-1949 (Diesel, 1949) and the Egyptian summer epidemic of 1989 . Similar epizootics have occurred in Kenya, the Sudan and Ruanda . The epizootics in different countries have shown much variation in morbidity and mortality, and the rates at which they spread . Such differences are thought to reflect the efficiency and population densities of vector populations . The rapid extension of LSD throughout Egypt in the summer of 1989 would suggest that large numbers of potential insect vectors were present, and this was the case . Similar rapid spread has been seen in Africa, although slower rates of transmission have generally been found in sub-Saharan countries, with periods of increased activity after the seasonal rains . The morbidity rates vary enormously; rates of 80% or more have been seen in South Africa and elsewhere but 2-20% is more usual . Both high and low rates may be found at different farms during the same epizootic . The mortality rates are usually low, in the range 2-10%, but rates of 40-75% have been recorded . There are differences in the susceptibility of Bos taurus and Bos indicus cattle . Breeds such as the Jersey and Guernsey appear to be clinically more severely affected by LSD than others (Ayre-Smith, 1960) ; Friesian and Ayrshire cattle are also highly susceptible .
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The breeds of zebu type indigenous to Africa are generally less susceptible . They may develop extensive skin lesions, but have less severe clinical disease and lower mortality rates than cattle exotic to Africa . Occasional cases of LSD have appeared in cattle in Kenya, during inter-epizootic periods, when no clinical disease has been identified for periods of several years . These have often been in relatively small isolated herds with no history of movement nor contact with other cattle . The source of the virus in those outbreaks was not apparent. Investigations showed that many African buffalo (Syncerus caffer) in the adjacent forest areas, had serum antibody to Capripoxvirus, but not to cowpox virus (Davies, 1982) . The antibody may have followed an infection with LSD virus or another unknown Capripoxvirus strain . It was not possible specifically to identify antibody to LSD . A recent investigation of .a buffalo population from an area, where LSD had been prevalent, found no antibody to LSD (Hamblin et al., 1990) . Natural infections have not been observed in any other ruminant species during epizootics of LSD, with the possible exception of a recent report of five cases in the Asian water buffalo (Bubalis bubalis) in Egypt (Ali et al., 1990) . Clinical disease has not been seen in the African buffalo (Syncerus caffer) nor in by far the majority of the water buffalo (Bubalis bubalis), when these were present in considerable numbers and closely associated with cattle affected with LSD in the Egyptian epizootic . No sero-surveys have been carried out to determine whether the buffalo suffered inapparent infections with seroconversion . Experimental pathogenicity studies are necessary to clarify this point . Antibody to Capripoxvirus has been found in the African buffalo, but may not be the result of an LSD infection . Camels are insusceptible . Rodents and lagomorphs are refractory to experimental laboratory infection . Experimental infections with the development of LSD-like skin lesions have been produced in impala (Aepyceros melampus), Thomson's gazelle (Gazella thomsonii) and the giraffe (Giraffa camelopardalis) (Young et al., 1968 ; Davies, unpublished data) . The first observation of LSD in Kenya in 1957 was made at a farm where there was an outbreak of sheep and goat pox (Burdin & Prydie, 1959) . There was a suggestion that the LSD had arisen following a change in the host affinities of the KSGPV . The lesions produced by these viruses are very similar, they are serologically identical and recent DNA restriction enzyme analysis has shown no differences . Intradermal inoculation of LSD into sheep or KSGPV into cattle produces a similar skin nodule (Capstick, 1961) . This explanation cannot be discounted . No sheep and goat pox has occurred in South Africa, however, during the LSD epizootics of the last 50 years, despite the presence of 40 million sheep .
PREVENTION Introduction of LSD by cattle movements New foci of infection can be created by the movement of cattle, from an area where there has been recent LSD virus activity (Diesel, 1949 ; Haig, 1957) . The animals may be infected or have recovered from recent infections . Statutory control regulations have been drawn up by the Office International des Epizooties (OIE)
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and are designated to prevent such occurrences . It is strongly recommended that they be followed . Many new outbreaks of LSD in sub-Saharan Africa have been related to cattle movements both within a country and to other countries . The introduction of LSD to Egypt followed the importation of cattle from Somalia, where there had been outbreaks of LSD . Such animals may not have shown more than a few old skin lesions of LSD, which a cursory inspection might not detect . They could, however, be infective . Clearly it is unsafe to import animals from areas where there has been LSD activity within the previous 1-3 years . Introduction of LSD by vector movements Much of the epidemiological evidence from Africa suggests that the movement of insect vectors has been responsible for the extension of LSD from disease foci, and has occurred despite the existence of strict movement control regulations (Haig, 1957) . An analysis of all the possibilities supports a conclusion that the recent outbreak of LSD in Israel followed the aerial movement of infected insect vectors (Shimshony, 1989) . LSD virus transmission was occurring between 70 and 300 km away to the west in Egypt. The meteorological data suggest that winds existed at the critical period which could have carried infected insects from the Nile delta . A previous experience of aerial movement of vectors from the delta, which introduced Anopheles pharoensis infected with malaria, was described by GarretJones (1962) . The potential for the spread of LSD from enzootic areas by such means, creates a need for continuous surveillance in all potentially receptive countries, and the establishment of contingency plans to deal with such an emergency . Vaccination Two vaccines have been widely used to protect cattle against LSD, both of which have been successful . More recently two further strains of vaccine virus have been used . Cross-protection between SGPV and LSD was first described in Kenya (Capstick & Coackley, 1961a), and the strains of SGPV so far studied have proved to be serologically identical with LSD (Davies & Atema, 1981) . A Kenyan strain has been used to protect cattle against LSD (Capstick & Coackley, 1961a, b ; Davies & Mbugwa, 1985) . The virus was cultured in lamb testis or fetal muscle cells for 16 passages and used as a freeze dried live virus vaccine . It can be used for the prophylaxis of SGPV and of LSD . In South Africa, the Neethling strain was passaged 60 times in lamb kidney cells and then 20 times in embryonated eggs (Weiss, 1968) . This vaccine, produced in tissue culture, is a satisfactory immunogenic modified live virus vaccine . It produces a granulomatous lump at the site of inoculation in up to 50% of cattle, which makes it unpopular with some cattle owners . An immunizing dose of approximately 10 35 tissue culture infective dose 50s (TCID50) is used for both vaccines in the field, although good protection is usually obtained with 10 2 TCID 50s . The Kenya KSGPV vaccine is absolutely safe in Bos indicus cattle but may produce a regional lymph node enlargement and on occasion mild LSD skin lesions in Bos taurus cattle . This complication has not been reproduced under experimental conditions despite the inoculation of up to 1000 times the field dose, and the
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possibility remains that it may be due to needle transmission of virulent LSD in the field . The reactions are seen in approximately 0 .02% of cattle and justify the continued use of the vaccine in the face of epizootics . In Egypt in 1989, a Roumanian sheep pox vaccine strain produced in sheep was successfully used to immunize cattle . No complications were recorded following its use (Ali-Moussa and Barsoum, personal communication) . The immunity induced by the live vaccines is considered to last for at least 2 years (KSGPV) and 3 years (South African strain) ; they probably produce a lifelong immunity in common with other poxvirus vaccines (Weiss, 1968) . Passive immunity in calves born to cows which have recovered from the disease persists for up to 6 months . The immunity may be assessed by serological methods (Weiss, 1968 ; Davies & Atema, 1978) . An indirect fluorescent antibody and a virus serum neutralization test have both been extensively used . After vaccination, however, a proportion of cattle with no detectable serum antibody to LSD will be refractory to challenge . An hypersensitivity test was developed at Kabete to detect this cellular immunity to LSD and is most valuable in determining the results of field vaccination campaigns (Capstick & Coackley, 1962) . Serological testing will show a much lower proportion of positive reactions .
CONTROL AND ERADICATION LSD has not been eradicated from any country iii sub-Saharan Africa in which it has appeared. The rapid response of the Israeli authorities, both in the diagnosis of LSD and in slaughtering all the diseased and in-contact cattle, appeared to have eradicated it for the first time . Elsewhere, an enzootic situation has developed in all the affected countries, with periodic cycles of sporadic and epizootic virus activity. In enzootic countries LSD is classified as a group A infectious disease by the OIE, and it is recommended that all countries at risk follow OIE's recommendations . Most countries have their own infectious disease control regulations, restricting all animal movement within a certain radius of an infected focus of disease . Cattle in the controlled area should be vaccinated against LSD as soon as possible and kept under close surveillance for at least 6 months . These measures can exert some control over outbreaks, but their success or otherwise may depend upon the climatic conditions and prevalence of insect vectors . Extension can and does occur despite these measures . For this reason, a much wider vaccination cover should be attempted and should include larger areas sharing the same biotype as the disease focus . All vaccination campaigns should be followed by a repeated vaccination at 4-6 month intervals to include calves born subsequent to the previous visit . Calves over 10 days of age may be vaccinated, although some may be passively protected by maternal antibody at the time . An annual vaccination policy should be maintained for 2-3 years . A reduction in clinical cases of LSD will follow such policies .
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Wherever possible, limited surveys should be carried out to determine the effectiveness of the vaccination campaigns . More intensive efforts to eliminate remaining foci of infection by slaughter could reduce the disease to the point where it might be eliminated . In practice, the resources which are required to achieve this have not been available on the African continent, where there are many other demanding animal disease problems simultaneously competing for the resources . Control in uninfected countries Efforts should be made to ensure that LSD does not become established in any country where it appears for the first time . In countries beyond the usual range of LSD in Africa, all infected and in-contact animals at the initial focus of disease should be slaughtered and the carcases disposed of by burial or burning, as soon as possible after the disease has been identified . Whether any vaccination should take place around the infected focus is debatable . If cattle in the surrounding areas are left unvaccinated, any residual infection in vector populations will quickly become apparent and further slaughter will be necessary . Vaccination will probably ensure that no further clinical cases occur, but there is a risk that the virus may persist and the disease may reappear at a later period . Which policy is followed will depend upon the resources available, the cattle populations at risk and other climatic and ecological factors . The costs of further slaughter could be avoided, however, and may make the second approach the more attractive .
GLOBAL IMPORTANCE OF LSD LSD must rank with foot and mouth disease, as an economically important disease of cattle . The debility which follows LSD may persist for 4-6 months and where there has been a high morbidity, the results can be disastrous (Diesel, 1949) . The chronic effects (anorexia due to mouth lesions, pneumonia, lameness, infertility, mastitis, cellulitis, infected lesions, etc .) all cause considerable consequential economic losses in affected herds . The hide damage is permanent, for the holes in the hide persist and greatly reduce their value to the leather industry (Green, 1959) . The prospect of the coexistence of LSD with the screw-worm fly Cochliomyia hominivorax creates a further dimension to its importance . The cycles of sporadic disease and epizootics are a continual drain upon the resources available for animal disease control in African countries . There is potential for extension into North Africa, the eastern and northern Mediterranean area, and possibly Asia . An important observation of LSD epizootics in the past 50 years is that it has the ability to spread rapidly in a wide range of ecotypes both in Africa and elsewhere . The delta and irrigated lands associated with the Nile have provided ideal conditions for epizootics of LSD . Well-head irrigation systems create a similar habitat and the summer populations of biting flies associated with cattle throughout the Mediterranean region could well propagate LSD . No studies have been made to identify the insect vectors capable of transmitting LSD virus in different situations . Such information is crucial to the control of the disease should it extend its range . There is no knowledge of the factors which pre-
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dispose to epizootics of the disease in Africa . There may be a relationship with rainfall and the characteristics of the intertropical convergence zone, as has been suggested for other African insect borne epizootic diseases such as Rift Valley fever and ephemeral fever (Davies et al., 1986, 1990) . These factors can be monitored and with the help of remote sensing satellite scanning, could allow some predictions to be made of the possibility of LSD epizootics . Cost effective vaccination campaigns could then be mounted to prevent their occurrence .
ACKNOWLEDGEMENTS The author wishes to thank the Directors of the Egyptian and Israeli Veterinary Services for many of the photographs.
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Infection 105, 585 . HUSTON, P . D . (1945) . Annual Report of the Chief Veterinary Surgeon, Southern Rhodesia . KITCHING, P . R . & MELLOR, P . S . (1986) . Research in Veterinary Science 40, 255 . KITCHING, P . R., BHAT, P . H . & BLACK, D . N . (1989) . Epidemiology and Infection 102, 335 .
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MACDONALD, R . A . S . (1931) . Northern Rhodesia Department of Animal Health. Annual Report, 1930, 20 . MACOWAN, K. D . S . (1959) . Bulletin of Epizootic Diseases ofAfrica7, 7 . MARTIN, W. B ., HAY, D ., CRAWFORD, L . V., LE BOUVIER, G . L . & CRAWFORD, E . M . (1966) . Journal of General Microbiology 45, 325 . MORRIS, J . P . A . (1931) . Northern Rhodesia Department of Animal Health, Annual Report, 1930, 20 . PLOWRIGHT, W . & WITCOMI, M . A . (1959) . Journal of Pathology and Bacteriology 78, 397 . SNIMSFIONY, A . (1989) . Proceedings of the 93rd Annual Meeting of the United States Animal Health Association, p . 334 . THOMAS, A. D . & MARE, C . V . E . (1945) . Journal of the South African Veterinary Medical Association 16, 36 . VAN ROOYEN, P . J ., MLNZ, B . K. & WEISS, K . E . (1968) . OnderstepoortJournal of Veterinary Research 36,165 . VON BACKSTROM, U . (1945) . Journal of the South African Veterinary Medical Association 16, 29 . WEISS, K . E . (1968) . In Virology Monographs, Vol . 3, p . 111 . Vienna & New York : Springer-
Verlag . YOUNG, E ., BASSON, P . A. & WEISS, K. (1968) . Onderstepoort journal of the Veterinary Research 37, 79 . (Accepted /brpublication 28 February 1991)