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1 Department of Veterinary Microbiology, Western College of Veterinary Medicine, 52 Campus Drive, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5B4, Canada
2 Department of Environment and Natural Resources, Government of the Northwest Territories, Box 130, Norman Wells, Northwest Territories X0E 0V0, Canada
3 Department of Veterinary Pathology, Western College of Veterinary Medicine, 52 Campus Drive, University of Saskatchewan, Saskatoon S7N 5B4, Canada
4 Department of Environment and Natural Resources, Government of the Northwest Territories, 5102-50th Avenue, Yellowknife, Northwest Territories X1A 1P4, Canada
5 United States Department of Agriculture, Agricultural Research Service, US National Parasite Collection, Animal Parasitic Disease Laboratory, BARC East No. 1180, 10300 Baltimore Avenue, Beltsville, Maryland 20705, USA
7 Corresponding author (email: emily.jenkins{at}usask.ca or emily.jenkins{at}ec.gc.ca)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Protostrongylid lungworms (P. stilesi, P. rushi, and, less commonly, Muellerius capillaris) play a controversial role in a multifactorial pneumonia complex that causes "die-offs" in bighorn sheep (Ovis canadensis) throughout North America (Demartini and Davies, 1977; Spraker et al., 1984; Samson et al., 1987; Monello et al., 2001). The combination of lungworms, bacteria, viruses, and natural or anthropogenic stressors is likely of greater importance in pneumonia in wild sheep than any single pathogen alone, other than introduced bacterial strains, which may act as primary pathogens in naïve populations. Parelaphostrongylus odocoilei may cause respiratory disease in ungulate hosts (e.g., cervids and mountain goats), because adults in the muscles produce eggs that travel through the vasculature to the lungs (Brunetti, 1969; Platt and Samuel, 1978; Pybus and Samuel, 1984; Pybus et al., 1984). In Dall’s sheep in the Mackenzie Mountains, P. odocoilei and P. stilesi may cause additive or even synergistic pulmonary damage (Kutz et al., 2001).
To isolate the effects of P. odocoilei, we experimentally infected thinhorn sheep and described the first neurological syndrome associated with P. odocoilei in any definitive host (Jenkins et al., 2005b). This led us to hypothesize a neural migration for this "muscleworm," but confirmation awaited demonstration of parasites and characteristic lesions in the central nervous system. Based on respiratory and neural pathogenicity in thinhorn sheep, P. odocoilei has the potential to contribute to fatal pneumonia and impact health at the population level.
In the current study, we describe parasite localization and associated pathology in three captive Stone’s sheep experimentally infected with P. odocoilei, and in >50 wild Dall’s sheep collected from the Mackenzie Mountains between 1998 and 2002. This is the first investigation of the pathology and seasonal patterns of prevalence and intensity of protostrongylid parasites in the lungs of thinhorn sheep, and it complements descriptions of seasonal patterns of larval shedding in feces of Dall’s sheep in the Mackenzie Mountains (Jenkins et al., 2006). In addition, we investigated mortality due to disease in this population of Dall’s sheep, with targeted sampling for pathogens that have been implicated in respiratory disease in domestic, bighorn, and captive Dall’s sheep (for review, see Garde et al., 2005), or for pathogens that have been identified in wild Dall’s sheep in Alaska through serologic surveys (Foreyt et al., 1983; Zarnke, 2000). To our knowledge, this is the first investigation of population health in wild Dall’s sheep.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Three adult Stone’s sheep (SS1, SS2, and SS3) were each infected with 200 third-stage larvae of P. odocoilei originating from first-stage larvae (L1) in feces of Dall’s sheep in the Mackenzie Mountains (Jenkins et al., 2005b). Before infection with P. odocoilei, larvae of Protostrongylus spp. were present at low densities in pooled feces from SS2 and SS3. Each sheep was examined using bronchoscopy and bronchoalveolar lavage (BAL) on at least two occasions before infection. Sheep were chemically immobilized, the trachea and larger bronchi were visually examined with a fiberoptic endoscope, and 3 x 50 ml of sterile saline was inserted rapidly into the lower lung via a plastic tube and immediately aspirated (adapted from Begin et al., 1981; Silflow and Foreyt, 1988; Meyer et al., 1998). After infection, SS1 was examined using bronchoscopy and BAL 7 times at 2- to 4-wk intervals from 28 to 161 days postinfection (dpi), whereas SS2 and SS3 were examined on 35, 63, and, for SS3 only, 91 dpi.
Analyses of BAL fluid included determination of total protein levels, a differential count of 100 (SS1) or 200 (SS2 and SS3) nucleated cells stained with Diff-Quick® (Dade Behring, Newark, Delaware, USA), and, in SS2 and SS3, the proportion of 100 macrophages that stained with Perl’s Prussian blue for hemosiderin (hemosiderophages). BAL fluid from SS1 was examined for L1 at 25x magnification, whereas BAL fluid from SS2 and SS3 was centrifuged at 1,200 rpm at 4 C for 12 min. Sediment was resuspended in 2% formalin, centrifuged again, and resuspended in a drop of methylene blue and examined for protostrongylid eggs and L1 at 40–100x (modified Knott’s test).
For SS1, thoracic radiographs were obtained at each BAL procedure. Images of transverse sections (7 mm, and also 1 mm at 126 and 173 dpi) through the entire lung field were obtained using a High Speed Computed Tomography/i® helical scanner (GE Medical Systems, Milwaukee, Wisconsin, USA) at the Royal University Hospital, Saskatoon, Saskatchewan (SK), Canada, at 37 days before infection, at 70 and 126 dpi, and on excised, inflated lungs at 173 dpi (Cadore et al., 1994; Kutz et al., 1999; Kutz et al., 2004).
A routine necropsy was performed on each sheep, involving gross and histologic examination of all major organs, including the brain and meninges. The spinal cord and canal were examined under a dissecting scope, and for SS3 only, sections of spinal cord were taken for histology. Adult nematodes were recovered from characteristic hemorrhages in 5-mm sections of the entire skeletal musculature (as per Kutz et al., 2001) and identified as P. odocoilei as described in Jenkins et al. (2005b). Larvae were recovered from rectal feces and quantified using a modified beaker Baermann technique (Forrester and Lankester, 1997; Jenkins et al., 2005a). Samples from lungs of all three sheep were cultured for bacteria, and for SS2 and SS3; samples were examined using immunohistochemistry for respiratory syncytial virus (RSV), parainfluenza III (PI3), and infectious bovine rhinotracheitis (IBR) (Haines and Chelack, 1991).
"Healthy" Dall’s sheep from Mackenzie Mountains
In 2000, 2001, and 2002, we sent kits and instructions to the eight outfitters who guide nonresident hunters in the Mackenzie Mountains, NT, who collected 2x2-cm samples from the dorsum of the left cranial and caudal (diaphragmatic) lung lobes, feces, blood (pooled in the chest cavity), and throat swabs (Cary-Blair or Amies transport media, BBL CultureSwab®, Becton Dickinson, Sparks, Maryland, USA). We also received submissions of lungs (entire) and feces from Dall’s sheep taken by resident hunters. Samples were frozen within 48 hr of collection and shipped by air to the Western College of Veterinary Medicine (WCVM), Saskatoon, SK.
In November 2001, three ewes and one young ram (FC23–26) were opportunistically selected from the Katherine Creek area in the Mackenzie Mountains (65°01'N, 127°35'W) and shot. Samples were collected as described above but processed immediately, including direct inoculation of swabs on to agar plates for culture, and separation of sera from whole blood. Washes of the airways were examined for adult lungworms (Oakley, 1980), and adult P. odocoilei were recovered from the left half of each carcass and archived in the United States National Parasite Collection (USNPC accession nos. 98890–98893). The brains, meninges, spinal cords, and spinal canals were examined grossly, and sections of the brains were fixed for histology. We also obtained data and slides of lung tissue from six adult ewes from the Mackenzie Mountains collected in October 1998 (FC1–3) and April 1999 (FC5–7) (Kutz et al., 2001). These "straggler" ewes, slower than others in the herd, were selected to maximize chances of parasite recovery.
Sick and dead Dall’s sheep from Mackenzie Mountains
Reports of one to three coughing, dyspneic, and lethargic rams with nasal discharge were received in each of September 1999, July 2001, and September 2002. In two of these instances, we inspected lung samples from affected rams (SK13 and SK18) by using histologic examination and culture. Twelve carcasses of Dall’s sheep found in the Mackenzie Mountains between 1999 and 2002 (SC15, FC21, FC27, and all with identifier MO [for mortality]) were kept cool and shipped within a few days to WCVM for routine necropsy. We determined cause of death and pathogens present (by using bacterial culture, immunohistochemistry, and parasitologic techniques), and we estimated body condition by using the marrow-fat technique (Mech and Delgiudice, 1985).
Processing of samples
When the entire lungs were available, the parenchyma was examined grossly for lesions, and airways were examined for adult lung-worms. Tissue samples for histologic examination were fixed in 10% formalin, embedded in paraffin, sectioned, and stained with hematoxylin and eosin by Prairie Diagnostic Services (PDS), Saskatoon, SK, Canada. Histologic sections from cranial and caudal lung were examined for protostrongylid eggs, larvae, and adult nematodes at 100x magnification. Adult nematodes in lung parenchyma and Protostrongylus spp. larvae in feces were assumed to be P. stilesi, the only lung-dwelling nematode identified in Dall’s sheep in the Mackenzie Mountains, NT (Kutz et al., 2001). Basophilic, loosely morulated eggs of P. odocoilei were clearly distinguishable from eosinophilic, dense, freshly deposited eggs of P. stilesi (Fig. 1
), although larvae could not be differentiated unless the tails were visible (Kutz et al., 2001). Protostrongylid eggs and larvae in 10 arbitrarily chosen 100x fields in the cranial lung section were counted to provide an estimate of parasite density. The dimensions (length and width) of the largest granuloma surrounding eggs of P. odocoilei observed in each section of cranial lung were measured using a calibrated slide micrometer (Pybus and Samuel, 1984). When examining histologic sections of lung, the observer did not know the results of Baermann analyses for protostrongylid larvae in matching fecal samples. For all Dall’s sheep, the prevalence of infection with each protostrongylid was determined separately for lung and fecal samples, and the two methods were combined to obtain the overall prevalence. For Dall’s sheep without bacterial pneumonia, density and dimensions of granulomas in the cranial lung were compared with intensity of L1 of P. odocoilei in feces by using Pearson’s correlation coefficient (two-tailed, significant at the 0.01 level) (Statistical Package for the Social Sciences 12.0, SPSS Inc., Chicago, Illinois, USA). Histologic sections were archived in the department of Veterinary Pathology at the Western College of Veterinary Medicine (accessible through the PDS database).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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There were no atypical findings on bronchoscopy or on thoracic radiography of SS1. A diffuse reticulonodular infiltrate was first observed on CT of the lungs at 126 dpi, and SS1 died of respiratory failure following exertion at 173 dpi (101 days after patency). At necropsy, nodules observed on computed tomography (CT) images of excised lungs corresponded with grossly visible granulomas that, on histological examination, consisted of lymphocytes, plasma cells, and macrophages surrounding three to 13 eggs and larvae of P. odocoilei (Fig. 2). In addition to this generalized, granulomatous, interstitial pneumonia, there was diffuse pulmonary edema and hemorrhage (acute and chronic), with petechial and ecchymotic hemorrhages visible on the pleural surface of the lung.
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). There was lymphoplasmacytic cuffing of blood vessels nearby and in the choroid plexus, and there were also plasma cells and hemosiderophages in the meninges of the medulla. Sheep SS3 died at 92 dpi (19 days after patency) of unknown causes, possibly as a complication of anesthesia, with high densities of eggs and larvae of P. odocoilei in the lungs (Table 1). There was no evidence of infection with respiratory viruses in SS2 or SS3.
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2.5%), and hemosiderophages (2 of 100 macrophages). In BAL fluid from SS1, increased numbers of hemosiderophages were observed starting at 56 dpi, larvae of P. odocoilei were detected at 77 dpi, and total protein increased starting at 105 dpi. In BAL fluid from SS2, 27 protostrongylid eggs (40 x 80 µm) were observed at 35 dpi, and one egg was present at 63 dpi. In BAL fluid from SS3, Protostrongylus spp. larvae were observed before, but not after, infection with P. odocoilei. One protostrongylid egg was recovered from BAL fluid at 63 dpi, and larvae of P. odocoilei and increased numbers of hemosiderophages (23 of 100 macrophages) were present in BAL fluid at 91 dpi.
In the muscles of SS1, SS2, and SS3, adult P. odocoilei were associated with inflammatory infiltrates and chronic hemorrhage in fascial planes. Tails of female nematodes were frequently observed inside blood vessels. In SS1, adult worms were evenly distributed throughout the body; in SS2, adults were predominantly in the axial and trunk muscles; and in SS3, adults were predominantly in the hindquarters (Table 1
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"Healthy" Dall’s sheep from Mackenzie Mountains
Lung samples from 47 Dall’s sheep with no evidence of bacterial pneumonia (Table 2) were examined from seven of the eight outfitting zones in the Mackenzie Mountains (primarily from the northern half). When the whole lungs were available (n=20), petechial and ecchymotic hemorrhages were observed throughout the pleural surface, and firm, pale, consolidated lesions typical of P. stilesi were grossly visible in the parenchyma of the dorsum and apices of the diaphragmatic lobes. Chronic fibrous adhesions between the lung and chest wall were reported or observed in a few sheep. No airway-dwelling nematodes, such as P. rushi or Dictyocaulus spp., were observed.
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). In straggler sheep collected in spring (SC5–7) but not those collected in fall (FC1–3), granulomas coalesced and surrounded terminal and respiratory bronchioles, leading to impairment of entire alveolar units. One yearling (SC15) died of respiratory failure after exertion, with gross and histologic lesions similar to those observed in SS1.
Locally severe, lymphoplasmacytic, granulomatous inflammation and fibrosis were associated with adults of P. stilesi that were actively producing eggs. Adult P. stilesi without associated eggs or developing larvae elicited little inflammatory reaction. In sheep collected in summer and fall, the prevalence of P. stilesi was greater based on detection in the lungs (86–93%) than in feces (57–67%) (Table 2). Prevalence of P. odocoilei was relatively consistent year-round, regardless of method of detection (lung or feces), and it was 100% overall when both methods were combined.
The majority of adult P. odocoilei were in the appendicular musculature of sheep collected in spring (81% of adult nematodes) and fall (66%) (Table 1). No nematodes were observed in the central nervous system (CNS) on gross examination. On histologic examination, focal eosinophilic and lymphoplasmacytic meningitis, as well as hemosiderophages, surrounded a blood vessel deep in the cerebral cortex of one wild Dall’s sheep (SC15) (Fig. 3C).
Arcanobacterium pyogenes was isolated from throat swabs plated immediately after collection from two healthy sheep. Other bacteria isolated were consistent with normal fauna of the upper respiratory and gastrointestinal tract (Streptococcus spp., usually alpha or nonhemolytic; Escherichia coli; Staphylococcus sp.; Enterobacter sp.; Enterococcus sp.; and Lactobacillus sp.). There were low titers (12–108) of antibody to a pestivirus related to BVD in three of 11 samples from "healthy" sheep; otherwise, there was no serologic evidence of exposure to bacterial or viral pathogens of domestic livestock.
Sick and dead Dall’s sheep from Mackenzie Mountains
We examined samples from two sick sheep that were shot (SK), and nine carcasses of sheep found dead (MO) with evidence of bacterial bronchopneumonia, septicemia, or both (Table 3). Older ewes were frequently lactating and in poor body condition. Most submissions were from the northern Mackenzie Mountains in the NT, with one from the southern Mackenzie Mountains (MO17) and one from the Yukon Territory (MO18). In September 1999, carcasses of three adult sheep with no visible signs of predation or trauma were found in proximity to each other in the northern Mackenzie Mountains; necropsies were not performed.
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). Fibrinous and fibrinopurulent pleuritis, as well as fibrous adhesions, also were observed. On histologic examination, normal lung architecture was obliterated by focal necrosis and abscesses. Protostrongylid eggs and larvae in various stages of decomposition occasionally could be seen on the periphery of "microabscesses," and larvae were observed in the midst of necrotic and bacterial debris in airways (Fig. 1B). Parenchymal lesions typical of P. stilesi were observed in the relatively unaffected caudal portions of the diaphragmatic lobes. In these sheep, prevalence of both P. odocoilei and P. stilesi was higher based on detection of parasite stages in the lungs (56% and 89%, respectively) compared with feces (11% and 67%, respectively) (Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Contrary to current understanding of the migration routes for species of Parelaphostrongylus (Platt and Samuel, 1978; Anderson, 2000; Lankester, 2001) and consistent with the hypothesis proposed in Jenkins et al. (2005b), our findings suggest that P. odocoilei undergoes a neural migration in thinhorn sheep. Although eggs of P. odocoilei in the brain of an experimentally infected Stone’s sheep (SS2) might have been of hematogenous origin, histologic lesions (hemorrhage, malacia, and inflammation) in the brains of experimentally and naturally infected sheep were similar to those reported for P. tenuis and E. rangiferi, which establish in or migrate through the CNS (Anderson, 1965; Lankester and Northcott, 1979; Handeland and Norberg, 1992; Lankester, 2001). Migration of developing P. odocoilei through the CNS during the prepatent and early patency periods might be followed by movement into the surrounding axial and trunk muscles and subsequent dissemination throughout the musculature of thinhorn sheep. This pattern is consistent with observations in cervids, in which adult P. odocoilei have been observed in connective tissue beneath the spine in black-tailed deer (Hobmaier and Hobmaier, 1934), and in the epidural space and in association with nervous tissue in muscles of mule deer killed in early patency (Pybus, 1983).
Three yearlings (FC24, FC27, and SC15), but no lambs, were naturally infected with P. odocoilei (Table 1), consistent with the hypothesis that infection occurs in late summer or fall of the first year (Samuel et al., 1985; Jenkins et al., 2006). If our hypothesis about the timing of the neural migration is correct, examination of lambs in fall should demonstrate the presence of developing fourth-stage larvae or adults in the CNS. Examination of the CNS of adult sheep in October–November did not reveal evidence of P. odocoilei; however, nematodes may only be present in the CNS of young of the year, as observed for E. alces (Handeland and Gibbons, 2001) and E. cervi (Handeland et al., 2000). Dall’s sheep infected as lambs may be subsequently immune to infection, as suggested for P. andersoni and P. tenuis (Lankester, 2001), with a long life span of adult P. odocoilei accounting for a prevalence approaching 100% in many populations of wild Dall’s sheep (Jenkins et al., 2005a).
Infections with P. stilesi were present in Dall’s lambs as young as 2 mo old (Table 3), suggesting that transplacental transmission, similar to that reported for this lungworm in bighorn sheep (Hibler et al., 1974), also may occur in thinhorn sheep. The prepatent period for P. stilesi in an experimentally infected thinhorn sheep was 45 days (Skific and Jenkins, pers. obs.), and newborn lambs less than 2 wk of age are unlikely to consume infected gastropods. Transplacentally derived P. stilesi have been implicated in a syndrome known as "summer lamb mortality" in bighorn sheep (Bunch et al., 1999); this syndrome has never been reported in thinhorn sheep, suggesting that other causes should be considered for this syndrome in bighorn sheep.
Verminous pneumonia in thinhorn sheep
Verminous pneumonia (granulomatous inflammation associated with eggs and larvae of P. stilesi and P. odocoilei, and adults of P. stilesi) was present in all "healthy" Dall’s sheep examined from the Mackenzie Mountains. Pathologic findings associated with adults and eggs of P. stilesi were focal and often limited to the caudal lung, whereas protostrongylid larvae and hematogenously derived eggs of P. odocoilei were distributed diffusely throughout the lung. Seasonal patterns in pathologic findings associated with larvae and eggs in the cranial lung corresponded with seasonal patterns in shedding of larvae in feces, with a trough in summer and a peak in spring (Jenkins et al., 2006). We used staging adapted from interstitial lung disease of humans (Crystal et al., 1981) to describe pathologic findings in lung associated with eggs of P. odocoilei. In stage 1, discrete granulomas caused focal distortions of alveolar septa, as in most sheep collected in summer and fall. In stage 2, coalescing granulomas caused diffuse distortion of the lung architecture and may have compromised vascular walls, terminal respiratory bronchioles, and associated alveolar units, as observed in "straggler" sheep collected in spring. In stage 3, increased blood pressure during exertion may have led to fatal pulmonary edema and hemorrhage in one experimentally (SS1) and one naturally infected sheep (SC15). Respiratory changes associated with P. odocoilei are likely a direct, if infrequent, cause of mortality in wild thinhorn sheep, similar to naturally infected black-tailed deer with "overwhelming parasitemia" (Brunetti, 1969) and "indurations" of the lungs (Hobmaier and Hobmaier, 1934).
In experimentally infected sheep, pathogenicity of P. odocoilei varied with stage of infection, magnitude and chronicity of the inflammatory reaction, and density of eggs and larvae. Respiratory damage may occur in the prepatent period, because protostrongylid eggs consistent with those of P. odocoilei (Hobmaier and Hobmaier, 1934), or possibly Protostrongylus spp., were present in lung washes of SS2 and SS3 before patency. As observed in experimentally infected mule deer (Pybus and Samuel, 1984), the size of granulomas associated with eggs of P. odocoilei increased with time since infection. Interestingly, one experimentally infected sheep with concomitant bacterial pneumonia (SS2) had little inflammatory response to eggs of P. odocoilei, suggesting that the immune response may have been overwhelmed in this animal, which also had eggs and larvae in the brain.
Bacterial pneumonia in thinhorn sheep
Chronic, fibrinopurulent bronchopneumonia in Dall’s sheep from the Mackenzie Mountains was associated with A. pyogenes and species of Mannheimia, and frequently also with lactation, emaciation, and dental disease (for details on MO8 and MO9, see Jenkins et al., 2000). The emaciated condition of these animals may be a predisposing factor, or a consequence of the disease process, as reported in free-ranging bighorn sheep and captive thin-horn sheep (Spraker and Hibler, 1982; Black et al., 1988). Other wild Dall’s sheep in good body condition died acutely with fibrinonecrotizing bronchopneumonia, also associated with A. pyogenes and Mannheimia spp. Differences in virulence of bacterial strains or host susceptibility may account for the different rates of progression of pneumonia. Observations of groups of two to three sick and dead sheep, often rams, suggest direct transmission of a primary pathogen or a common predisposing factor. Increased scrutiny by hunters may be responsible for this gender bias; however, older rams experienced greater losses in one pneumonia epizootic in bighorn sheep (Onderka and Wishart, 1984).
Arcanobacterium pyogenes was isolated from the upper respiratory tract of healthy Dall’s sheep and the lungs of pneumonic sheep in the Mackenzie Mountains, suggesting that this bacterium is an opportunistic pathogen in the lower respiratory tract. Elsewhere, A. pyogenes is commonly isolated from mandibular osteomyelitis (lumpy jaw) in Dall’s sheep (Glaze et al., 1982; Bowyer and Leslie, 1992). Species of Pasteurella and Mannheimia also may act as opportunistic invaders; however, we did not recover any of these bacteria from pharyngeal swabs, even those collected from freshly killed sheep and cultured immediately. For hunter-killed sheep, poor recovery may have been due to the choice of transport media, freezing, and long periods between collection and culture (Ward et al., 1997). Collection and freezing of tonsils or tonsillar biopsies (versus swabs) (Wild and Miller, 1991; Foreyt and Lagerquist, 1994) are recommended for future field investigations of the bacterial fauna of the upper respiratory tract of Dall’s sheep.
Bacterial species and lung pathology in sporadic cases of pneumonia in Dall’s sheep were similar to those observed during widespread outbreaks of pneumonia in bighorn sheep (Spraker et al., 1984; Schwantje, 1986; Aune et al., 1998). However, the strains of Mannheimia present in Dall’s sheep in the NT (based on serology and immunohistochemistry) differed from those described in bighorn and domestic sheep. As well, we found minimal evidence of exposure to respiratory viruses common in bighorn and domestic sheep, consistent with a history of isolation and philopatry of thinhorn sheep (Loehr et al., 2006). Most populations of thinhorn sheep have had few, if any, opportunities for contact with domestic sheep, although recent genetic characterization of Pasteurella and Mannheimia strains suggests that Dall’s sheep in Alaska may have had indirect contact with strains endemic in domestic sheep (Kelley et al., 2006); however, this interpretation may be complicated by the choice of locus used for molecular characterization. Further investigation of the diversity and significance of these bacterial species in Dall’s sheep is warranted, in light of their importance in bighorn sheep.
Management significance
Parelaphostrongylus odocoilei is a newly recognized pathogen in thinhorn sheep with a year-round prevalence approaching 100% in Dall’s sheep in the Mackenzie Mountains. High densities of eggs and larvae of P. odocoilei caused diffuse interstitial lung disease and even respiratory failure in naturally and experimentally infected thinhorn sheep. Based on observations of interstitial lung disease in other species (Crystal et al., 1981; Collie et al., 1993), hypoxemia due to reduced lung volumes, compliance, and gas exchange may compromise respiratory function of Dall’s sheep, especially at high altitudes and when escaping predation. Infection with P. odocoilei also has been linked to poor body condition and respiratory failure in mountain goats and mule deer (Pybus and Samuel, 1984; Pybus et al., 1984), and it caused weight loss and neurological disease in experimentally infected thinhorn sheep (Jenkins et al., 2005b). Therefore, P. odocoilei has the potential to cause subclinical disease and even overt mortality in wild thinhorn sheep, primarily among young of the year in their first winter. Exposure of thinhorn sheep to increasing numbers of infective larvae and range expansion of P. odocoilei in northern North America associated with climate warming may augment the impact of this parasite in the near future (Jenkins et al., 2006).
Verminous pneumonia, especially the diffuse pattern associated with P. odocoilei, may act as a predisposing factor in the development of bacterial pneumonia; however, lesions of verminous pneumonia were frequently obliterated by necrosis and inflammation associated with bacterial pneumonia, complicating interpretation of the role of protostrongylid parasites in the pathogenesis of pneumonia in thinhorn sheep. In bighorn sheep, the "role of any nematode in the pneumonia complex ... is poorly understood" (Demartini and Davies, 1977). If P. odocoilei becomes established in bighorn sheep, it could become yet another player in the multifactorial pneumonia complex. Because of their caudal distribution, lesions associated with the lungworm P. stilesi in bighorn and thinhorn sheep may compromise the only remaining functional lung tissue in sheep with concomitant bacterial bronchopneumonia (generally confined to the cranial lung).
In bighorn sheep, counts of protostrongylid larvae in feces are frequently used to monitor population health, and even to predict pneumonia outbreaks. In our study population of Dall’s sheep, relying on detection of larvae in feces would have underestimated the true prevalence of infection with P. stilesi in sheep collected in summer and fall. This may reflect seasonal decreases in parasite reproduction, or newly acquired infections with P. stilesi, which were not yet patent. In animals with concomitant verminous and bacterial pneumonia, shedding of both types of protostrongylid larvae in feces was reduced or eliminated, probably due to inflammatory destruction and mechanical trapping of larvae in the airways. Caution must be exercised when interpreting fecal larval counts as a measure of health in individuals and populations of wild sheep.
Finally, we observed a sporadic pattern of mortality due to pneumonia caused by pathogens endemic in Dall’s sheep in the Mackenzie Mountains. This differs markedly from the wide-scale die-offs due to pneumonia in bighorn sheep, often associated with pathogens introduced from domestic livestock. Thinhorn sheep occupy most of their historic range at high densities and abundance (Heimer et al., 1992; Worley et al., 2004), and no large-scale, disease-related die-offs have been reported (Hoefs and Cowan, 1979; Dau, 1981; Bowyer et al., 2000). Thinhorn sheep have not been extensively translocated (Hatter and Blower, 1996; Heimer and Taylor, 1996; Veitch, 1996) and have had minimal opportunities for contact with domestic livestock, which harbor parasites, viruses, and strains of bacteria to which thinhorn sheep are naïve (Nielsen and Neiland, 1974; Foreyt et al., 1996; Zarnke, 2000; Garde et al., 2005). Thinhorn sheep are likely susceptible to the same pathogens and ecologic factors (such as habitat fragmentation and degradation) that have hampered conservation efforts for bighorn sheep. Therefore, efforts to avoid translocation, contact with domestic animals, and anthropogenic stressors are a sound basis for proactive management for thinhorn sheep in North America, and they may forestall the breakdown of protective ecological barriers.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Received for publication 22 November 2005.
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