A comprehensive study of a pneumonic epizootic was initiated when the first signs of disease were noted in a metapopulation of bighorn sheep inhabiting Hells Canyon, bordering Idaho, Oregon, and Washington. A total of 92 bighorn sheep were tested for etiologic agents during the following 6-mo study period. The study population included bighorn sheep believed to be the subpopulation in which disease was first noted, and these sheep were translocated to a holding facility in an effort to contain the disease (group A1, n = 72); bighorn sheep in other subpopulations (group A2) with evidence of clinical disease were captured, sampled, given antibiotics, and released (n = 8) and those that were found dead were necropsied (n = 12). Samples, including oropharyngeal and nasal swabs, and lung and liver tissue were collected from the bighorn sheep identified above. Tissue was collected at necropsy from 60 group A1 bighorn sheep that died following translocation, and samples were cultured for bacteria and viruses. Blood samples were tested for antibodies against known respiratory viruses, and histopathology was conducted on tissue samples. The major cause of death in both group A1 and group A2 bighorn sheep was a rapidly developing fibrinous bronchopneumonia. Multiple biovariants of Pasteurella were isolated from oropharyngeal and nasal samples from both groups, and Mycoplasma ovipneumonia was isolated from five group A1 oropharyngeal samples. Organisms isolated from lung tissue included Pasteurella multocida multocida a and Pasteurella trehalosi, both of which differentiated into multiple strains by restriction enzyme analysis, and parainfluenza-3 virus (PI-3). Paired serum samples revealed > fourfold increases in titers against PI-3 and bovine respiratory syncytial viruses. It was concluded that this epizootic resulted from a complex of factors including multiple potential respiratory pathogens, none of which were identified as a primary pathogen, and possible stress factors.

Pasteurella multocida is a highly diverse group of bacteria recognized as important pathogens. Although P. multocida is not ordinarily associated with disease in Rocky Mountain bighorn sheep (Ovis canadensis canadensis), numerous isolates were cultured in high numbers from free-ranging bighorn sheep in the Hells Canyon area of Idaho, Washington, and Oregon (USA) during the winter of 1995-96. Animals captured in Hells Canyon and held in captivity, and their offspring, also harbored P. multocida. Biochemical utilization tests on 90 isolates identified three subspecies: P. multocida multocida a (n = 54); P. multocida multocida b (n = 13); and P. multocida gallicida (n = 15); and a non-speciated biotype, U6 (n = 8). Genomic DNA digestion with restriction endonuclease Hha I separated the isolates into 62 unique restriction fragment length polymorphism profiles. Capsular type A was predominant (72% of isolates). Only one isolate type, which may have been transmitted from a feral goat, was capsular type D, possessed the structural gene, toxA, for dermonecrotoxin detected by polymerase chain reaction, and produced toxin as determined by monoclonal antibody immunoblot. In conclusion, bighorn sheep in this study carried diverse types of generally non-toxigenic P. multocida associated with epizootic pneumonia.

Multiple strains of P haemolytica and P trehalosi belonging to nonhemolytic and beta-hemolytic biovariant groups were isolated from the pharynx of healthy domestic pack goats. Because hemolytic activity correlates with leukotoxin production, beta-hemolytic strains may have a greater potential to cause disease in naive populations of wild ruminants. However, vaccination with an A1 serotype bacterin did not decrease the proportion of culture-positive goats.

The efficacy of a Pasteurella haemolytica vaccine (serotypes A1, A2, and T10) to induce humoral antibodies and alter colonization of the upper respiratory tract by related P. haemolytica spp. strains was evaluated in 10 bighorn (Ovis canadensis canadensis) and 10 domestic (Ovis aries) sheep. Sheep of each species were divided into five pairs based on age and history of respiratory disease. One sheep in each pair was vaccinated twice 2 wk apart with 2 ml of vaccine (VAC group) and the remaining animals (NV group) were injected with 2 ml of sterile saline. Mild, transient lameness was the only observed adverse effect. Blood sera from the sheep were tested for agglutinating antibodies against whole cells of A1, A2, and T10 and for leukotoxin neutralizing antibodies. Antibody titers were expressed as the reciprocal log 2 of the highest reactive dilutions. Domestic sheep Ͼ1-yr-old and two bighorn sheep with a history of A1 infection had higher titers throughout the study against A1 cells than domestic sheep Ͻ1-yr-old and bighorns without a history of A1 infection. Both domestic and bighorn sheep had log 2 titers of 8 to 12 against A2 cells and 6 to 12 against T10 cells during this time. Bighorn sheep in the VAC group had 2 to 32 fold titer increases for A1 cells by 2 wk post-vaccination (PV) compared to 0 to 2 fold increases in VAC domestic sheep. Two to 16 and 0 to 8 fold increases in antibodies titers to A2 and T10 cells, respectively, were detected in sera of both VAC groups. Sera of bighorn sheep with a history of respiratory disease and all domestic sheep had log 2 leukotoxin neutralizing antibody titers of 4 to 14 in contrast to Յ2 in sera of bighorn sheep without a history of respiratory disease. Neutralizing antibody titers of two bighorns without a history of respiratory disease in the VAC group increased from log 2 0 to 5 in one and from 0 to 9 in the other 2 wk PV. Antibody increases in these animals were no longer evident at 16 wk PV while titers of animals with histories of disease remained relatively stable. The types and numbers of Pasteurella spp. isolated from nasal and pharyngeal swabs varied throughout the study without conclusive evidence of suppression of colonization. Although the animals were not experimentally challenged to determine the efficacy of the vaccine, one VAC and one NV bighorn sheep died following introduction of an A2 P. haemolytica strain when leukotoxin neutralizing antibodies had returned to pre-vaccination levels. This vaccine appeared to be safe for use in bighorn sheep and stimulated moderate but transient increases in antibody levels which should provide some protection against naturally occurring disease. A vaccine which would induce production of high and maintained antibodies against multiple strains of P. haemolytica would be valuable for use in bighorn sheep maintained in captivity or when captured for relocation.