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Practical relationships between the biology and control of cattle grubs
Veterinary Parasitology, 3 (1977) 000--000 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
251
PRACTICAL RELATIONSHIPS BETWEEN THE BIOLOGY AND CONTROL OF CATTLE GRUBS
W.N. BEESLEY
Department of Veterinary Parasitology, Liverpool School of Tropical Medicine, Liverpool L3 5QA (Great Britain) (Received 11 August 1976)
ABSTRACT Beesley, W.N., 1977. Practicalrelationshipsbetween the biology and control of cattle grubs. Vet. Parasitol., 3: 251--257. This paper deals with the toxic effects o f treatment of cattle against warble infestations, including the incidence of symptoms and whether these are due to death o f migrating larvae or insecticide poisoning, or both. It was concluded that the likelihood of deaths associated with treatment by systemic insecticides is much less than 1 in 5 000 and may be as rare as 1 in 100 000. The importance of a national scheme to eradicate cattle grubs in Britain was stressed in view of the successes in controlling cattle grubs in Northern Ireland and in Eire.
INTRODUCTION
The annual incidence of warbled hides in England and Wales reached 36.1% in 1941 and since then has frequently exceeded 20%. These figures relate to unhealed warble holes seen in "all cattle hides" at 40 representative hide markets, but they exclude hides with healed warble holes. If the data are abstracted to include only young animals, in the key hide-warble months of April to June, monthly figures of 60--70% were obtained (Beesley, 1971, 1973). In recent years, the warble incidence has fluctuated with annual "all hides" figures of 13.5% in 1972, 8.8% in 1973, and 11.8% in 1974 (Hide and Allied Trades Improvement Society, personal communication, 1976). Corresponding annual figures for steers and heifers alone were 14.8, 10.6 and 16.4%, and the peak summer figures for these animals were 33.8, 25.8 and 41.8% for May 1972, June 1973 and May 1974. The May 1974 peak number for younger animals was the worst since the summer of 1964. It appears that no significant progress was made in controlling cattle grubs since 1964, despite the annual use of well over 4 × l 0 s doses of systemic insecticides. By comparison, the Eireann figures for all hides were about 0.3% in 1973 and 0.05% in 1974.
252 The systemic insecticides now used in Britain are the pour-ons Tiguvon 1 (fenthion, O,O-dimethyl O-[4-(methylthio)-m-tolyl] phosphorothioate), Ruelene 2 (crufomate, 4-tert-butyl-2-chlorophenyl methyl methylphosphoramidate), and Prolate 3 (phosmet, O,O-dimethyl phosphorodithioate S-ester with N-(mercaptomethyl) phthaiimide); and the insecticidal mineral lick, Etrolene 4 (ronnel or fenchlorphos, O,O-dimethyl O-(2,4,5-trichlorophenyl) phosphorothioate). Tiguvon replaced the pour-on preparations Asuntol 5 (coumaphos, 3-chloro-7-hydroxy-4-methylcoumarin O-ester with O,Odiethyl phosphorothioate), Neguvon 6 (trichlorfon, dimethyl(2,2,2-trichloro1-hydroxyethyl) phosphonate) and Dyvon 7 (trichlorfon), while Ruelene replaced the original fenchlorphos boluses. All these insecticides are very effective. There is, at present, no statutory obligation in Britain to treat cattle with systemics. Suggestions have been made for a voluntary a u t u m n treatment, with m a n d a t o r y treatment of visibly infested cattle the next spring. It has been estimated that only about 30% of cattle in Britain are treated with systemics. A national plan for the treatment of warble is therefore long overdue. Five problems seem to prevent the successful control of warble. First, lack o f government or commercial funds to subsidize the cost of insecticide and its application to cattle. Second, real or assumed lack o f manpower to muster and treat animals, including decisions as to which animals shall have their treatment postponed because o f sickness, stress, age or pregnancy. Third, lack o f financial incentive for the farmer. The tanner may pay over £1 extra for a clean hide. This, however, does not reach the producer. Fourth, low priority for local or national warble control schemes in the overall programme of animal health improvement. This is especially evident when a warble control programme is presented in competition with proposals for the eradication of brucellosis, vesicular diseases, and tuberculosis. Fifth, uncertainty about possible toxic effects in cattle, and their cause. In this paper we are concerned mainly with the fifth item. TOXICITY Warble larvae spend 5--6 months migrating through the tissues of the host, overwintering in the wail o f the oesophagus (Hypoderma lineatum) or in the epidurai fat (H. boris). The larvae can cause severe reactions if they die in either site, but especially in the spinal cord. Larvae could die naturally or by host reaction in an animal already sensitized to Hypoderma antigen and this could stimulate a shock reaction (Beesley, 1971). Alternatively, successful treatment would have killed the larvae, liberating toxic material which could 1-7 Chemicals 1, 5, 6 and 7 are produced by Farbenfabriken Bayer A.G., Leverkusen, Federal Republic of Germany; chemicals 2 and 4 by Dow Chemical Corporation, Midland, Mich., U.S.A.; and chemical 3 by Stauffer Chemical Company, Richmond, Calif., U.S.A.
253
damage the host's spinal cord, oesophagus etc., again producing shock reaction in some animals (Beesley, 1971). There might be simultaneous toxicity by the organophosphorus insecticide as some individuals show idiosyncrasies to organophosphorus compounds. It is, therefore, useful to distinguish between the different types of toxic reactions that occasionally occur following treatment of warbled animals. In Britain, warble larvae arrive in the back of their host from January to July, the largest numbers being during March to June. The data obtained from examination of hides indicate slightly later dates due to the time needed to get the hides from the abattoir to the market, with possibly a storage period between. Larvae of H. lineatum reach the oesophagus from September to February (Beesley, 1966) and H. bovis reach the epidural fat from November to May (Beesley, 1962). It is undesirable to treat cattle while larvae are lodged in large numbers in the oesophagus or spinal canal. Manufacturers of systemic insecticides warn of the danger period and contraindicate treatment during late November to early March. The warble area o f the British Isles extends north and south for about 800 km, and it is cool and wet in the north and west. Mustering and treatment of cattle has to be fitted into the farming timetable, and in practice some treatments will be inevitably administered a little later than the recommended time. Individual idiosyncrasies resulting in a few cases exhibiting clinical signs of toxicity and deaths occur even when every posssible precaution has been taken to ensure correct dosing. This has happened in the well-planned programmes of Eire, Northern Ireland, and Switzerland. In East Germany, Hiepe et al. (1974) recorded only 24 sick cattle and 0.7 deaths per 100 000 treated with systemics in 1972. Before the discovery of the systemics, reactions in warbled cattle, whether treated with derris or left untreated, were reported occasionally. " R o s e fever", characterized by anaphylaxis-like symptoms, was related to the release of toxic material from dead first, second or third stage larvae. The same symptoms can be produced by injecting the contents of first stage larvae into cattle and rabbits (Anderson and Kirkwood, 1968; Beesley, 1971). In cattle treated with systemics, one must always remember that the animal may or may not be infested. From one to over 200 larvae, possibly of both H. lineatum and H. bovis, m a y be in the animal (Rich, 1965 a). Cox et al. (1970) reported t w o cases in which one treated and one untreated animal died at about the same time. The first animal was treated within the cut-off period and became ill 3 days after treatment. On post-mortem examination, conducted 27 days after treatment, it was found to harbour six live and one dead H. bovis larvae in the spine and two live and one dead H. lineatum larvae in the oesophagus. The second animal died from pneumonia and postmortem examination showed seven live H. bovis and eight live and five dead H. lineatum larvae. In evaluating the evidence about toxicity in treated cattle, one must be
254
critical of what was seen at post-mortem examination, if recovered larvae were alive or dead, etc. Many investigators claim that early toxic signs are due to the anti-cholinesterase effects of the organophosphorus insecticide, while later s y m p t o m s are most probably due to interactions between larval products and the host tissues (Magat et al., 1969). Others maintain that reactions from larval l~resence are rare and that the great majority of ill effects are simply due to organophosphorus poisoning (Khan, 1973). S y m p t o m s that develop 0.5--6 h after treatment with systemics are typical of organophosphorus poisoning and include salivation, lachrymation, myosis, muscle twitching, dyspnoea, colic, stiffness and paralysis. There is usually no oesophagal or pulmonary oedema or bloat. The clinical signs may resolve spontaneously or following treatment with atropine, 2-PAM (pralidoxime iodide), or TMB-4 (1,3-di(4-hydroxyiminomethylpyridinium) propane dibromide). These clinical signs may persist for 1--4 weeks, perhaps with fatal results (Khan, 1973). If the symptoms continue, they may overlap the start of secondary symptoms, i.e. those from host--larval reactions (Walsh, 1963). Toxic signs associated with the death of larvae in the animals' tissues include oedema of the oesophagus and bloat in the case of H. lineaturn, or ataxia and paralysis of the hindquarters in the case of H. bovis. Other signs include oedema of the eyelids and ear pinnae, congested conjunctivae, lachrymation, salivation, coughing, dyspnoea and urticaria. These signs are similar to those of shock in cattle following injection of egg albumen or histamine into uninfected calves (Anderson and Kirkwood, 1968; Aitken and Sanford, 1969). Systemic insecticides are found in the blood within 15 min (Fredriksson, 1962). Rapid kill of larvae and massive release of larval enzymes may therefore be expected. In practice this does not happen. Hypoderma larvae are remarkably resistant to organophosphorus systemic insecticides. First instar larvae of H. bovis and H. lineatum exposed to 32 ppm crufomate or trichlorfon in vitro survive for 4 and 32 days, respectively (Beesley, 1967). In naturally infested cattle, orally treated with dimethoate, first instar larvae died in 4--7 days (Hewitt et al., 1958). In calves where larvae were artificially implanted, all first instar larvae died within 14--27 days after pour-on treatments with trichlorfon, crufomate or fenchlorphos (Beesley, 1966). Anderson and Kirkwood (1968) considered that warble larvae exposed to systemic insecticides die simultaneously, but that the subsequent release o f larval toxin is insufficient to cause harm to the host. They based their suggestion on experiments in which material from up to 45 larvae, injected intravenously, failed to kill calves. The effect of a steady release of larval toxin into the system o f the host over a period of many days is not known. Rich (1965 a) reported the effect of systemic insecticide treatment in 11 537 cattle of different ages. Of 4 679 animals older than I year, three became ill and one died; while of 6 858 animals under 12 months, 71 were ill and four died. None of the toxic symptoms were typical of organophosphorus poisoning and nearly all the affected animals suffered from bloat. On
255
post-mortem examination of dead animals extensive oesophagopharyngeal necrosis was a c o m m o n lesion. Poor nutrition, stress following vaccination, dehorning, or branding did n o t seem to affect the incidence o f development of toxic signs. It was concluded that the reactions of the host to release of larval toxins were responsible for all the clinical signs observed. Deliberate overdosing of cattle with pour-on insecticides has frequently failed to produce poisoning (Mozier, 1969). In Eire, it has been suggested that, in the later years of the eradication programme, an increasing proportion of toxic effects was due to reactions to dead larvae rather than to organophosphorus poisoning (H. Thornberry, personal communication, 1976). This was supported by the finding in many districts that some animals became ill after treatment with systemics in 1964 b u t were unaffected in the next year, when they were warble free. In the same areas, cattle that were brought in from districts where warble infestation was c o m m o n developed toxicity in only a few cases. PROBLEMS IN ERADICATION
Apart from problems of toxic effects of warble infestation other features should be considered in the control of this parasite. Some biologists believe that once the warble population falls below some threshold value the complete biomass automatically disappears. Wide fluctuations have occurred, however, in the degree of warble infestation both in Eire and in England and Wales. The incidence of warbled hides can increase by 50% in only 12 months, as it did in 1969--1970 and 1973--1974 in Britain (Beesley, 1973). Control programmes therefore must be continuous, thorough, and well supervised. Even an overall hide-warble incidence of 0.3% (the latest available from Eire) indicates that warble has not been eradicated. The same difficulty was encountered by Rich (1965 b), who used systemics on an isolated range herd in Canada. He reduced infestation from 30.2 to 0.2 grubs/head, b u t when treatment was discontinued the infestation rate rose to 10.2 grubs/head, in only two seasons. Khan (1968}, in a 3-year programme in Alberta, Canada, virtually eradicated warble from 5 000 cattle; no grubs remained in untreated control cattle within the same area, and even cattle just outside the trial area had fewer warbles than at the start of the programme. Despite this encouraging result, Khan decided to spring treat any cattle seen infested rather than wait until the fall. Workers were n o w realizing that, with the warble fly, drastic reduction and complete elimination are very different. Resilience in insect populations is a feature in other pests, such as tsetse fly and mosquitoes. This may account for the recent difficulties encountered in the programme to control screw-worm in the United States. The World Health Organization (WHO, 1970) recommended that special attention be given to the problem of small residual populations of insects that appear to be withstanding good control measures. It may be that a few individual
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i n s e c t s c a n f l y f u r t h e r o r l a y m o r e eggs t h a n h a d b e e n e s t i m a t e d p r e v i o u s l y . F l i g h t r a n g e , t h e r e f o r e , is i m p o r t a n t in r e g i o n a l c o n t r o l s c h e m e s . I n e x p e r i m e n t s w i t h Hypoderma G r e g s o n ( 1 9 5 6 ) b r e d , m a r k e d , r e l e a s e d a n d r e c a p t u r e d w a r b l e flies. T h e m a x i m u m d i s t a n c e t r a v e l l e d in his e x p e r i m e n t s w a s 3 0 0 m , b u t as t h i s w a s o b s e r v e d in 9 5 m i n i t s e e m s s a f e t o a s s u m e t h a t u n d e r n a t u r a l c o n d i t i o n s w a r b l e flies g o m u c h f u r t h e r , p a r t i c u l a r l y w h e n a t t r a c t e d t o m o v i n g c a t t l e o r b l o w n b y t h e w i n d . G r e g s o n a l s o s t a t e d t h a t a d u l t Hypoderma c a n f l y u p t o 1 4 . 5 k m .
REFERENCES Aitken, M.M. and Sanford, J., 1969. Experimental anaphylaxis in cattle. J. Comp. Pathol. Ther., 79: 131--139. Anderson, P.H. and Kirkwood, A.C., 1968. A reaction in cattle to toxins of Hypoderma bovis (warble fly)larvae. Br. Vet. J., 124: 569--575. Beesley, W.N., 1962. Observations on the development of Hypoderma bovis de Geer (Diptera, Oestridae) in bovine host. Res. Vet. Sci., 3: 203--208. Beesley, W.N., 1966. The use in Britain of systemic insecticides for the control of Hypoderma (Diptera: Oestridae). Vet. Med. Rev., 1966: 37--38. Beesley, W.N.,1967. Observations on the biology of the ox warble fly (Hypoderma, Diptera: Oestridae). I: In vitro culture of the first instar larva. Ann. Trop. Med. Parasitol., 61: 175--181. Beesley, W.N., 1971. Observations on the biology of the ox warble fly (Hypoderma, Diptera: Oestridae). V: Anaphylactoid shock in laboratory animals and calves following exposure to extracts o f larvae of Hypoderma. Ann. Trop. Med. Parasitol., 65: 567--572. Beesley, W.N., 1973. Recent developments in the control of ectoparasites of animals. Proc. 7th Br. Insecticide Fungicide Conf. Br. Crop Prot. Counc., London, pp. 856--875 Cox, D.D., Mozier, J.O. and Mullee, M.T., 1970. Posterior paralysis in a calf caused by cattle grubs (Hypoderma bovis) after treatment with a systemic insecticide for grub control. J. Am. Vet. Med. Assoc., 157: 1088--1092. Fredriksson, T., 1962. Studies on the percutaneous absorption of parathion and paroxon. V. Rate of absorption of paroxon. J. Invest. Dermatol., 38: 233--236. Gregson, J.D., 1956. Recent cattle grub life-history studies at Kamloops, British Columbia and Lethbridge, Alberta. Proc. 10th Int. Entomol. Congr., 3: 725--734. Hewitt, R., Emro, J., Entwistle, J., Pankavich, J., Thorson, R., Wallace, W. and Waletzky, E., 1958. Carbamoyl alkyl phosphordithioates as chemotherapeutic agents: effects of dimethoate against grubs in cattle. J. Econ. Entomol., 51 : 445--450. Hiepe, T., Ribbeck, R., Mieth, K. and Garz, I., 1974. Ergebnisse der staatlich geleiteten, planm~ssigen Hypodermosebek~mpfung in der Deutschen Demokratischen Republik. Angew. Parasitol., 15: 57--67. Khan, M.A., 1968. Extermination of cattle grubs (Hypoderma spp.) on a regional basis. Vet. Rec., 83: 97--101. Khan, M.A., 1973. Toxicity o f systemic insecticides. Vet. Rec., 92: 411--419. Magat, A., Cotterau, P. and Faure, N., 1969. Accidents after the treatment o f bovine hypodermatosis with organophosphorus compounds. Rev. Med. Vet. (Toulouse), 119: 595--605. Mozier, J.O., 1969. Advances in cattle ectoparasite control with a discussion of some problems related to treatment with systemic insecticides. J. Am. Vet. Med. Assoc., 154: 1206--1213.
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Rich, G.B., 1965 a. Post-treatment reactions in cattle during extensive field tests of systemic organophosphate insecticides. Can. J. Comp. Med. Vet. Sci., 29: 30--37. Rich, G.B., 1965 b. Systemic treatment for control of cattle grubs, Hypoderma spp., in an isolated range herd. Can. J. Anita. Sci., 45: 165--172. Walsh, W.C., 1963. Posterior paralysis in a steer treated with a systemic insecticide for warble fly control. Can. Vet. J., 4: 245--247. WHO, 1970. Insecticide Resistance and Vector Control. World Health Organization, Geneva. Tech. Ref. Ser. 443, 279 pp.
251
PRACTICAL RELATIONSHIPS BETWEEN THE BIOLOGY AND CONTROL OF CATTLE GRUBS
W.N. BEESLEY
Department of Veterinary Parasitology, Liverpool School of Tropical Medicine, Liverpool L3 5QA (Great Britain) (Received 11 August 1976)
ABSTRACT Beesley, W.N., 1977. Practicalrelationshipsbetween the biology and control of cattle grubs. Vet. Parasitol., 3: 251--257. This paper deals with the toxic effects o f treatment of cattle against warble infestations, including the incidence of symptoms and whether these are due to death o f migrating larvae or insecticide poisoning, or both. It was concluded that the likelihood of deaths associated with treatment by systemic insecticides is much less than 1 in 5 000 and may be as rare as 1 in 100 000. The importance of a national scheme to eradicate cattle grubs in Britain was stressed in view of the successes in controlling cattle grubs in Northern Ireland and in Eire.
INTRODUCTION
The annual incidence of warbled hides in England and Wales reached 36.1% in 1941 and since then has frequently exceeded 20%. These figures relate to unhealed warble holes seen in "all cattle hides" at 40 representative hide markets, but they exclude hides with healed warble holes. If the data are abstracted to include only young animals, in the key hide-warble months of April to June, monthly figures of 60--70% were obtained (Beesley, 1971, 1973). In recent years, the warble incidence has fluctuated with annual "all hides" figures of 13.5% in 1972, 8.8% in 1973, and 11.8% in 1974 (Hide and Allied Trades Improvement Society, personal communication, 1976). Corresponding annual figures for steers and heifers alone were 14.8, 10.6 and 16.4%, and the peak summer figures for these animals were 33.8, 25.8 and 41.8% for May 1972, June 1973 and May 1974. The May 1974 peak number for younger animals was the worst since the summer of 1964. It appears that no significant progress was made in controlling cattle grubs since 1964, despite the annual use of well over 4 × l 0 s doses of systemic insecticides. By comparison, the Eireann figures for all hides were about 0.3% in 1973 and 0.05% in 1974.
252 The systemic insecticides now used in Britain are the pour-ons Tiguvon 1 (fenthion, O,O-dimethyl O-[4-(methylthio)-m-tolyl] phosphorothioate), Ruelene 2 (crufomate, 4-tert-butyl-2-chlorophenyl methyl methylphosphoramidate), and Prolate 3 (phosmet, O,O-dimethyl phosphorodithioate S-ester with N-(mercaptomethyl) phthaiimide); and the insecticidal mineral lick, Etrolene 4 (ronnel or fenchlorphos, O,O-dimethyl O-(2,4,5-trichlorophenyl) phosphorothioate). Tiguvon replaced the pour-on preparations Asuntol 5 (coumaphos, 3-chloro-7-hydroxy-4-methylcoumarin O-ester with O,Odiethyl phosphorothioate), Neguvon 6 (trichlorfon, dimethyl(2,2,2-trichloro1-hydroxyethyl) phosphonate) and Dyvon 7 (trichlorfon), while Ruelene replaced the original fenchlorphos boluses. All these insecticides are very effective. There is, at present, no statutory obligation in Britain to treat cattle with systemics. Suggestions have been made for a voluntary a u t u m n treatment, with m a n d a t o r y treatment of visibly infested cattle the next spring. It has been estimated that only about 30% of cattle in Britain are treated with systemics. A national plan for the treatment of warble is therefore long overdue. Five problems seem to prevent the successful control of warble. First, lack o f government or commercial funds to subsidize the cost of insecticide and its application to cattle. Second, real or assumed lack o f manpower to muster and treat animals, including decisions as to which animals shall have their treatment postponed because o f sickness, stress, age or pregnancy. Third, lack o f financial incentive for the farmer. The tanner may pay over £1 extra for a clean hide. This, however, does not reach the producer. Fourth, low priority for local or national warble control schemes in the overall programme of animal health improvement. This is especially evident when a warble control programme is presented in competition with proposals for the eradication of brucellosis, vesicular diseases, and tuberculosis. Fifth, uncertainty about possible toxic effects in cattle, and their cause. In this paper we are concerned mainly with the fifth item. TOXICITY Warble larvae spend 5--6 months migrating through the tissues of the host, overwintering in the wail o f the oesophagus (Hypoderma lineatum) or in the epidurai fat (H. boris). The larvae can cause severe reactions if they die in either site, but especially in the spinal cord. Larvae could die naturally or by host reaction in an animal already sensitized to Hypoderma antigen and this could stimulate a shock reaction (Beesley, 1971). Alternatively, successful treatment would have killed the larvae, liberating toxic material which could 1-7 Chemicals 1, 5, 6 and 7 are produced by Farbenfabriken Bayer A.G., Leverkusen, Federal Republic of Germany; chemicals 2 and 4 by Dow Chemical Corporation, Midland, Mich., U.S.A.; and chemical 3 by Stauffer Chemical Company, Richmond, Calif., U.S.A.
253
damage the host's spinal cord, oesophagus etc., again producing shock reaction in some animals (Beesley, 1971). There might be simultaneous toxicity by the organophosphorus insecticide as some individuals show idiosyncrasies to organophosphorus compounds. It is, therefore, useful to distinguish between the different types of toxic reactions that occasionally occur following treatment of warbled animals. In Britain, warble larvae arrive in the back of their host from January to July, the largest numbers being during March to June. The data obtained from examination of hides indicate slightly later dates due to the time needed to get the hides from the abattoir to the market, with possibly a storage period between. Larvae of H. lineatum reach the oesophagus from September to February (Beesley, 1966) and H. bovis reach the epidural fat from November to May (Beesley, 1962). It is undesirable to treat cattle while larvae are lodged in large numbers in the oesophagus or spinal canal. Manufacturers of systemic insecticides warn of the danger period and contraindicate treatment during late November to early March. The warble area o f the British Isles extends north and south for about 800 km, and it is cool and wet in the north and west. Mustering and treatment of cattle has to be fitted into the farming timetable, and in practice some treatments will be inevitably administered a little later than the recommended time. Individual idiosyncrasies resulting in a few cases exhibiting clinical signs of toxicity and deaths occur even when every posssible precaution has been taken to ensure correct dosing. This has happened in the well-planned programmes of Eire, Northern Ireland, and Switzerland. In East Germany, Hiepe et al. (1974) recorded only 24 sick cattle and 0.7 deaths per 100 000 treated with systemics in 1972. Before the discovery of the systemics, reactions in warbled cattle, whether treated with derris or left untreated, were reported occasionally. " R o s e fever", characterized by anaphylaxis-like symptoms, was related to the release of toxic material from dead first, second or third stage larvae. The same symptoms can be produced by injecting the contents of first stage larvae into cattle and rabbits (Anderson and Kirkwood, 1968; Beesley, 1971). In cattle treated with systemics, one must always remember that the animal may or may not be infested. From one to over 200 larvae, possibly of both H. lineatum and H. bovis, m a y be in the animal (Rich, 1965 a). Cox et al. (1970) reported t w o cases in which one treated and one untreated animal died at about the same time. The first animal was treated within the cut-off period and became ill 3 days after treatment. On post-mortem examination, conducted 27 days after treatment, it was found to harbour six live and one dead H. bovis larvae in the spine and two live and one dead H. lineatum larvae in the oesophagus. The second animal died from pneumonia and postmortem examination showed seven live H. bovis and eight live and five dead H. lineatum larvae. In evaluating the evidence about toxicity in treated cattle, one must be
254
critical of what was seen at post-mortem examination, if recovered larvae were alive or dead, etc. Many investigators claim that early toxic signs are due to the anti-cholinesterase effects of the organophosphorus insecticide, while later s y m p t o m s are most probably due to interactions between larval products and the host tissues (Magat et al., 1969). Others maintain that reactions from larval l~resence are rare and that the great majority of ill effects are simply due to organophosphorus poisoning (Khan, 1973). S y m p t o m s that develop 0.5--6 h after treatment with systemics are typical of organophosphorus poisoning and include salivation, lachrymation, myosis, muscle twitching, dyspnoea, colic, stiffness and paralysis. There is usually no oesophagal or pulmonary oedema or bloat. The clinical signs may resolve spontaneously or following treatment with atropine, 2-PAM (pralidoxime iodide), or TMB-4 (1,3-di(4-hydroxyiminomethylpyridinium) propane dibromide). These clinical signs may persist for 1--4 weeks, perhaps with fatal results (Khan, 1973). If the symptoms continue, they may overlap the start of secondary symptoms, i.e. those from host--larval reactions (Walsh, 1963). Toxic signs associated with the death of larvae in the animals' tissues include oedema of the oesophagus and bloat in the case of H. lineaturn, or ataxia and paralysis of the hindquarters in the case of H. bovis. Other signs include oedema of the eyelids and ear pinnae, congested conjunctivae, lachrymation, salivation, coughing, dyspnoea and urticaria. These signs are similar to those of shock in cattle following injection of egg albumen or histamine into uninfected calves (Anderson and Kirkwood, 1968; Aitken and Sanford, 1969). Systemic insecticides are found in the blood within 15 min (Fredriksson, 1962). Rapid kill of larvae and massive release of larval enzymes may therefore be expected. In practice this does not happen. Hypoderma larvae are remarkably resistant to organophosphorus systemic insecticides. First instar larvae of H. bovis and H. lineatum exposed to 32 ppm crufomate or trichlorfon in vitro survive for 4 and 32 days, respectively (Beesley, 1967). In naturally infested cattle, orally treated with dimethoate, first instar larvae died in 4--7 days (Hewitt et al., 1958). In calves where larvae were artificially implanted, all first instar larvae died within 14--27 days after pour-on treatments with trichlorfon, crufomate or fenchlorphos (Beesley, 1966). Anderson and Kirkwood (1968) considered that warble larvae exposed to systemic insecticides die simultaneously, but that the subsequent release o f larval toxin is insufficient to cause harm to the host. They based their suggestion on experiments in which material from up to 45 larvae, injected intravenously, failed to kill calves. The effect of a steady release of larval toxin into the system o f the host over a period of many days is not known. Rich (1965 a) reported the effect of systemic insecticide treatment in 11 537 cattle of different ages. Of 4 679 animals older than I year, three became ill and one died; while of 6 858 animals under 12 months, 71 were ill and four died. None of the toxic symptoms were typical of organophosphorus poisoning and nearly all the affected animals suffered from bloat. On
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post-mortem examination of dead animals extensive oesophagopharyngeal necrosis was a c o m m o n lesion. Poor nutrition, stress following vaccination, dehorning, or branding did n o t seem to affect the incidence o f development of toxic signs. It was concluded that the reactions of the host to release of larval toxins were responsible for all the clinical signs observed. Deliberate overdosing of cattle with pour-on insecticides has frequently failed to produce poisoning (Mozier, 1969). In Eire, it has been suggested that, in the later years of the eradication programme, an increasing proportion of toxic effects was due to reactions to dead larvae rather than to organophosphorus poisoning (H. Thornberry, personal communication, 1976). This was supported by the finding in many districts that some animals became ill after treatment with systemics in 1964 b u t were unaffected in the next year, when they were warble free. In the same areas, cattle that were brought in from districts where warble infestation was c o m m o n developed toxicity in only a few cases. PROBLEMS IN ERADICATION
Apart from problems of toxic effects of warble infestation other features should be considered in the control of this parasite. Some biologists believe that once the warble population falls below some threshold value the complete biomass automatically disappears. Wide fluctuations have occurred, however, in the degree of warble infestation both in Eire and in England and Wales. The incidence of warbled hides can increase by 50% in only 12 months, as it did in 1969--1970 and 1973--1974 in Britain (Beesley, 1973). Control programmes therefore must be continuous, thorough, and well supervised. Even an overall hide-warble incidence of 0.3% (the latest available from Eire) indicates that warble has not been eradicated. The same difficulty was encountered by Rich (1965 b), who used systemics on an isolated range herd in Canada. He reduced infestation from 30.2 to 0.2 grubs/head, b u t when treatment was discontinued the infestation rate rose to 10.2 grubs/head, in only two seasons. Khan (1968}, in a 3-year programme in Alberta, Canada, virtually eradicated warble from 5 000 cattle; no grubs remained in untreated control cattle within the same area, and even cattle just outside the trial area had fewer warbles than at the start of the programme. Despite this encouraging result, Khan decided to spring treat any cattle seen infested rather than wait until the fall. Workers were n o w realizing that, with the warble fly, drastic reduction and complete elimination are very different. Resilience in insect populations is a feature in other pests, such as tsetse fly and mosquitoes. This may account for the recent difficulties encountered in the programme to control screw-worm in the United States. The World Health Organization (WHO, 1970) recommended that special attention be given to the problem of small residual populations of insects that appear to be withstanding good control measures. It may be that a few individual
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i n s e c t s c a n f l y f u r t h e r o r l a y m o r e eggs t h a n h a d b e e n e s t i m a t e d p r e v i o u s l y . F l i g h t r a n g e , t h e r e f o r e , is i m p o r t a n t in r e g i o n a l c o n t r o l s c h e m e s . I n e x p e r i m e n t s w i t h Hypoderma G r e g s o n ( 1 9 5 6 ) b r e d , m a r k e d , r e l e a s e d a n d r e c a p t u r e d w a r b l e flies. T h e m a x i m u m d i s t a n c e t r a v e l l e d in his e x p e r i m e n t s w a s 3 0 0 m , b u t as t h i s w a s o b s e r v e d in 9 5 m i n i t s e e m s s a f e t o a s s u m e t h a t u n d e r n a t u r a l c o n d i t i o n s w a r b l e flies g o m u c h f u r t h e r , p a r t i c u l a r l y w h e n a t t r a c t e d t o m o v i n g c a t t l e o r b l o w n b y t h e w i n d . G r e g s o n a l s o s t a t e d t h a t a d u l t Hypoderma c a n f l y u p t o 1 4 . 5 k m .
REFERENCES Aitken, M.M. and Sanford, J., 1969. Experimental anaphylaxis in cattle. J. Comp. Pathol. Ther., 79: 131--139. Anderson, P.H. and Kirkwood, A.C., 1968. A reaction in cattle to toxins of Hypoderma bovis (warble fly)larvae. Br. Vet. J., 124: 569--575. Beesley, W.N., 1962. Observations on the development of Hypoderma bovis de Geer (Diptera, Oestridae) in bovine host. Res. Vet. Sci., 3: 203--208. Beesley, W.N., 1966. The use in Britain of systemic insecticides for the control of Hypoderma (Diptera: Oestridae). Vet. Med. Rev., 1966: 37--38. Beesley, W.N.,1967. Observations on the biology of the ox warble fly (Hypoderma, Diptera: Oestridae). I: In vitro culture of the first instar larva. Ann. Trop. Med. Parasitol., 61: 175--181. Beesley, W.N., 1971. Observations on the biology of the ox warble fly (Hypoderma, Diptera: Oestridae). V: Anaphylactoid shock in laboratory animals and calves following exposure to extracts o f larvae of Hypoderma. Ann. Trop. Med. Parasitol., 65: 567--572. Beesley, W.N., 1973. Recent developments in the control of ectoparasites of animals. Proc. 7th Br. Insecticide Fungicide Conf. Br. Crop Prot. Counc., London, pp. 856--875 Cox, D.D., Mozier, J.O. and Mullee, M.T., 1970. Posterior paralysis in a calf caused by cattle grubs (Hypoderma bovis) after treatment with a systemic insecticide for grub control. J. Am. Vet. Med. Assoc., 157: 1088--1092. Fredriksson, T., 1962. Studies on the percutaneous absorption of parathion and paroxon. V. Rate of absorption of paroxon. J. Invest. Dermatol., 38: 233--236. Gregson, J.D., 1956. Recent cattle grub life-history studies at Kamloops, British Columbia and Lethbridge, Alberta. Proc. 10th Int. Entomol. Congr., 3: 725--734. Hewitt, R., Emro, J., Entwistle, J., Pankavich, J., Thorson, R., Wallace, W. and Waletzky, E., 1958. Carbamoyl alkyl phosphordithioates as chemotherapeutic agents: effects of dimethoate against grubs in cattle. J. Econ. Entomol., 51 : 445--450. Hiepe, T., Ribbeck, R., Mieth, K. and Garz, I., 1974. Ergebnisse der staatlich geleiteten, planm~ssigen Hypodermosebek~mpfung in der Deutschen Demokratischen Republik. Angew. Parasitol., 15: 57--67. Khan, M.A., 1968. Extermination of cattle grubs (Hypoderma spp.) on a regional basis. Vet. Rec., 83: 97--101. Khan, M.A., 1973. Toxicity o f systemic insecticides. Vet. Rec., 92: 411--419. Magat, A., Cotterau, P. and Faure, N., 1969. Accidents after the treatment o f bovine hypodermatosis with organophosphorus compounds. Rev. Med. Vet. (Toulouse), 119: 595--605. Mozier, J.O., 1969. Advances in cattle ectoparasite control with a discussion of some problems related to treatment with systemic insecticides. J. Am. Vet. Med. Assoc., 154: 1206--1213.
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Rich, G.B., 1965 a. Post-treatment reactions in cattle during extensive field tests of systemic organophosphate insecticides. Can. J. Comp. Med. Vet. Sci., 29: 30--37. Rich, G.B., 1965 b. Systemic treatment for control of cattle grubs, Hypoderma spp., in an isolated range herd. Can. J. Anita. Sci., 45: 165--172. Walsh, W.C., 1963. Posterior paralysis in a steer treated with a systemic insecticide for warble fly control. Can. Vet. J., 4: 245--247. WHO, 1970. Insecticide Resistance and Vector Control. World Health Organization, Geneva. Tech. Ref. Ser. 443, 279 pp.