Immunologic Responses of Domestic and Bighorn Sheep to a Multivalent Pasteurella Haemolytica Vaccine

Abstract

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.

Comparison of Passively Transferred Antibodies in Bighorn and Domestic Lambs Reveals One Factor in Differential Susceptibility of These Species to Mannheimia haemolytica-Induced Pneumonia

“…Furthermore, the serum titers of Ab against M. haemolytica surface antigens were not as low as their Lktneutralizing Ab titers, which suggests that the ability of BHS to mount an Ab response is not impaired. Bighorn sheep are capable of producing levels of Lkt-neutralizing Abs comparable to those produced by DS (34). So why then are the Lktneutralizing serum Ab titers extremely low in BH ewes, in comparison with those for domestic ewes?…”

Pathogen surveillance and epidemiology in endangered peninsular bighorn sheep (Ovis canadensis nelsoni)

“…Recent research indicates that Mycoplasma ovipneumoniae infection can cause pneumonia by decreasing respiratory immune function and allowing colonization by other pathogens (Besser et al, 2012(Besser et al, , 2014Dassanayake et al, 2010). Numerous management tools including vaccination, population reduction, and supplemental feeding have failed to prevent or control pneumonia outbreaks in bighorn sheep (Cassirer et al, 2001(Cassirer et al, , 2018Ward et al, 1999) but recent efforts to test and remove chronic M. ovipneumoniae carriers demonstrate promising results, including improved lamb survival (Garwood et al, 2020).…”

Incorporating foray behavior into models estimating contact risk between bighorn sheep and areas occupied by domestic sheep

“…Managers have been unable to stop die‐offs once they begin. No vaccine or antibiotic treatment is available that protects bighorn sheep (Ward et al , Cassirer et al ); and even in the event one becomes available, treating wild sheep can be logistically difficult or impossible. To reduce the risk of outbreaks, most wildlife, livestock, and land management professionals recommend physically separating the species using buffers around occupied bighorn habitat (Singer and Gudorf , Schommer and Woolever , Garde et al , CAST , WAFWA ).…”