Combatting Anthrax (Bacillus anthracis) in Texas Wildlife with Continued Development of an Oral Anthrax Vaccine and Serosurveillance of Sentinel Species

Abstract

Anthrax, Bacillus anthracis, is an endospore-forming zoonotic bacterium naturally existing in calcium rich soils around the world, including Texas, USA. The disease is severely underreported; outbreaks cause thousands of acute wildlife mortalities while only dozens are reported. Although anthrax is considered an ancient disease, the ecology and epidemiology are largely unknown. Anthrax is treatable and vaccine-preventable, but these are not viable for wildlife. The Veterinary Feed Directive prevents prophylactic antibiotic treatment of wildlife, and the acute pathogenesis complicates treatment of clinically infected wildlife. Furthermore, the veterinary anthrax vaccine, using naturally attenuated Sterne 34F2 strain anthrax spores, is only labeled for subcutaneous use in livestock species and is not feasible for mass wildlife vaccination efforts. Reports of effective wildlife vaccination via intramuscular dart-injection indicate alternative routes of administration, though oral self-vaccination is the primary goal for anthrax prevention and management. For wildlife vaccination protocols to succeed, the spatial patterns of disease exposure and target species should be identified. We conducted serosurveillance for anthrax-specific antibodies via species-specific enzyme-linked immunosorbent assays and toxin neutralizing assays using hunter-harvested white-tailed deer (Odocoileus virginianus) and feral hogs (Sus scrofa) across 24 counties in Texas during a six-month period following the summer anthrax season. Seroprevalence was 9.0–24.6% among white-tailed deer and 19.7–50.0% among feral hogs. Understanding the potential biases introduced with hunter-harvested sampling, we also conducted experiments to determine the impacts of unideal storage and subsequent blood sample hemolysis on diagnostic accuracy. Hemolysis was significantly higher among blood samples stored in refrigeration for 28 days post-collection, with no significant effects noticed following up to 24 hours of ambient storage. Neither storage treatment nor hemolysis impacted diagnostic determination (positive/negative), though trends indicated that more severe hemolysis may be influential. We also expanded the understanding of wildlife vaccination by continuing development of an alginate-microencapsulated anthrax vaccine and establishing the efficacy and in vitro protection following three vaccine administration routes in white-tailed deer and axis deer (Axis axis): subcutaneous hand-injection, intramuscular remote-dart-delivered injection, and oral administration of the novel formulation. In both species, injected vaccines were consistently efficacious and oral vaccines, though less efficacious, were immunogenic.