Abstract
In the United States, elk (Cervus elaphus) serve as a reservoir for Brucella abortus, and pose a significant problem for wildlife managers and the cattle industry. Over the last 27 years, seroprevalence among female elk surveyed on feedgrounds in the Greater Yellowstone Area has averaged approximately 34%.9 Ballistic vaccination programs of feedground elk with B. abortus strain 19 (s19) were implemented in the 1980s and continue with notable success.9 However, research has been ongoing to produce an alternative vaccine regimen to s19, given the fact that it produces erroneous positive results on routine brucellosis surveillance tests in animals that have received the vaccine.3 B. abortus strain RB51 (sRB51) is an attenuated rough mutant that is currently being used in domestic cattle as a calfhood vaccine. An sRB51 does not induce antibodies to the lipopolysacharide O-side chain epitopes and thus does not elicit false positives on serologic tests.8 The results of research conducted by many of our colleagues in the last decade have indicated that, although safe, parenteral sRB51 is not efficacious in protecting elk from abortion in the face of challenge with virulent B. abortus.1,2,5,6 This observation comes despite this species’ ability to mount a significant IgG antibody response to the vaccine.6,7
At the National Animal Disease Center, we have recently investigated the immune response to both sRB51 and s19 in captive elk, specifically looking at total IgG titers, as well as at antigen stimulated proliferation of peripheral blood mononuclear cells (PBMC) in vaccinated animals. Results show that IgG levels appeared to peek 4–6 weeks after both single vaccination and booster given 65 weeks post-prime. Proliferation of PBMC in the face of both RB51 and S19 antigens was predominantly of the B cell type. These data indicate that elk produce an immune response to current Brucella vaccines that is clearly of a humoral quality. It is widely believed that elimination of Brucella abortus infection requires a strong cell mediated immune (CMI) response.7 Therefore, a vaccine regimen capable of producing such a response will likely be more effective in protecting elk from Brucella-induced abortions. Future work to be done at the National Wildlife Research Center (NWRC) includes the use of an adjuvant (AdjuVac) developed at the NWRC, consisting of modified Johne’s vaccine in an oil and water emulsion, to determine whether sRB51 administered with this adjuvant may elicit the desired CMI response with long-lasting duration. Various recombinants of RB51 will also be tested for their efficacy in elk as well.
Literature Cited
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