Sharing of Pasteurella spp. between free-ranging bighorn sheep and feral goats

Bighorn sheep associations: understanding tradeoffs of sociality and implications for disease transmission

Sociality directly influences mating success, survival rates, and disease, but ultimately likely evolved for its fitness benefits in a challenging environment. The tradeoffs between the costs and benefits of sociality can operate at multiple scales, resulting in different interpretations of animal behavior. We investigated the influence of intrinsic (e.g., relatedness, age) and extrinsic factors (e.g., land cover type, season) on direct contact (simultaneous GPS locations ≤ 25 m) rates of bighorn sheep (Ovis canadensis) at multiple scales near the Waterton-Glacier International Peace Park. During 2002-2012, male and female bighorn were equipped with GPS collars. Indirect contact (GPS locations ≤ 25 m regardless of time) networks identified two major breaks whereas direct contact networks identified an additional barrier in the population, all of which corresponded with prior disease exposure metrics. More direct contacts occurred between same-sex dyads than female-male dyads and between bighorn groups with overlapping summer home ranges. Direct contacts occurred most often during the winter-spring season when bighorn traveled at low speeds and when an adequate number of bighorn were collared in the area. Direct contact probabilities for all dyad types were inversely related to habitat quality, and differences in contact probability were driven by variables related to survival such as terrain ruggedness, distance to escape terrain, and canopy cover. We provide evidence that probabilities of association are higher when there is greater predation risk and that contact analysis provides valuable information for understanding fitness tradeoffs of sociality and disease transmission potential.

Exposure of bighorn sheep to domestic goats colonized with Mycoplasma ovipneumoniae induces sub-lethal pneumonia

BACKGROUND

Bronchopneumonia is a population limiting disease of bighorn sheep (Ovis canadensis) that has been associated with contact with domestic Caprinae. The disease is polymicrobial but is initiated by Mycoplasma ovipneumoniae, which is commonly carried by both domestic sheep (O. aries) and goats (Capra aegagrus hircus). However, while previous bighorn sheep comingling studies with domestic sheep have resulted in nearly 100% pneumonia mortality, only sporadic occurrence of fatal pneumonia was reported from previous comingling studies with domestic goats. Here, we evaluated the ability of domestic goats of defined M. ovipneumoniae carriage status to induce pneumonia in comingled bighorn sheep.

METHODOLOGY/PRINCIPAL FINDINGS

In experiment 1, three bighorn sheep naïve to M. ovipneumoniae developed non-fatal respiratory disease (coughing, nasal discharge) following comingling with three naturally M. ovipneumoniae-colonized domestic goats. Gross and histological lesions of pneumonia, limited to small areas on the ventral and lateral edges of the anterior and middle lung lobes, were observed at necropsies conducted at the end of the experiment. A control group of three bighorn sheep from the same source housed in isolation during experiment 1 remained free of observed respiratory disease. In experiment 2, three bighorn sheep remained free of observed respiratory disease while comingled with three M. ovipneumoniae-free domestic goats. In experiment 3, introduction of a domestic goat-origin strain of M. ovipneumoniae to the same comingled goats and bighorn sheep used in experiment 2 resulted in clinical signs of respiratory disease (coughing, nasal discharge) in both host species. At the end of experiment 3, gross and histological evidence of pneumonia similar to that observed in experiment 1 bighorn sheep was observed in both affected bighorn sheep and domestic goats.

CONCLUSIONS/SIGNIFICANCE

M. ovipneumoniae strains carried by domestic goats were transmitted to comingled bighorn sheep, triggering development of pneumonia. However, the severity of the disease was markedly milder than that seen in similar experiments with domestic sheep strains of the bacterium.

Potential disease agents in domestic goats and relevance to bighorn sheep (Ovis canadensis) management

Domestic goats are raised for meat, milk and hair production, in herds for rangeland weed control, and as pack animals. Domestic sheep, goats and wild bighorn sheep are all susceptible to a multifactorial pneumonia. We sampled 43 herd goats from 7 herds and 48 pack goats from 11 herds for viral and bacterial serology, parasitology, and Pasteurellaceae microbiology. The goats in this study were in generally good health, although most goats did harbor various pathogens and parasites including several bacteria, specifically Pasteurellaceae, which have been associated with pneumonia in free-ranging bighorn sheep. It is not known if domestic goats can transmit the Pasteurellaceae or other pathogens found in this study readily to wild bighorn sheep. However, due the possibility of transmission, domestic goats in areas in or near bighorn sheep habitat should be managed to minimize the risk of spreading disease agents to bighorn sheep.

Epizootic pneumonia of bighorn sheep following experimental exposure to Mycoplasma ovipneumoniae

Background

Bronchopneumonia is a population limiting disease of bighorn sheep (Ovis canadensis). The cause of this disease has been a subject of debate. Leukotoxin expressing Mannheimia haemolytica and Bibersteinia trehalosi produce acute pneumonia after experimental challenge but are infrequently isolated from animals in natural outbreaks. Mycoplasma ovipneumoniae, epidemiologically implicated in naturally occurring outbreaks, has received little experimental evaluation as a primary agent of bighorn sheep pneumonia.

Methodology/Principal Findings

In two experiments, bighorn sheep housed in multiple pens 7.6 to 12 m apart were exposed to M. ovipneumoniae by introduction of a single infected or challenged animal to a single pen. Respiratory disease was monitored by observation of clinical signs and confirmed by necropsy. Bacterial involvement in the pneumonic lungs was evaluated by conventional aerobic bacteriology and by culture-independent methods. In both experiments the challenge strain of M. ovipneumoniae was transmitted to all animals both within and between pens and all infected bighorn sheep developed bronchopneumonia. In six bighorn sheep in which the disease was allowed to run its course, three died with bronchopneumonia 34, 65, and 109 days after M. ovipneumoniae introduction. Diverse bacterial populations, predominantly including multiple obligate anaerobic species, were present in pneumonic lung tissues at necropsy.

Conclusions/Significance

Exposure to a single M. ovipneumoniae infected animal resulted in transmission of infection to all bighorn sheep both within the pen and in adjacent pens, and all infected sheep developed bronchopneumonia. The epidemiologic, pathologic and microbiologic findings in these experimental animals resembled those seen in naturally occurring pneumonia outbreaks in free ranging bighorn sheep.

Bighorn sheep pneumonia: sorting out the cause of a polymicrobial disease

Pneumonia of bighorn sheep (Ovis canadensis) is a dramatic disease of high morbidity and mortality first described more than 80 years ago. The etiology of the disease has been debated since its initial discovery, and at various times lungworms, Mannheimia haemolytica and other Pasteurellaceae, and Mycoplasma ovipneumoniae have been proposed as primary causal agents. A multi-factorial "respiratory disease complex" has also been proposed as confirmation of causation has eluded investigators. In this paper we review the evidence for each of the candidate primary agents with regard to causal criteria including strength of association, temporality, plausibility, experimental evidence, and analogy. While we find some degree of biological plausibility for all agents and strong experimental evidence for M. haemolytica, we demonstrate that of the alternatives considered, M. ovipneumoniae is the best supported by all criteria and is therefore the most parsimonious explanation for the disease. The strong but somewhat controversial experimental evidence implicating disease transmission from domestic sheep is consistent with this finding. Based on epidemiologic and microbiologic data, we propose that healthy bighorn sheep populations are naïve to M. ovipneumoniae, and that its introduction to susceptible bighorn sheep populations results in epizootic polymicrobial bacterial pneumonia often followed by chronic infection in recovered adults. If this hypothesized model is correct, efforts to control this disease by development or application of vectored vaccines to Pasteurellaceae are unlikely to provide significant benefits, whereas efforts to ensure segregation of healthy bighorn sheep populations from M. ovipneumoniae-infected reservoir hosts are crucial to prevention of new disease epizootics. It may also be possible to develop M. ovipneumoniae vaccines or other management strategies that could reduce the impact of this devastating disease in bighorn sheep.

Pasteurellaceae isolated from bighorn sheep (Ovis canadensis) from Idaho, Oregon, and Wyoming

OBJECTIVE

To elucidate the species and biovariants of Pasteurellaceae isolated from clinically normal bighorn sheep (Ovis canadensis) or bighorn sheep with evidence of respiratory disease.

SAMPLE

675 Pasteurellaceae isolates from 290 free-ranging bighorn sheep in Idaho, Oregon and Wyoming.

PROCEDURES

Nasal and oropharyngeal swab specimens were inoculated onto selective and nonselective blood agar media. Representatives of each colony type were classified via a biovariant scheme. The association of respective β-hemolytic isolates with respiratory disease was evaluated via χ(2) analyses.

RESULTS

Bacterial isolates belonged to 4 species: Histophilus somni, Mannheimia haemolytica, Pasteurella multocida, and Bibersteinia (Pasteurella) trehalosi. Within the latter 3 species, 112 subspecies, biotypes, and biovariants were identified. Bibersteinia trehalosi 2 and B trehalosi 2B constituted 345 of 675 (51%) isolates. Most (597/618 [97%]) isolates from adult sheep were from clinically normal animals, whereas most (47/57 [82%]) isolates from lambs were from animals with evidence of respiratory disease. Twenty-two Pasteurellaceae biovariants were isolated from sheep with respiratory disease; 17 of these biovariants were also isolated from clinically normal sheep. The ability of isolates to cause β-hemolysis on blood agar was associated with respiratory disease in adult bighorn sheep (OR, 2.59; 95% confidence interval, 1.10 to 6.07).

CONCLUSIONS AND CLINICAL RELEVANCE

Bighorn lambs appeared more susceptible to respiratory disease caused by Pasteurellaceae than did adult sheep. β-Hemolytic Pasteurellaceae isolates were more likely to be associated with respiratory disease than were non-β-hemolytic isolates in adult sheep. Identification of Pasteurellaceae with the greatest pathogenic potential will require studies to estimate the risk of disease from specific biovariants.

A Review of Hypothesized Determinants Associated with Bighorn Sheep (Ovis canadensis) Die-Offs

Multiple determinants have been hypothesized to cause or favor disease outbreaks among free-ranging bighorn sheep (Ovis canadensis) populations. This paper considered direct and indirect causes of mortality, as well as potential interactions among proposed environmental, host, and agent determinants of disease. A clear, invariant relationship between a single agent and field outbreaks has not yet been documented, in part due to methodological limitations and practical challenges associated with developing rigorous study designs. Therefore, although there is a need to develop predictive models for outbreaks and validated mitigation strategies, uncertainty remains as to whether outbreaks are due to endemic or recently introduced agents. Consequently, absence of established and universal explanations for outbreaks contributes to conflict among wildlife and livestock stakeholders over land use and management practices. This example illustrates the challenge of developing comprehensive models for understanding and managing wildlife diseases in complex biological and sociological environments.

Association of Mycoplasma ovipneumoniae infection with population-limiting respiratory disease in free-ranging Rocky Mountain bighorn sheep (Ovis canadensis canadensis)

Bronchopneumonia is a population-limiting disease in bighorn sheep in much of western North America. Previous investigators have isolated diverse bacteria from the lungs of affected sheep, but no single bacterial species is consistently present, even within single epizootics. We obtained high-quality diagnostic specimens from nine pneumonic bighorn sheep in three populations and analyzed the bacterial populations present in bronchoalveolar lavage specimens of seven by using a culture-independent method (16S rRNA gene amplification and clone library analyses). Mycoplasma ovipneumoniae was detected as a predominant member of the pneumonic lung flora in lambs with early lesions of bronchopneumonia. Specific PCR tests then revealed the consistent presence of M. ovipneumoniae in the lungs of pneumonic bighorn sheep in this study, and M. ovipneumoniae was isolated from lung specimens of five of the animals. Retrospective application of M. ovipneumoniae PCR to DNA extracted from archived formalin-fixed, paraffin-embedded lung tissues of historical adult bighorn sheep necropsy specimens supported the association of this agent with bronchopneumonia (16/34 pneumonic versus 0/17 nonpneumonic sheep were PCR positive [P < 0.001]). Similarly, a very strong association was observed between the presence of one or more M. ovipneumoniae antibody-positive animals and the occurrence of current or recent historical bronchopneumonia problems (seropositive animals detected in 9/9 versus 0/9 pneumonic and nonpneumonic populations, respectively [P < 0.001]). M. ovipneumoniae is strongly associated with bronchopneumonia in free-ranging bighorn sheep and is a candidate primary etiologic agent for this disease.

Phylogenetic diversity of Pasteurellaceae and horizontal gene transfer of leukotoxin in wild and domestic sheep

Wild and domestic animal populations are known to be sources and reservoirs of emerging diseases. There is also a growing recognition that horizontal genetic transfer (HGT) plays an important role in bacterial pathogenesis. We used molecular phylogenetic methods to assess diversity and cross-transmission rates of Pasteurellaceae bacteria in populations of bighorn sheep, Dall's sheep, domestic sheep and domestic goats. Members of the Pasteurellaceae cause an array of deadly illnesses including bacterial pneumonia known as "pasteurellosis", a particularly devastating disease for bighorn sheep. A phylogenetic analysis of a combined dataset of two RNA genes (16S ribosomal RNA and RNAse P RNA) revealed remarkable evolutionary diversity among Pasteurella trehalosi and Mannheimia (Pasteurella) haemolytica bacteria isolated from sheep and goats. Several phylotypes appeared to associate with particular host species, though we found numerous instances of apparent cross-transmission among species and populations. Statistical analyses revealed that host species, geographic locale and biovariant classification, but not virulence, correlated strongly with Pasteurellaceae phylogeny. Sheep host species correlated with P. trehalosi isolates phylogeny (PTP test; P=0.002), but not with the phylogeny of M. haemolytica isolates, suggesting that P. trehalosi bacteria may be more host specific. With regards to populations within species, we also discovered a strong correlation between geographic locale and isolate phylogeny in the Rocky Mountain bighorn sheep (PTP test; P=0.001). We also investigated the potential for HGT of the leukotoxin A (lktA) gene, which produces a toxin that plays an integral role in causing disease. Comparative analysis of the combined RNA gene phylogeny and the lktA phylogenies revealed considerable incongruence between the phylogenies, suggestive of HGT. Furthermore, we found identical lktA alleles in unrelated bacterial species, some of which had been isolated from sheep in distantly removed populations. For example, lktA sequences from P. trehalosi isolated from remote Alaskan Dall's sheep were 100% identical over a 900-nucleotide stretch to sequences determined from M. haemolytica isolated from domestic sheep in the UK. This extremely high degree of sequence similarity of lktA sequences among distinct bacterial species suggests that HGT has played a role in the evolution of lktA in wild hosts.