- Email: [email protected]
Risk analysis of ectoparasites acting as vectors for chronic wasting disease
Medical Hypotheses (2005) 65, 47–54
http://intl.elsevierhealth.com/journals/mehy
Risk analysis of ectoparasites acting as vectors for chronic wasting disease Omar Lupi
*
Department of Medical Clinics, Microcirculation Research Laboratory, Universidade do Estado do Rio de Janeiro, Rua Frei Leandro, 16n501, 22.470-210 Rio de Janeiro, RJ, Brazil Universidade Federal do Estado do Rio de Janeiro (UNIRIO), Rio de Janeiro, RJ, Brazil Received 7 January 2005; accepted 12 January 2005
Summary Prion diseases are rare neurodegenerative diseases of humans and animals with a lethal evolution. Animal prion infections, such as chronic wasting disease (CWD) and scrapie (sheep) have shown a pattern of horizontal transmission. CWD is an endemic disease that has been affecting thousands of domestic and wild cervids in US for the last three decades. The mode of contamination is not known, although direct contact between infected and non-infected animals via saliva, urine and feces have been considered. Increasing spread of CWD has raised concerns about the potential transmission to humans and the conversion of human prion protein by CWD-associated prions has been demonstrated in laboratory experiments. Fly larvae exposed to brain infected material were able to readily transmit scrapie to hamsters. Prion rods were identified in both larvae and fly pupae. New lines of evidence confirmed that adult flies are also able to express prion proteins. The most prevalent species of myiasis in cattle, sheep and wild cervids (Hypoderma spp.) present a very different life cycle from human myiasis, with a long contact with neurologic structures, such as the spinal canal and epidural fat, that are potentially rich in prion rods. Considering the huge amount of fly larvae that affects each animal, it is important to discuss the possibility that these ectoparasites could theoretically act as reservoirs and vectors for CWD and other prion diseases. It is critical to recognize all the possible factors involved in CWD transmission since ectoparasites could be handled in an easier way than the environmental persistence of infectious prions. c 2005 Elsevier Ltd. All rights reserved.
Introduction Prion diseases, also known as transmissible spongiform encephalopathies (TSE), are rare fatal neurodegenerative disorders that occur in animals and *
Tel.: +55 21 2537 7665; fax: +55 21 2521 5812. E-mail address: [email protected].
man [1]. The prototypic disease is scrapie, a naturally occuring disease affecting sheep and goats [2]. More recently recognized animal diseases include bovine spongiform encephalopathy (BSE or ‘‘mad cow disease’’), and spongiform encephalopathies of wild and domestic animals, such as transmissible mink encephalopathy, chronic wasting disease (CWD) in cervids, and feline spongiform
0306-9877/$ - see front matter c 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2005.01.039
48 encephalopathy [3,4]. Scrapie has been recognized in Europe for over 200 years and was first transmitted, experimentally, in 1936 [5]. Traditionally, human prion diseases have been classified as Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler syndrome (GSS), fatal familial insomnia, or kuru [1,3]. Each disease shares histopathological features: spongiform vacuolation, neuronal loss, and astrocytic proliferation. They are very rare neurodegenerative disorders, affecting about one person per million, each year, worlwide [3]. These diseases have had remarkable attention focused on them recently because of the unique biology of the transmissible agent, and also because of fears that through dietary exposure to infected tissues, BSE could affect humans [3,6]. A novel form of human prion disease, new variant CJD (nvCJD) was recognized in the United Kingdom (UK) [3,6]. Research on this variant including epidemiological and molecular studies has indicated a link between BSE and nvCJD, through ingestion of contaminated meat [6]. PrPC is the normal, cellular prion protein, and it is converted into PrPSc (from scrapie). The normal protein consists of mainly alpha helices with a spiral backbone, but the new, mutated prion protein is predominately formed by beta strands with a fully extended backbone [7].The mechanism of replication involves the synthesis of polypeptides in the absence of nucleic acid templates and the post-translational modifications of cellular proteins [1,3,7]. For the prion, replication involves converting conventional proteins into prions. Prions replicate by recruiting normal proteins to their cause, ’’flipping’’ them into a rogue prionlike shape that can go on to infect other cells and animals. This change initiates a chain reaction, and newly converted prions convert other proteins which they come into contact with on the interior of their respective cell membrane [1,7]. The PrPSc isoform is resistent to proteases and affects all the cells that presents the normal form PrPC. Neurological tissues and lymphocytes are two of the most important targets for the aberrant isoform [3,8]; neurons express large amounts of the normal isoform and are very sensitive to prion replication [9]. Lymphocytes and macrophages are the putative route of acess to the central nervous system (CNS) [8,10]. The only prion infection that is currently endemic to a large geographic area, affecting both domestic and wild animals, is CWD [4,11]. The mode of transmission of CWD is unknown. There is no evidence that CWD is a food-borne disease associated with rendered ruminant meat and bonemeal as was the case in BSE [6]. Occurrence of the
Lupi disease among captive deer and elk, many of which were acquired as neonates, fawns, or adults, provides strong evidence of lateral transmission [11–13]. Maternal transmission may also occur; however, this has not been definitively determined [13]. It is likely transmission occurred from mule deer to elk in an yet unknown way [12]. Since CWD is the only prion infection that occurs naturally in wild animals, it is critical to understand better its exact method of transmission in order to access any potential risk to human beings. Several studies suggested that flies and mites might harbor scrapie in farms and could act as vectors to infect previous healthy animals [9,14–17]. The skin and mucous membranes of both humans and animals are a natural target for both parasites and also express PrP protein in large amounts [18,19]. Further studies are necessary to clarify the real risk of ectoparasites acting as prion vectors, but there is a theoretical possibility that these ectoparasites could be associated to the transmission of CWD, and also scrapie, in farms and in wild animals in US and Europe, respectively [18,19].
Chronic wasting disease CWD is a prion disease that attacks the CNS of many species of cervids, such as white-tailed deer, mule deer and Rocky Mountain elk, and causes fatal damage to the brain [4,11,12]. CWD is similar to, but significantly different from, scrapie (documented in domestic sheep for over 200 years) [4], BSE and nvCJD found in humans [3] In the later stages of infection, deer and elk infected with CWD will show signs of progressive weight loss, listlessness, excessive salivation and urination, increased water intake, depression and, eventually, death [11,20]. Animals can be infected with CWD for months or years before outward signs of infection are evident [4]. CWD is a disease unique to North America. It has been described in wild deer and or elk in Colorado, Wyoming, Nebraska, South Dakota, Wisconsin and Saskatchewan [12,20,21]. In captive deer and or elk, it has been found in Colorado, Montana, South Dakota, Oklahoma, Kansas, Nebraska, Saskatchewan and Alberta [12,21]. The disease’s persistance has permanently contaminated an area of about 15,000 square miles in northeastern Colorado, southeastern Wyoming, and southwestern Nebraska [20,21]. The incidence of CWD in wild animals is of great concern in the so-called endemic area; it averages
Risk analysis of ectoparasites acting as vectors for chronic wasting disease about 5% but has reached 18% in some places [11– 13]. The disease was originally described in captive animals, 35 years ago, in Colorado [11,12]. However, over the last five years, CWD has been found in wild herds in several surrounding states and Canada [22]. In early 2002, CWD has been detected in wild deer in South Dakota, Wisconsin and New Mexico [4,21,22]. Researchers speculate that CWD could have been transported long distance as a result of interstate shipment of infected animals [21,21]. The prion agent is found in many lymphoid tissues of affected deer and elk, including those of the digestive tract [2,4], suggesting the agent may be shed through the alimentary tract. Lymphoid tissues contain PrPSc; thus, alimentary tract shedding may also occur in CWD. The TSE agents are extremely resistant in the environment [9]; pasture contamination has been suspected of being the source of scrapie agent in some outbreaks of sheep scrapie [2,4]. Concentration of deer and elk in captivity or by artificial feeding may increase the likelihood of transmission between individuals [20]. Biologists in Fort Collins, Colorado, where the disease was first discovered, found out how resilient prions can be. They set out in an intensive effort to rid the research facilities of CWD. All captive deer and elk were killed and buried. Personnel then plowed up the soil in the pens in an effort to bury possible disease organisms and structures and pastures were repeatedly treated with a powerful disinfectant. A year later, 12 elk calves from the wild were released in the sanitized holding areas. In the next five years, two of these elk died from CWD [23]. The mode of transmission of CWD between animals is not known, although direct contact between infected and non-infected animals via saliva, urine and feces is the most likely route of transmission [11]. Animals that have a social system that includes close contact with herd mates also have a higher chance of becoming infected, since there is a high possibility that CWD prions can survive in the environment after infected and exposed animals are removed [13,23]. Elk females lick males that have sprayed themselves with urine. Saliva could be a vector too; in both deer and elk, individual meet and greet by licking each other’s mouths and noses. Especially, during mating season, wild cervids nosed up to captives through the chain-link fences. Ranched elk may swap saliva when they feed in close quarters [11,12]. Contamination of soil by excreta from infected animals is thought to be another route of transmission, since most cervids ingest dirt to supplement
49
their diets with minerals. Bucks also lick soil on which does have urinated to ascertain their mating status. Pedersen and Smith [24] have described that abnormally folded prions stick to the surface of some soil types, such as clay, resisting environmental and chemical damage. However, the implication of environmental contamination in free-ranging animals is not clearly understood [13,20]. It is possible that animals take in the pathogen while grazing in areas where sick animals have shed prions on the ground in their feces, urine and saliva. Preliminary modeling suggested lateral transmission is necessary to maintain CWD at the prevalence observed in surveillance programs. Maternal transmission may occur, but in the model this route of transmission alone was not adequate to maintain the disease at observed levels [11]. According to Miller et al. [12], both field and model data supported the crucial importance of lateral transmission in CWD dynamics. Based on prevalence ans spatial distribution, they suggest CWD has been occurring in Colorado and Wyoming for more than 30 years, and may be best represented as an epizootic with a protracted time-scale [12]. Other tantalizing possibility to explain CWD dynamics would be the possibility of prion transmission by ectoparasites. Other animal prion infections, such as scrapie and BSE have shown a pattern of horizontal transmission in farm conditions [9] and several ectoparasites have been proven to harbor prion rods in laboratory experiments [14,17]. Fly larvae [14] and mites [15–17] were exposed to brain infected material and were able to readily transmit scrapie to hamsters. New lines of evidence confirmed that adult flies are also able to express prion proteins [25].
The myiasis hypothesis Infestations by ectoparasites are very common among domestic and wild animals. The invasion of mammalian tissues by the larvae of Diptera (fly) is known as myiasis [26]. The eggs, living larvae, or both are deposited on the skin or mucous membranes, where they hatch and produce larvae, which burrow into the skin and produce mild or severe inflammatory changes [26,27]. Some varieties of flies puncture the skin and extrude the ova beneath the surface (furuncular myiasis), whereas others deposit their eggs on open wounds and ulcers. Myiasis can also arose in any natural cavity of the body, such as the ear, eye, paranasal sinuses, mouth, anus and vagina (cavitary myiasis), and cause extensive destruction
50 of healthy tissue [9]. Ophthalmomyiasis is a common manifestation of the disease, when the larvae are deposited on the eyes, directly by the insect or after a minor ocular damage caused by sand particles or wind [9]. Myiasis-inducing flies are member of a super family Oestrodiae. Oestrodiae consists of three major families: Oestridae which includes four subfamilies (Oestrinae, Gasterophilinae, Hypodermatinae, and Cuterebrinae), Calliphoridae, and Sarcophagidae. All Oestridae, at least 151 species, are obligate parasites. The families Calliphoridae, greater than 1000 species, and Sarcophagidae, greater than 2000 species, contain both obligate and facultative organisms [27]. A brief discussion of two of the most important agents of myiasis in animals will clearly show that they are able to infest several animals, and even humans, and that ocular lesions are quite common. Hypodermosis – Larvae of the genus Hypoderma spp. are the most important agents of myiasis in cattle [28,29], but also affect many mammals, including most cervids [30–32]. Hypoderma bovis causes the ‘‘cattle botfly’’ and is found in temperate regions including North America and Europe; it is a major cause of morbidity in cattle [28]. Humans are occasionally infested [33,34]. After penetrating the skin the larvae produce migratory subcutaneous swellings in a life cycle very different from the classical one observed in human myiasis [35–38]. They may also invade the eye, producing severe damage [38–42]. Oestriosis - Infestation caused by Oestrus ovis, the ‘‘sheep nasal botfly’’, is found in all major sheep-raising regions [43,44]. It has been particularly implicated in ophthalmomyiasis in humans [45–50]. Female flies directly deposit first-instar larvae in the nostrils of sheep for obligate development in the upper respiratory tract. The condition is relatively common among shepherds of middleeastern countries, sometimes with very severe ocular involvement [51]. Lupi [9] suggested the possibility that ectoparasites, such as myiasis and mites, could act as vectors for prion diseases, in order to explain the endemicity of scrapie for many centuries, despite the huge efforts to sacrifice infected animals as well as quarentine affected farms [14–17]. Post et al. [14] analyzed experimental transmissibility of the scrapie agent by Sarcophaga carnaria, a common agent of myiasis in humans. Scrapie was transmitted to monkeys, mice and hamsters, either orally or after intracerebral injections, in the past [5]. In this study [14], larvae of S. carnaria were fed with brains of scrapie-infected hamsters. Ten days after being fed with infected brains, six fly pu-
Lupi pae were given orally to four hamsters [14]. Two developed clinical signs of scrapie after 215 days and three were positive to PrPSc. Other eight hamsters were fed with a macerate of infected larvae and five died of scrapie. These results confirm that PrPSc was presented, in both larvae and pupae, in high enough levels to infect hamsters [14]. This study supports the conception that fly larvae can harbor the prion agent but it was not possible to know, however, if the PrPSc replicated in S. carnaria [14]. Raeber et al. [25] developed a new evidence that supported the conception of a prion replication in adult flies. They were able to detect the expression of prion proteins in transgenic Drosophila melanogaster (‘‘fruit fly’’) exposed to heat pulses. Since prions proteins can also be largely expressed in cutaneous cells according to Pammer et al. [18,19], especially in chronic ulcers, it is reasonable to discuss the possibility of contamination of fly larvae fed on cutaneous ulcers of prion infected sheep and cattle. Since the adult form of flies is also able to express prion proteins [25], a new generation of flies could, theoretically, transmit the disease to new hosts during their life cycle. Post et al.’s experiment [14] did not reproduce the natural conditions where fly larvae infect human or animals, however, it is important to consider that iatrogenic CJD was already transmitted following corneal transplantation in humans [1,3]. Scott et al. [52] transmitted scrapie to mice by instilling brain homogenate into the conjunctiva of healthy animals. Since then, some ophthalmologic procedures, especially corneal transplantation, were considered at high risk for prion transmission due to the physical proximity to the brain and because of the almost direct contact between the eye and the CNS [53]. Ophthalmomyiasis is a quite common manifestation of myiasis in humans and animals [39–42,46–51]. The disease is far more common in tropical areas such as Hawaii [46,48] and Tunisia [50], where it can reach epidemic proportions, but has been described worldwide [39–51]. It is important to point out that a great proportion of cases of ocular myiasis in humans are caused by the larvae of O. ovis and H. bovis, respectively, the most common cause of myiasis in sheep and cattle. Both are the only domestic animals that were commonly infected by prions. Even wound and ulcer myiasis are candidates for the dissemination of prion infections [9]. The immune system displays a natural tolerance to PrPSc and lymphocytes are probably involved in the dissemination of the disease to the central nervous system [2,8]. Tabouret et al. [44] investigated
Risk analysis of ectoparasites acting as vectors for chronic wasting disease the cellular and humoral responses in sheep experimentally infected with O. ovis and detected a huge local recruitment of either lymphocytes and macrophages. The local humoral response was mainly directed against larval salivary gland antigens and the sinusal mucosa of infected animals was extremely thickened, and the epithelium exhibited hyperplasia, metaplasia and eosinophilic exocytosis. The high expression of PrPC on the skin and mucosa and the common occurrence of ocular myiasis could readily increase the efficiency of acquisition and transmission of prions in both animals and humans.
Hypodermosis Human prion diseases are very rare neurodegenerative disorders with a transmission associated with dietary exposure to infected tissues or inherited as genetic disorders [1,3]. Both the epidemiological and histopathological data indicate that nvCJD was originated from BSE [6]. Cattle have been tested for prion infection around the world, in the last decade, and the recognition of the real impact of nvCJD in many European populations is probably far from be concluded [6]. It is not very probable to have a new BSE epidemy in the near future but some isolated cases of the disease are still happening in UK, Canada and US [54,55]. Since cattle was the source of the most disseminated contamination by prions in human population, it is critical to better understand the natural history of hypodermosis, the most common cause of myiasis in cattle. According to Haine et al. [28] the herd seroprevalence to Hypoderma spp., among 390 animals, was 48.7% in Belgian. Cattle hypodermosis in China reaches 98% of the animals and maximum intensities exceeds 400 warbles for each animal [56]. Hypoderma is mainly found in the Northern Hemisphere. The adult fly is 15 mm long, covered in hair. The third instar larva is walnut sized and is found in the lump, or warble, on the back of the cow. It is 25 mm long, white to light brown, and appears segmented [27,35,36]. The female can deposit up to 100–800 eggs on a single cow. There are two species of Hypoderma that affect cattle. H. lineatum appears at the start of warm weather and remains for two months. H. bovis appears when H. lineatum is done and will persist throughout the summer months. The eggs of H. lineatum and H. bovis hatch within one week and the larvae burrow thru the skin migrating in the connective tissue. Five months later the larvae of H. lineatum accumulate in tissues of the esophagus where they stay for three months [27].
51
The larvae of H. bovis, however, accumulate in the spinal canal and epidural fat, anatomical structures potentially rich in prion rods, where they remain for about 3 months (‘‘winter resting sites’’) [35–38]. The larvae migrate once more to the back of the cow. Once on the dorsum, the larvae will cut breathing holes and press their posterior spiracles against for air. The larvae will continue to grow while spending 2 months in the warble. Finally, the larvae will enlarge the hole and fall to the ground where they will spend one month as pupae before emerging as an adult. The whole life cycle takes about one year [38]. It is important to point out that ataxia and posterior paralysis are commonly observed in animals infested by H. bovis, due to invasion of the spinal cord [35,38]. Aberrant migration intracranially or into the brain may cause acute neurological disease [33,34]; blindness is a common complication due to the invasion of the eye [39–42]. The huge amount of fly larvae in each animal and the very specific life cycle pattern of H. bovis in cattle, with a very long period of accumulation in nervous structures, that are very susceptible to prion infections, raise the possibility of a BSE transmission to other animals and even to humans.
Myiasis in cervids Few studies have examined the infestation of cervids by myiasis and the pathogenicity of larvae for host deer remains unknown. Since Miller and Williams [11] believe that horizontal transmission of CWD is the critical mechanism involved in the dissemination of the disease and myiasis and mites could be possible involved in the transmission of scrapie [9,14–17], it seems reasonable to perform a closer evaluation of myiasis among cervids in the endemic area of CWD in US. Several Hypoderma species are found in deer, but the three main parasites are Hypoderma diana, Hypoderma actaeon and Hypoderma tarandi. H. diana affects, in addition to red deer (Cervus elaphus), a range of other cervids including roe deer (Capreolus capreolus), fallow deer (Dama dama), elk (Alces alces) and reindeer (Rangifer rangifer), and lives in a great diversity of ecosystems [30]. Nonetheless, a checklist of animals examined during the hunting season (October–February) permitted the monitoring of commonly encountered diseases; this included testing for hypodermosis, recognized as one of the most prevalent diseases (98%), and for intensity of parasite infestation (up to 400 larvae per animal) [30,31]. However, little
52 is known about the biology of deer-infesting Hypoderma spp. H. Diana, one of the most common parasite in cervids, is present in a great variety of habitats, overlapping the territory of its hosts. The extent of parasitism, in respect to the number of larvae per animal, varies greatly in H. diana. In Spain [31], values ranging from 1 to 400 larvae per animal have been recorded, where over half of the animals contained more than 50 larvae [30]. In the former Czechoslovakia cases have been reported of up to 300 larvae per animal [57], in Algeria 350–450 larvae [58] and in Poland up to 150 larvae [59]. It has been shown that H. diana can infest other hosts in addition to deer. In Europe, cases have been reported of sheep living near deer-populated forests being affected [60]. The main factors influencing the flight and oviposition of female flies of H. diana species are ambient air temperature and light, so they are most active at midday [30]. In contrast, their endogenous cycle has yet to be fully described. Some authors [30,31,61] claim that they arrive at the dorsal area via the spinal canal, exhibiting a cycle similar to that of H. bovis in cattle, when is possible a closer and longer contact of the larvae with potencially infected nervous tissues. H. tarandi is more specific and is found in just one host (R. rangifer). Its biology has been studied in great detail due to the importance of the reindeer in cold northern countries [62]. Cases of ophthalmomyiasis produced by H. tarandi have been described in children [63], always in the vicinity of rein deer-populated areas, reinforcing the potential of these parasites to cause ophthal momyiasis. The other important species in red deer is H. actaeon, which reaches very high levels of prevalence in Southern and Central Spain, up to 92% [64], with a mean intensity of parasitisation close to 40 larvae per animal. Its participation as a prion vector is unlikely since only sheep and goats have been affected by scrapie, over the centuries, in Spain. However, McMahon and Bunch [65] described several cases of wild mule deer (Odocoileus hemionos), from Utah, infested by larvae of Cephenemya spp., (Oestridae family), reinforcing the possibility that fly larvae could be acting as vectors for CWD in some areas of the US.
Lupi sion of BSE to humans indicates that species barrier may not completely protect humans from animal prion diseases. Conversion of human prion protein by CWD-associated prions has been demonstrated already in an in vitro cell-free experiment [22,66]. Some isolated cases of prion infection in young humans who consumed venison regularly in US [22,26] have suggested a risk of CWD transmission to humans, in a similar way to BSE in UK. However, the absence of an increase in CJD and nvCJD in Colorado and Wyoming [12] suggest that the risk, if any, of transmission of CWD to humans is low. Other authors [20,22] argue that because CWD has occurred in a limited geographic area for decades, an adequate number of people may not have been exposed to the CWD agent to result in a clinically recognizable human disease. The level and frequency of human exposure to the CWD agent may increase with the spread of CWD in the US [22]. Deer farms have proved extremely successful in many countries, offering venison as an alternative to beef, pork and lamb. A satisfactory carcass is produced in 16 months, so animals are normally slaughtered at 16–18 months, before any possible manifestation of CWD [20]. Miller et al. [13] described, however, under experimental conditions that mule deer became infected in 2 of 3 paddocks containing naturally infected deer, in 2 of 3 paddocks where infected deer carcasses had decomposed in situ for more than 18 months earlier, and in 1 of 3 paddocks where infected deer had last resided 2 years earlier. It is a fact that CWD has been transmitted in both farm and wild animals. It is critical to recognize all the possible factors involved in its transmission in order to control the dissemination of the disease. Indirect transmission and environmental persistence of infectious prions would complicate efforts to control CWD but a possible transmission by ectoparasites could be handled in an easier way.
Conflict of interest statement The authors have no conflict of interest to disclose.
Discussion Acknowledgement Increasing spread of CWD has raised concerns about the potential for increasing human exposure to the CWD agent [66]. The foodborne transmis-
This work was supported by CNPq (Brazilian National Research Council).
Risk analysis of ectoparasites acting as vectors for chronic wasting disease
References [1] Collinge J. Prion diseases of human and animals: their causes and molecular basis. Annu Rev Neurosci 2001;24:519–50. [2] Parsonson IM. Scrapie: recent trends. Aust Vet J 1996;74(5):383–7. [3] Lupi O. Prions in dermatology. J Am Acad Dermatol 2002;46(5):790–3. [4] Williams ES. Scrapie and chronic wasting disease. Clin Lab Med 2003;23(1):139–59. [5] Gibbs Jr CJ, Amyx HL, Bacote A, Masters CL, Gajdusek DC. Oral transmission of kuru, Creutzfeldt-Jakob disease, and scrapie to nonhuman primates. J Infect dis 1980;142(2):205–8. [6] Calza L, Manfredi R, Chiodo F. Epidemics of BSE and nvCJD in humans. Most recent findings on prion diseases. Recent Prog Med 2001;92(2):140–9. [7] Prusiner SB. Novel proteinaceous infectious particles cause scrapie. Science 1982;216:136–44. [8] Aucouturier P, Carp RI, Carnaud C, Wisniewski T. Prion diseases and the immune system. Clin Immunol 2000;289(3):173–6. [9] Lupi O. Could ectoparasites act as vectors for prion diseases? Int J Dermatol 2003;42:425–9. [10] Taylor DM, McConnell I, Fraser H. Scrapie infection can be established readily through skin scarification in immunocompetent but not immunodeficient mice. J Gen Virol 1996;77:1595–9. [11] Miller MW, Williams ES. Chronic wasting disease of cervids. Curr Top Microbiol Immunol 2004;284:193–214. [12] Miller MW, Williams ES, McCarty CW, Spraker TR, Kreeger TJ, Larsen CT, et al. Epizootiology of chronic wasting disease in free-ranging cervids in Colorado and Wyoming. J Wildlife Dis 2000;36(4):676–90. [13] Miller MW, Williams ES, Hobbs NT, Wolfe LL. Environmental sources of prion transmission in mule deer. Emerg Infect Dis 2004;10(6):1003–6. [14] Post K, Riesner D, Walldorf V, Mehlhorn R. Fly larvae and pupae as vectors for scrapie. Lancet 1999;122: 199–204. [15] Carp RI, Meeker HC, Rubenstein R, Sigudarson S, Papini M, et al. Characteristics of scrapie isolates derived from hay mites. J Neurovirol 2000;6(2):137–44. [16] Wisniewski H, Sigudarson S, Rubenstein R, Kascsak RJ, Carp RI. Mites as vectors for scrapie. Lancet 1996;347(9008): 1114. [17] Rubenstein R, Kascsak RJ, Crp RI, Papini MC, La Fauci G, Sigudarson S, et al. Potential role of mites as vector for scrapie transmission. Alzheimer Dis Rev 1998;3:52–6. [18] Pammer J, Weninger W, Tschachler E. Human keratinocytes express cellular prion-related protein in vitro during inflammatory skin disease. Am J Pathol 1998;153(5): 1353–8. [19] Pammer J, Tschachler E. A possible role of keratinocytes of skin and mucous membranes in prion propagation and transmission. J Invest Dermatol Symp Proc 2002;7(1): 59–63. [20] Quirk M. Chronic wasting disease and neurological disease cluster link investigated. Lancet Infect Dis 2002;2(11):652. [21] Bonetta L. Chronic wasting disease research increases as US concern grows. Nat Med 2002;8(12):1338. [22] Belay ED, Maddox RA, Williams ES, Miller MW, Gambetti P, Schonberger LB. Chronic wasting disease and potential transmission to humans. Emerg Infect Dis 2004;10(6): 977–84.
53
[23] Salman MD. Chronic wasting disease in deer and elk: scientific facts and findings. J Vet Med Sci 2003;65(7): 761–8. [24] Pedersen NS, Smith E. Prion diseases: epidemiology in man. APMIS 2002;110(1):14–22. [25] Raeber AJ, Muramoto T, Kornberg TB, Prusiner SB. Expression and targeting of Syrian hamster prion protein induced by heat shock in transgenic Drosophila melanogaster. Mech Dev 1995;51(2–3):317–27. [26] Tabouret G, Lacroux C, Andreoletti O, Bergeaud JP, HailuTolosa Y, Hoste H, et al. Cellular and humoral local immune responses in sheep experimentally infected with Oestrus ovis (Diptera: Oestridae). Vet Res 2003;34(2):231–41. [27] Noutsis C, Milikan LE. Myiasis. Dermatol Clin 1994;12(4): 729–36. [28] Haine D, Boelaert F, Pfeiffer DU, Saegerman C, Lonneux JF, Losson B, et al. Herd-level seroprevalence and risk-mapping of bovine hypodermosis in Belgian cattle herds. Prev Vet Med 2004;65(1–2):93–104. [29] Frangipane di Regalbono A, Capelli G, Otranto D, Pietrobelli M. Assessment of cattle grub (Hypoderma spp.) prevalence in northeastern Italy: an immunoepidemiological survey on bulk milk samples using ELISA. Vet Parasitol 2003;111(4):343–50. [30] Martinez-Moreno FJ, Navarrete I, Reina D, HernandezRodriguez S. Deer hypodermosis. Parassitologia 1997; 39(4):419–22. [31] San Miguel JM, Alvarez G, Luzon M. Hypodermosis of red deer in Spain. J Wildlife Dis 2001;37(2):342–6. [32] Vicente J, Fierro Y, Martinez M, Gortazar C. Long-term epidemiology, effect on body condition and interspecific interactions of concomitant infection by nasopharyngeal bot fly larvae (Cephenemyia auribarbis and Pharyngomyia picta, Oestridae) in a population of Iberian red deer (Cervus elaphus hispanicus). Parasitology 2004;129(Pt. 3):349–61. [33] Kalelioglu M, Akturk G, Akturk F, Komsuoglu SS, Kuzeyli K, Tigin Y, et al. Intracerebral myiasis from Hypoderma bovis larva in a child. Case report. J Neurosurg 1989;71(6): 929–31. [34] Labbe A, Desvignes V, Meyer M, Campagne D, Cohen F, Dechelotte P. Hypoderma bovis meningitis. Apropos of new pediatric case. Ann Pediatr (Paris) 1983;30(4):277–80. [35] Meyer L. Wandering larvae of Hypoderma bovis in the spinal canal of young cattle. Dtsch Tierarztl Wochenschr 1973;80(17):397. [36] Derylo A. Effect of climate on the extensity of invasion with subcutaneous larvae of Hypoderma bovis (de Geer, 1776). Wiad Parazytol 1972;18(4):543–6. [37] Maranski C. Studies on the incidence of the larvae of Hypoderma bovis in the spinal canal of cattle. Acta Neuroveg (Wien) 1967;30(1):585–90. [38] Hiepe T, Ribbeck R, Gahtow I, Gramann F, Horst J. Dynamics of infestation of cattle with Hypoderma bovis De Geer, 1776. 2. Occurrence of larvae I in the spinal canal and localization of the grubs in cattle. Monatsh Veterinarmed 1969;24(8):289–93. [39] Vit H. Gadfly larva in the anterior chamber. Klin Monatsbl Augenheilkd 1980;177(1):88–90. [40] Feigelson J, Mathe JC, Regnault F, Tourte-Schaefer C, Lapierre J, Polonovski C. Ocular myiasis Hypoderma bovis in a child with mucoviscidosis. Pediatrie 1976;31(1):77–9. [41] Sachs W, Feldman-Muhsam B. A case of intra-ocular myiasis due to Hypoderma bovis. Isr J Med Sci 1966;2(6):778–80. [42] Rapoza PA, Michels RG, Semeraro RJ, Green WR. Vitrectomy for excision of intraocular larva (Hypoderma species). Retina 1986;6(2):99–104.
54 [43] Amin AR, Morsy TA, Shoukry A, Mazyad SA. Oestrid head maggots in slaughtered sheep in Cairo abattoir. J Egypt Soc Parasitol 1997;27(3):855–61. [44] Tabouret G, Jacquiet P, Scholl P, Dorchies P. Oestrus ovis in sheep: relative third-instar populations, risks of infection and parasitic control. Vet Res 2001;32(6):525–31. [45] Sigauke E, Beebe WE, Gander RM, Cavuoti D, Southern PM. Case report: ophthalmomyiasis externa in Dallas County, Texas. Am J Trop Med Hyg 2003;68(1):46–7. [46] Kajioka EH, Nagao CF, Karas S, Hardman JM, Navin JJ. Ophthalmomyiasis in Hawaii. Hawaii Med J 2004;63(3):78–9. [47] Weinand FS, Bauer C. Ophthalmomyiasis externa acquired in Germany: case report and review of the literature. Ophthalmologica 2001;215(5):383–6. [48] Yoshimoto CM, Goff ML. External ophthalmomyiasis, a disease established in Hawaii. Hawaii Med J 1997;56(1): 10–2. [49] Macdonald P, Chan C, Dickson J, Jean-Louis F, Heath A. Ophthalmomyiasis and nasal myiasis in New Zealand: a case series. N Z Med J 1999;112(1100):445–7. [50] Zayani A, Chaabouni M, Gouiaa R, Ben Hamida F, Fki J. Conjunctival myiasis. 23 cases in the Tunisian Sahel. Arch Inst Pasteur Tunis 1989;66(3–4):289–92. [51] Pampiglione S, Giannetto S, Virga A. Persistence of human myiasis by Oestrus ovis L. (Diptera: Oestridae) among shepherds of the Etnean area (Sicily) for over 150 years. Parassitologia 1997;39(4):415–8. [52] Scott JR, Foster JD, Fraser H. Conjunctival instillation of scrapie in mice can produce disease. Vet Microbiol 1993;34(4):305–9. [53] Kozlowski PB, Moretz RC, Carp RI, Wisniewski HM. Retinal damage in scrapie mice. Acta Neuropathol (Berl) 1982;56(1):9–12. [54] Stack MJ, Balachandran A, Chaplin M, Davis L, Czub S, Miller B. The first Canadian indigenous case of bovine spongiform encephalopathy (BSE) has molecular characteristics for prion protein that are similar to those of BSE in the United Kingdom but differ from those of chronic wasting disease in captive elk and deer. Can Vet J 2004;45(10):825–30.
Lupi [55] Nolen RS. Washington state dairy cow nation’s first case of BSE. J Am Vet Med Assoc 2004;224(3):345–6. [56] Otranto D, Traversa D, Colwell DD, Guan G, Giangaspero A, Boulard C et al. A third species of Hypoderma (Diptera: Oestridae) affecting cattle and yaks in China: molecular and morphological evidence. J Parasitol 2004;90(5): 958–65. [57] Minar J, Breev KA. Studies on the low and fundamental populations of the warble fly Hypoderma bovis. Folia Parasitol (Praha) 1983;30(1):57–71. [58] Benakhla A, Lonneux JF, Mekroud A, Losson B, Boulard C. Bovine hypodermosis in north-eastern Algeria: prevalence and intensity of infestation. Vet Res 1999;30(5): 539–45. [59] Piotrowski F, Szczukocka K. Grubs of Hypoderma bovis de Geer and H. lineatum de Vill. in slaughter cattle in Gdansk. Wiad Parazytol 1972;18(4):547–9. [60] Boulard C, Villejoubert C, Moire N. Cross-reactive, stagespecific antigens in the Oestridae family. Vet Res 1996;27(4–5):535–44. [61] Martynowicz T. Hypoderma bovis, cattle warble fly (morphology and biology). Wiad Parazytol 1975;21(1):113–6. [62] Anderson JR, Nilssen AC. Trapping oestrid parasites of reindeer: the response of Cephenemyia trompe and Hypoderma tarandi to baited traps. Med Vet Entomol 1996;10(4):337–46. [63] Kearney MS, Nilssen AC, Lyslo A, Syrdalen P, Dannevig L. Ophthalmomyiasis caused by the reindeer warble fly larva. J Clin Pathol 1991;44(4):276–84. [64] Perez JM, Granados JE, Ruiz-Martinez I. Studies on the hypodermosis affecting red deer in central and southern Spain. J Wildlife Dis 1995;31(4):486–90. [65] McMahon DC, Bunch TD. Bot fly larvae (Oestridae) in mule deer (Odocoileus hemionus) from Utah. J Wildlife Dis 1989;25(4):636–8. [66] Belay ED, Gambetti P, Schonberger LB, Parchi P, Lyon DR, Capellari S et al. Creutzfeldt-Jakob disease in unusually young patients who consumed venison. Arch Neurol 2001;58(10):1673–8.
http://intl.elsevierhealth.com/journals/mehy
Risk analysis of ectoparasites acting as vectors for chronic wasting disease Omar Lupi
*
Department of Medical Clinics, Microcirculation Research Laboratory, Universidade do Estado do Rio de Janeiro, Rua Frei Leandro, 16n501, 22.470-210 Rio de Janeiro, RJ, Brazil Universidade Federal do Estado do Rio de Janeiro (UNIRIO), Rio de Janeiro, RJ, Brazil Received 7 January 2005; accepted 12 January 2005
Summary Prion diseases are rare neurodegenerative diseases of humans and animals with a lethal evolution. Animal prion infections, such as chronic wasting disease (CWD) and scrapie (sheep) have shown a pattern of horizontal transmission. CWD is an endemic disease that has been affecting thousands of domestic and wild cervids in US for the last three decades. The mode of contamination is not known, although direct contact between infected and non-infected animals via saliva, urine and feces have been considered. Increasing spread of CWD has raised concerns about the potential transmission to humans and the conversion of human prion protein by CWD-associated prions has been demonstrated in laboratory experiments. Fly larvae exposed to brain infected material were able to readily transmit scrapie to hamsters. Prion rods were identified in both larvae and fly pupae. New lines of evidence confirmed that adult flies are also able to express prion proteins. The most prevalent species of myiasis in cattle, sheep and wild cervids (Hypoderma spp.) present a very different life cycle from human myiasis, with a long contact with neurologic structures, such as the spinal canal and epidural fat, that are potentially rich in prion rods. Considering the huge amount of fly larvae that affects each animal, it is important to discuss the possibility that these ectoparasites could theoretically act as reservoirs and vectors for CWD and other prion diseases. It is critical to recognize all the possible factors involved in CWD transmission since ectoparasites could be handled in an easier way than the environmental persistence of infectious prions. c 2005 Elsevier Ltd. All rights reserved.
Introduction Prion diseases, also known as transmissible spongiform encephalopathies (TSE), are rare fatal neurodegenerative disorders that occur in animals and *
Tel.: +55 21 2537 7665; fax: +55 21 2521 5812. E-mail address: [email protected].
man [1]. The prototypic disease is scrapie, a naturally occuring disease affecting sheep and goats [2]. More recently recognized animal diseases include bovine spongiform encephalopathy (BSE or ‘‘mad cow disease’’), and spongiform encephalopathies of wild and domestic animals, such as transmissible mink encephalopathy, chronic wasting disease (CWD) in cervids, and feline spongiform
0306-9877/$ - see front matter c 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2005.01.039
48 encephalopathy [3,4]. Scrapie has been recognized in Europe for over 200 years and was first transmitted, experimentally, in 1936 [5]. Traditionally, human prion diseases have been classified as Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler syndrome (GSS), fatal familial insomnia, or kuru [1,3]. Each disease shares histopathological features: spongiform vacuolation, neuronal loss, and astrocytic proliferation. They are very rare neurodegenerative disorders, affecting about one person per million, each year, worlwide [3]. These diseases have had remarkable attention focused on them recently because of the unique biology of the transmissible agent, and also because of fears that through dietary exposure to infected tissues, BSE could affect humans [3,6]. A novel form of human prion disease, new variant CJD (nvCJD) was recognized in the United Kingdom (UK) [3,6]. Research on this variant including epidemiological and molecular studies has indicated a link between BSE and nvCJD, through ingestion of contaminated meat [6]. PrPC is the normal, cellular prion protein, and it is converted into PrPSc (from scrapie). The normal protein consists of mainly alpha helices with a spiral backbone, but the new, mutated prion protein is predominately formed by beta strands with a fully extended backbone [7].The mechanism of replication involves the synthesis of polypeptides in the absence of nucleic acid templates and the post-translational modifications of cellular proteins [1,3,7]. For the prion, replication involves converting conventional proteins into prions. Prions replicate by recruiting normal proteins to their cause, ’’flipping’’ them into a rogue prionlike shape that can go on to infect other cells and animals. This change initiates a chain reaction, and newly converted prions convert other proteins which they come into contact with on the interior of their respective cell membrane [1,7]. The PrPSc isoform is resistent to proteases and affects all the cells that presents the normal form PrPC. Neurological tissues and lymphocytes are two of the most important targets for the aberrant isoform [3,8]; neurons express large amounts of the normal isoform and are very sensitive to prion replication [9]. Lymphocytes and macrophages are the putative route of acess to the central nervous system (CNS) [8,10]. The only prion infection that is currently endemic to a large geographic area, affecting both domestic and wild animals, is CWD [4,11]. The mode of transmission of CWD is unknown. There is no evidence that CWD is a food-borne disease associated with rendered ruminant meat and bonemeal as was the case in BSE [6]. Occurrence of the
Lupi disease among captive deer and elk, many of which were acquired as neonates, fawns, or adults, provides strong evidence of lateral transmission [11–13]. Maternal transmission may also occur; however, this has not been definitively determined [13]. It is likely transmission occurred from mule deer to elk in an yet unknown way [12]. Since CWD is the only prion infection that occurs naturally in wild animals, it is critical to understand better its exact method of transmission in order to access any potential risk to human beings. Several studies suggested that flies and mites might harbor scrapie in farms and could act as vectors to infect previous healthy animals [9,14–17]. The skin and mucous membranes of both humans and animals are a natural target for both parasites and also express PrP protein in large amounts [18,19]. Further studies are necessary to clarify the real risk of ectoparasites acting as prion vectors, but there is a theoretical possibility that these ectoparasites could be associated to the transmission of CWD, and also scrapie, in farms and in wild animals in US and Europe, respectively [18,19].
Chronic wasting disease CWD is a prion disease that attacks the CNS of many species of cervids, such as white-tailed deer, mule deer and Rocky Mountain elk, and causes fatal damage to the brain [4,11,12]. CWD is similar to, but significantly different from, scrapie (documented in domestic sheep for over 200 years) [4], BSE and nvCJD found in humans [3] In the later stages of infection, deer and elk infected with CWD will show signs of progressive weight loss, listlessness, excessive salivation and urination, increased water intake, depression and, eventually, death [11,20]. Animals can be infected with CWD for months or years before outward signs of infection are evident [4]. CWD is a disease unique to North America. It has been described in wild deer and or elk in Colorado, Wyoming, Nebraska, South Dakota, Wisconsin and Saskatchewan [12,20,21]. In captive deer and or elk, it has been found in Colorado, Montana, South Dakota, Oklahoma, Kansas, Nebraska, Saskatchewan and Alberta [12,21]. The disease’s persistance has permanently contaminated an area of about 15,000 square miles in northeastern Colorado, southeastern Wyoming, and southwestern Nebraska [20,21]. The incidence of CWD in wild animals is of great concern in the so-called endemic area; it averages
Risk analysis of ectoparasites acting as vectors for chronic wasting disease about 5% but has reached 18% in some places [11– 13]. The disease was originally described in captive animals, 35 years ago, in Colorado [11,12]. However, over the last five years, CWD has been found in wild herds in several surrounding states and Canada [22]. In early 2002, CWD has been detected in wild deer in South Dakota, Wisconsin and New Mexico [4,21,22]. Researchers speculate that CWD could have been transported long distance as a result of interstate shipment of infected animals [21,21]. The prion agent is found in many lymphoid tissues of affected deer and elk, including those of the digestive tract [2,4], suggesting the agent may be shed through the alimentary tract. Lymphoid tissues contain PrPSc; thus, alimentary tract shedding may also occur in CWD. The TSE agents are extremely resistant in the environment [9]; pasture contamination has been suspected of being the source of scrapie agent in some outbreaks of sheep scrapie [2,4]. Concentration of deer and elk in captivity or by artificial feeding may increase the likelihood of transmission between individuals [20]. Biologists in Fort Collins, Colorado, where the disease was first discovered, found out how resilient prions can be. They set out in an intensive effort to rid the research facilities of CWD. All captive deer and elk were killed and buried. Personnel then plowed up the soil in the pens in an effort to bury possible disease organisms and structures and pastures were repeatedly treated with a powerful disinfectant. A year later, 12 elk calves from the wild were released in the sanitized holding areas. In the next five years, two of these elk died from CWD [23]. The mode of transmission of CWD between animals is not known, although direct contact between infected and non-infected animals via saliva, urine and feces is the most likely route of transmission [11]. Animals that have a social system that includes close contact with herd mates also have a higher chance of becoming infected, since there is a high possibility that CWD prions can survive in the environment after infected and exposed animals are removed [13,23]. Elk females lick males that have sprayed themselves with urine. Saliva could be a vector too; in both deer and elk, individual meet and greet by licking each other’s mouths and noses. Especially, during mating season, wild cervids nosed up to captives through the chain-link fences. Ranched elk may swap saliva when they feed in close quarters [11,12]. Contamination of soil by excreta from infected animals is thought to be another route of transmission, since most cervids ingest dirt to supplement
49
their diets with minerals. Bucks also lick soil on which does have urinated to ascertain their mating status. Pedersen and Smith [24] have described that abnormally folded prions stick to the surface of some soil types, such as clay, resisting environmental and chemical damage. However, the implication of environmental contamination in free-ranging animals is not clearly understood [13,20]. It is possible that animals take in the pathogen while grazing in areas where sick animals have shed prions on the ground in their feces, urine and saliva. Preliminary modeling suggested lateral transmission is necessary to maintain CWD at the prevalence observed in surveillance programs. Maternal transmission may occur, but in the model this route of transmission alone was not adequate to maintain the disease at observed levels [11]. According to Miller et al. [12], both field and model data supported the crucial importance of lateral transmission in CWD dynamics. Based on prevalence ans spatial distribution, they suggest CWD has been occurring in Colorado and Wyoming for more than 30 years, and may be best represented as an epizootic with a protracted time-scale [12]. Other tantalizing possibility to explain CWD dynamics would be the possibility of prion transmission by ectoparasites. Other animal prion infections, such as scrapie and BSE have shown a pattern of horizontal transmission in farm conditions [9] and several ectoparasites have been proven to harbor prion rods in laboratory experiments [14,17]. Fly larvae [14] and mites [15–17] were exposed to brain infected material and were able to readily transmit scrapie to hamsters. New lines of evidence confirmed that adult flies are also able to express prion proteins [25].
The myiasis hypothesis Infestations by ectoparasites are very common among domestic and wild animals. The invasion of mammalian tissues by the larvae of Diptera (fly) is known as myiasis [26]. The eggs, living larvae, or both are deposited on the skin or mucous membranes, where they hatch and produce larvae, which burrow into the skin and produce mild or severe inflammatory changes [26,27]. Some varieties of flies puncture the skin and extrude the ova beneath the surface (furuncular myiasis), whereas others deposit their eggs on open wounds and ulcers. Myiasis can also arose in any natural cavity of the body, such as the ear, eye, paranasal sinuses, mouth, anus and vagina (cavitary myiasis), and cause extensive destruction
50 of healthy tissue [9]. Ophthalmomyiasis is a common manifestation of the disease, when the larvae are deposited on the eyes, directly by the insect or after a minor ocular damage caused by sand particles or wind [9]. Myiasis-inducing flies are member of a super family Oestrodiae. Oestrodiae consists of three major families: Oestridae which includes four subfamilies (Oestrinae, Gasterophilinae, Hypodermatinae, and Cuterebrinae), Calliphoridae, and Sarcophagidae. All Oestridae, at least 151 species, are obligate parasites. The families Calliphoridae, greater than 1000 species, and Sarcophagidae, greater than 2000 species, contain both obligate and facultative organisms [27]. A brief discussion of two of the most important agents of myiasis in animals will clearly show that they are able to infest several animals, and even humans, and that ocular lesions are quite common. Hypodermosis – Larvae of the genus Hypoderma spp. are the most important agents of myiasis in cattle [28,29], but also affect many mammals, including most cervids [30–32]. Hypoderma bovis causes the ‘‘cattle botfly’’ and is found in temperate regions including North America and Europe; it is a major cause of morbidity in cattle [28]. Humans are occasionally infested [33,34]. After penetrating the skin the larvae produce migratory subcutaneous swellings in a life cycle very different from the classical one observed in human myiasis [35–38]. They may also invade the eye, producing severe damage [38–42]. Oestriosis - Infestation caused by Oestrus ovis, the ‘‘sheep nasal botfly’’, is found in all major sheep-raising regions [43,44]. It has been particularly implicated in ophthalmomyiasis in humans [45–50]. Female flies directly deposit first-instar larvae in the nostrils of sheep for obligate development in the upper respiratory tract. The condition is relatively common among shepherds of middleeastern countries, sometimes with very severe ocular involvement [51]. Lupi [9] suggested the possibility that ectoparasites, such as myiasis and mites, could act as vectors for prion diseases, in order to explain the endemicity of scrapie for many centuries, despite the huge efforts to sacrifice infected animals as well as quarentine affected farms [14–17]. Post et al. [14] analyzed experimental transmissibility of the scrapie agent by Sarcophaga carnaria, a common agent of myiasis in humans. Scrapie was transmitted to monkeys, mice and hamsters, either orally or after intracerebral injections, in the past [5]. In this study [14], larvae of S. carnaria were fed with brains of scrapie-infected hamsters. Ten days after being fed with infected brains, six fly pu-
Lupi pae were given orally to four hamsters [14]. Two developed clinical signs of scrapie after 215 days and three were positive to PrPSc. Other eight hamsters were fed with a macerate of infected larvae and five died of scrapie. These results confirm that PrPSc was presented, in both larvae and pupae, in high enough levels to infect hamsters [14]. This study supports the conception that fly larvae can harbor the prion agent but it was not possible to know, however, if the PrPSc replicated in S. carnaria [14]. Raeber et al. [25] developed a new evidence that supported the conception of a prion replication in adult flies. They were able to detect the expression of prion proteins in transgenic Drosophila melanogaster (‘‘fruit fly’’) exposed to heat pulses. Since prions proteins can also be largely expressed in cutaneous cells according to Pammer et al. [18,19], especially in chronic ulcers, it is reasonable to discuss the possibility of contamination of fly larvae fed on cutaneous ulcers of prion infected sheep and cattle. Since the adult form of flies is also able to express prion proteins [25], a new generation of flies could, theoretically, transmit the disease to new hosts during their life cycle. Post et al.’s experiment [14] did not reproduce the natural conditions where fly larvae infect human or animals, however, it is important to consider that iatrogenic CJD was already transmitted following corneal transplantation in humans [1,3]. Scott et al. [52] transmitted scrapie to mice by instilling brain homogenate into the conjunctiva of healthy animals. Since then, some ophthalmologic procedures, especially corneal transplantation, were considered at high risk for prion transmission due to the physical proximity to the brain and because of the almost direct contact between the eye and the CNS [53]. Ophthalmomyiasis is a quite common manifestation of myiasis in humans and animals [39–42,46–51]. The disease is far more common in tropical areas such as Hawaii [46,48] and Tunisia [50], where it can reach epidemic proportions, but has been described worldwide [39–51]. It is important to point out that a great proportion of cases of ocular myiasis in humans are caused by the larvae of O. ovis and H. bovis, respectively, the most common cause of myiasis in sheep and cattle. Both are the only domestic animals that were commonly infected by prions. Even wound and ulcer myiasis are candidates for the dissemination of prion infections [9]. The immune system displays a natural tolerance to PrPSc and lymphocytes are probably involved in the dissemination of the disease to the central nervous system [2,8]. Tabouret et al. [44] investigated
Risk analysis of ectoparasites acting as vectors for chronic wasting disease the cellular and humoral responses in sheep experimentally infected with O. ovis and detected a huge local recruitment of either lymphocytes and macrophages. The local humoral response was mainly directed against larval salivary gland antigens and the sinusal mucosa of infected animals was extremely thickened, and the epithelium exhibited hyperplasia, metaplasia and eosinophilic exocytosis. The high expression of PrPC on the skin and mucosa and the common occurrence of ocular myiasis could readily increase the efficiency of acquisition and transmission of prions in both animals and humans.
Hypodermosis Human prion diseases are very rare neurodegenerative disorders with a transmission associated with dietary exposure to infected tissues or inherited as genetic disorders [1,3]. Both the epidemiological and histopathological data indicate that nvCJD was originated from BSE [6]. Cattle have been tested for prion infection around the world, in the last decade, and the recognition of the real impact of nvCJD in many European populations is probably far from be concluded [6]. It is not very probable to have a new BSE epidemy in the near future but some isolated cases of the disease are still happening in UK, Canada and US [54,55]. Since cattle was the source of the most disseminated contamination by prions in human population, it is critical to better understand the natural history of hypodermosis, the most common cause of myiasis in cattle. According to Haine et al. [28] the herd seroprevalence to Hypoderma spp., among 390 animals, was 48.7% in Belgian. Cattle hypodermosis in China reaches 98% of the animals and maximum intensities exceeds 400 warbles for each animal [56]. Hypoderma is mainly found in the Northern Hemisphere. The adult fly is 15 mm long, covered in hair. The third instar larva is walnut sized and is found in the lump, or warble, on the back of the cow. It is 25 mm long, white to light brown, and appears segmented [27,35,36]. The female can deposit up to 100–800 eggs on a single cow. There are two species of Hypoderma that affect cattle. H. lineatum appears at the start of warm weather and remains for two months. H. bovis appears when H. lineatum is done and will persist throughout the summer months. The eggs of H. lineatum and H. bovis hatch within one week and the larvae burrow thru the skin migrating in the connective tissue. Five months later the larvae of H. lineatum accumulate in tissues of the esophagus where they stay for three months [27].
51
The larvae of H. bovis, however, accumulate in the spinal canal and epidural fat, anatomical structures potentially rich in prion rods, where they remain for about 3 months (‘‘winter resting sites’’) [35–38]. The larvae migrate once more to the back of the cow. Once on the dorsum, the larvae will cut breathing holes and press their posterior spiracles against for air. The larvae will continue to grow while spending 2 months in the warble. Finally, the larvae will enlarge the hole and fall to the ground where they will spend one month as pupae before emerging as an adult. The whole life cycle takes about one year [38]. It is important to point out that ataxia and posterior paralysis are commonly observed in animals infested by H. bovis, due to invasion of the spinal cord [35,38]. Aberrant migration intracranially or into the brain may cause acute neurological disease [33,34]; blindness is a common complication due to the invasion of the eye [39–42]. The huge amount of fly larvae in each animal and the very specific life cycle pattern of H. bovis in cattle, with a very long period of accumulation in nervous structures, that are very susceptible to prion infections, raise the possibility of a BSE transmission to other animals and even to humans.
Myiasis in cervids Few studies have examined the infestation of cervids by myiasis and the pathogenicity of larvae for host deer remains unknown. Since Miller and Williams [11] believe that horizontal transmission of CWD is the critical mechanism involved in the dissemination of the disease and myiasis and mites could be possible involved in the transmission of scrapie [9,14–17], it seems reasonable to perform a closer evaluation of myiasis among cervids in the endemic area of CWD in US. Several Hypoderma species are found in deer, but the three main parasites are Hypoderma diana, Hypoderma actaeon and Hypoderma tarandi. H. diana affects, in addition to red deer (Cervus elaphus), a range of other cervids including roe deer (Capreolus capreolus), fallow deer (Dama dama), elk (Alces alces) and reindeer (Rangifer rangifer), and lives in a great diversity of ecosystems [30]. Nonetheless, a checklist of animals examined during the hunting season (October–February) permitted the monitoring of commonly encountered diseases; this included testing for hypodermosis, recognized as one of the most prevalent diseases (98%), and for intensity of parasite infestation (up to 400 larvae per animal) [30,31]. However, little
52 is known about the biology of deer-infesting Hypoderma spp. H. Diana, one of the most common parasite in cervids, is present in a great variety of habitats, overlapping the territory of its hosts. The extent of parasitism, in respect to the number of larvae per animal, varies greatly in H. diana. In Spain [31], values ranging from 1 to 400 larvae per animal have been recorded, where over half of the animals contained more than 50 larvae [30]. In the former Czechoslovakia cases have been reported of up to 300 larvae per animal [57], in Algeria 350–450 larvae [58] and in Poland up to 150 larvae [59]. It has been shown that H. diana can infest other hosts in addition to deer. In Europe, cases have been reported of sheep living near deer-populated forests being affected [60]. The main factors influencing the flight and oviposition of female flies of H. diana species are ambient air temperature and light, so they are most active at midday [30]. In contrast, their endogenous cycle has yet to be fully described. Some authors [30,31,61] claim that they arrive at the dorsal area via the spinal canal, exhibiting a cycle similar to that of H. bovis in cattle, when is possible a closer and longer contact of the larvae with potencially infected nervous tissues. H. tarandi is more specific and is found in just one host (R. rangifer). Its biology has been studied in great detail due to the importance of the reindeer in cold northern countries [62]. Cases of ophthalmomyiasis produced by H. tarandi have been described in children [63], always in the vicinity of rein deer-populated areas, reinforcing the potential of these parasites to cause ophthal momyiasis. The other important species in red deer is H. actaeon, which reaches very high levels of prevalence in Southern and Central Spain, up to 92% [64], with a mean intensity of parasitisation close to 40 larvae per animal. Its participation as a prion vector is unlikely since only sheep and goats have been affected by scrapie, over the centuries, in Spain. However, McMahon and Bunch [65] described several cases of wild mule deer (Odocoileus hemionos), from Utah, infested by larvae of Cephenemya spp., (Oestridae family), reinforcing the possibility that fly larvae could be acting as vectors for CWD in some areas of the US.
Lupi sion of BSE to humans indicates that species barrier may not completely protect humans from animal prion diseases. Conversion of human prion protein by CWD-associated prions has been demonstrated already in an in vitro cell-free experiment [22,66]. Some isolated cases of prion infection in young humans who consumed venison regularly in US [22,26] have suggested a risk of CWD transmission to humans, in a similar way to BSE in UK. However, the absence of an increase in CJD and nvCJD in Colorado and Wyoming [12] suggest that the risk, if any, of transmission of CWD to humans is low. Other authors [20,22] argue that because CWD has occurred in a limited geographic area for decades, an adequate number of people may not have been exposed to the CWD agent to result in a clinically recognizable human disease. The level and frequency of human exposure to the CWD agent may increase with the spread of CWD in the US [22]. Deer farms have proved extremely successful in many countries, offering venison as an alternative to beef, pork and lamb. A satisfactory carcass is produced in 16 months, so animals are normally slaughtered at 16–18 months, before any possible manifestation of CWD [20]. Miller et al. [13] described, however, under experimental conditions that mule deer became infected in 2 of 3 paddocks containing naturally infected deer, in 2 of 3 paddocks where infected deer carcasses had decomposed in situ for more than 18 months earlier, and in 1 of 3 paddocks where infected deer had last resided 2 years earlier. It is a fact that CWD has been transmitted in both farm and wild animals. It is critical to recognize all the possible factors involved in its transmission in order to control the dissemination of the disease. Indirect transmission and environmental persistence of infectious prions would complicate efforts to control CWD but a possible transmission by ectoparasites could be handled in an easier way.
Conflict of interest statement The authors have no conflict of interest to disclose.
Discussion Acknowledgement Increasing spread of CWD has raised concerns about the potential for increasing human exposure to the CWD agent [66]. The foodborne transmis-
This work was supported by CNPq (Brazilian National Research Council).
Risk analysis of ectoparasites acting as vectors for chronic wasting disease
References [1] Collinge J. Prion diseases of human and animals: their causes and molecular basis. Annu Rev Neurosci 2001;24:519–50. [2] Parsonson IM. Scrapie: recent trends. Aust Vet J 1996;74(5):383–7. [3] Lupi O. Prions in dermatology. J Am Acad Dermatol 2002;46(5):790–3. [4] Williams ES. Scrapie and chronic wasting disease. Clin Lab Med 2003;23(1):139–59. [5] Gibbs Jr CJ, Amyx HL, Bacote A, Masters CL, Gajdusek DC. Oral transmission of kuru, Creutzfeldt-Jakob disease, and scrapie to nonhuman primates. J Infect dis 1980;142(2):205–8. [6] Calza L, Manfredi R, Chiodo F. Epidemics of BSE and nvCJD in humans. Most recent findings on prion diseases. Recent Prog Med 2001;92(2):140–9. [7] Prusiner SB. Novel proteinaceous infectious particles cause scrapie. Science 1982;216:136–44. [8] Aucouturier P, Carp RI, Carnaud C, Wisniewski T. Prion diseases and the immune system. Clin Immunol 2000;289(3):173–6. [9] Lupi O. Could ectoparasites act as vectors for prion diseases? Int J Dermatol 2003;42:425–9. [10] Taylor DM, McConnell I, Fraser H. Scrapie infection can be established readily through skin scarification in immunocompetent but not immunodeficient mice. J Gen Virol 1996;77:1595–9. [11] Miller MW, Williams ES. Chronic wasting disease of cervids. Curr Top Microbiol Immunol 2004;284:193–214. [12] Miller MW, Williams ES, McCarty CW, Spraker TR, Kreeger TJ, Larsen CT, et al. Epizootiology of chronic wasting disease in free-ranging cervids in Colorado and Wyoming. J Wildlife Dis 2000;36(4):676–90. [13] Miller MW, Williams ES, Hobbs NT, Wolfe LL. Environmental sources of prion transmission in mule deer. Emerg Infect Dis 2004;10(6):1003–6. [14] Post K, Riesner D, Walldorf V, Mehlhorn R. Fly larvae and pupae as vectors for scrapie. Lancet 1999;122: 199–204. [15] Carp RI, Meeker HC, Rubenstein R, Sigudarson S, Papini M, et al. Characteristics of scrapie isolates derived from hay mites. J Neurovirol 2000;6(2):137–44. [16] Wisniewski H, Sigudarson S, Rubenstein R, Kascsak RJ, Carp RI. Mites as vectors for scrapie. Lancet 1996;347(9008): 1114. [17] Rubenstein R, Kascsak RJ, Crp RI, Papini MC, La Fauci G, Sigudarson S, et al. Potential role of mites as vector for scrapie transmission. Alzheimer Dis Rev 1998;3:52–6. [18] Pammer J, Weninger W, Tschachler E. Human keratinocytes express cellular prion-related protein in vitro during inflammatory skin disease. Am J Pathol 1998;153(5): 1353–8. [19] Pammer J, Tschachler E. A possible role of keratinocytes of skin and mucous membranes in prion propagation and transmission. J Invest Dermatol Symp Proc 2002;7(1): 59–63. [20] Quirk M. Chronic wasting disease and neurological disease cluster link investigated. Lancet Infect Dis 2002;2(11):652. [21] Bonetta L. Chronic wasting disease research increases as US concern grows. Nat Med 2002;8(12):1338. [22] Belay ED, Maddox RA, Williams ES, Miller MW, Gambetti P, Schonberger LB. Chronic wasting disease and potential transmission to humans. Emerg Infect Dis 2004;10(6): 977–84.
53
[23] Salman MD. Chronic wasting disease in deer and elk: scientific facts and findings. J Vet Med Sci 2003;65(7): 761–8. [24] Pedersen NS, Smith E. Prion diseases: epidemiology in man. APMIS 2002;110(1):14–22. [25] Raeber AJ, Muramoto T, Kornberg TB, Prusiner SB. Expression and targeting of Syrian hamster prion protein induced by heat shock in transgenic Drosophila melanogaster. Mech Dev 1995;51(2–3):317–27. [26] Tabouret G, Lacroux C, Andreoletti O, Bergeaud JP, HailuTolosa Y, Hoste H, et al. Cellular and humoral local immune responses in sheep experimentally infected with Oestrus ovis (Diptera: Oestridae). Vet Res 2003;34(2):231–41. [27] Noutsis C, Milikan LE. Myiasis. Dermatol Clin 1994;12(4): 729–36. [28] Haine D, Boelaert F, Pfeiffer DU, Saegerman C, Lonneux JF, Losson B, et al. Herd-level seroprevalence and risk-mapping of bovine hypodermosis in Belgian cattle herds. Prev Vet Med 2004;65(1–2):93–104. [29] Frangipane di Regalbono A, Capelli G, Otranto D, Pietrobelli M. Assessment of cattle grub (Hypoderma spp.) prevalence in northeastern Italy: an immunoepidemiological survey on bulk milk samples using ELISA. Vet Parasitol 2003;111(4):343–50. [30] Martinez-Moreno FJ, Navarrete I, Reina D, HernandezRodriguez S. Deer hypodermosis. Parassitologia 1997; 39(4):419–22. [31] San Miguel JM, Alvarez G, Luzon M. Hypodermosis of red deer in Spain. J Wildlife Dis 2001;37(2):342–6. [32] Vicente J, Fierro Y, Martinez M, Gortazar C. Long-term epidemiology, effect on body condition and interspecific interactions of concomitant infection by nasopharyngeal bot fly larvae (Cephenemyia auribarbis and Pharyngomyia picta, Oestridae) in a population of Iberian red deer (Cervus elaphus hispanicus). Parasitology 2004;129(Pt. 3):349–61. [33] Kalelioglu M, Akturk G, Akturk F, Komsuoglu SS, Kuzeyli K, Tigin Y, et al. Intracerebral myiasis from Hypoderma bovis larva in a child. Case report. J Neurosurg 1989;71(6): 929–31. [34] Labbe A, Desvignes V, Meyer M, Campagne D, Cohen F, Dechelotte P. Hypoderma bovis meningitis. Apropos of new pediatric case. Ann Pediatr (Paris) 1983;30(4):277–80. [35] Meyer L. Wandering larvae of Hypoderma bovis in the spinal canal of young cattle. Dtsch Tierarztl Wochenschr 1973;80(17):397. [36] Derylo A. Effect of climate on the extensity of invasion with subcutaneous larvae of Hypoderma bovis (de Geer, 1776). Wiad Parazytol 1972;18(4):543–6. [37] Maranski C. Studies on the incidence of the larvae of Hypoderma bovis in the spinal canal of cattle. Acta Neuroveg (Wien) 1967;30(1):585–90. [38] Hiepe T, Ribbeck R, Gahtow I, Gramann F, Horst J. Dynamics of infestation of cattle with Hypoderma bovis De Geer, 1776. 2. Occurrence of larvae I in the spinal canal and localization of the grubs in cattle. Monatsh Veterinarmed 1969;24(8):289–93. [39] Vit H. Gadfly larva in the anterior chamber. Klin Monatsbl Augenheilkd 1980;177(1):88–90. [40] Feigelson J, Mathe JC, Regnault F, Tourte-Schaefer C, Lapierre J, Polonovski C. Ocular myiasis Hypoderma bovis in a child with mucoviscidosis. Pediatrie 1976;31(1):77–9. [41] Sachs W, Feldman-Muhsam B. A case of intra-ocular myiasis due to Hypoderma bovis. Isr J Med Sci 1966;2(6):778–80. [42] Rapoza PA, Michels RG, Semeraro RJ, Green WR. Vitrectomy for excision of intraocular larva (Hypoderma species). Retina 1986;6(2):99–104.
54 [43] Amin AR, Morsy TA, Shoukry A, Mazyad SA. Oestrid head maggots in slaughtered sheep in Cairo abattoir. J Egypt Soc Parasitol 1997;27(3):855–61. [44] Tabouret G, Jacquiet P, Scholl P, Dorchies P. Oestrus ovis in sheep: relative third-instar populations, risks of infection and parasitic control. Vet Res 2001;32(6):525–31. [45] Sigauke E, Beebe WE, Gander RM, Cavuoti D, Southern PM. Case report: ophthalmomyiasis externa in Dallas County, Texas. Am J Trop Med Hyg 2003;68(1):46–7. [46] Kajioka EH, Nagao CF, Karas S, Hardman JM, Navin JJ. Ophthalmomyiasis in Hawaii. Hawaii Med J 2004;63(3):78–9. [47] Weinand FS, Bauer C. Ophthalmomyiasis externa acquired in Germany: case report and review of the literature. Ophthalmologica 2001;215(5):383–6. [48] Yoshimoto CM, Goff ML. External ophthalmomyiasis, a disease established in Hawaii. Hawaii Med J 1997;56(1): 10–2. [49] Macdonald P, Chan C, Dickson J, Jean-Louis F, Heath A. Ophthalmomyiasis and nasal myiasis in New Zealand: a case series. N Z Med J 1999;112(1100):445–7. [50] Zayani A, Chaabouni M, Gouiaa R, Ben Hamida F, Fki J. Conjunctival myiasis. 23 cases in the Tunisian Sahel. Arch Inst Pasteur Tunis 1989;66(3–4):289–92. [51] Pampiglione S, Giannetto S, Virga A. Persistence of human myiasis by Oestrus ovis L. (Diptera: Oestridae) among shepherds of the Etnean area (Sicily) for over 150 years. Parassitologia 1997;39(4):415–8. [52] Scott JR, Foster JD, Fraser H. Conjunctival instillation of scrapie in mice can produce disease. Vet Microbiol 1993;34(4):305–9. [53] Kozlowski PB, Moretz RC, Carp RI, Wisniewski HM. Retinal damage in scrapie mice. Acta Neuropathol (Berl) 1982;56(1):9–12. [54] Stack MJ, Balachandran A, Chaplin M, Davis L, Czub S, Miller B. The first Canadian indigenous case of bovine spongiform encephalopathy (BSE) has molecular characteristics for prion protein that are similar to those of BSE in the United Kingdom but differ from those of chronic wasting disease in captive elk and deer. Can Vet J 2004;45(10):825–30.
Lupi [55] Nolen RS. Washington state dairy cow nation’s first case of BSE. J Am Vet Med Assoc 2004;224(3):345–6. [56] Otranto D, Traversa D, Colwell DD, Guan G, Giangaspero A, Boulard C et al. A third species of Hypoderma (Diptera: Oestridae) affecting cattle and yaks in China: molecular and morphological evidence. J Parasitol 2004;90(5): 958–65. [57] Minar J, Breev KA. Studies on the low and fundamental populations of the warble fly Hypoderma bovis. Folia Parasitol (Praha) 1983;30(1):57–71. [58] Benakhla A, Lonneux JF, Mekroud A, Losson B, Boulard C. Bovine hypodermosis in north-eastern Algeria: prevalence and intensity of infestation. Vet Res 1999;30(5): 539–45. [59] Piotrowski F, Szczukocka K. Grubs of Hypoderma bovis de Geer and H. lineatum de Vill. in slaughter cattle in Gdansk. Wiad Parazytol 1972;18(4):547–9. [60] Boulard C, Villejoubert C, Moire N. Cross-reactive, stagespecific antigens in the Oestridae family. Vet Res 1996;27(4–5):535–44. [61] Martynowicz T. Hypoderma bovis, cattle warble fly (morphology and biology). Wiad Parazytol 1975;21(1):113–6. [62] Anderson JR, Nilssen AC. Trapping oestrid parasites of reindeer: the response of Cephenemyia trompe and Hypoderma tarandi to baited traps. Med Vet Entomol 1996;10(4):337–46. [63] Kearney MS, Nilssen AC, Lyslo A, Syrdalen P, Dannevig L. Ophthalmomyiasis caused by the reindeer warble fly larva. J Clin Pathol 1991;44(4):276–84. [64] Perez JM, Granados JE, Ruiz-Martinez I. Studies on the hypodermosis affecting red deer in central and southern Spain. J Wildlife Dis 1995;31(4):486–90. [65] McMahon DC, Bunch TD. Bot fly larvae (Oestridae) in mule deer (Odocoileus hemionus) from Utah. J Wildlife Dis 1989;25(4):636–8. [66] Belay ED, Gambetti P, Schonberger LB, Parchi P, Lyon DR, Capellari S et al. Creutzfeldt-Jakob disease in unusually young patients who consumed venison. Arch Neurol 2001;58(10):1673–8.