Immunogenicity and protectivity of a live spore Bacillus anthracis vaccine in goats

Abstract

Anthrax is a zoonotic disease affecting most warm-blooded mammals. Primarily recognized as a disease of herbivores, it is caused by a spore-forming, rod-shaped bacterium, Bacillus anthracis. The disease has a worldwide distribution though only of a sporadic nature in developed countries due to effective vaccination and control measures. The current anthrax live spore veterinary vaccine was developed by Max Sterne and its introduction in the 1940s made the control of the disease possible. The principal virulence factors of the B. anthracis are located on two plasmids, pXO1 and pXO2. The pXO1 encodes the toxic factors; protective antigen (PA), lethal and oedema factors (LF and EF) respectively while pXO2 contains the encapsulation genes. Attenuated strains lack either of the two virulence plasmids and consequently have reduced virulence. The Sterne 34F2 strain (tox+, cap-) therefore produces the PA, LF and EF components of the anthrax toxin but lacks the plasmid pXO2 encoding capsule formation and is therefore relatively safe, albeit with some residual virulence. During the development of the avirulent Sterne vaccine, few studies were conducted on the immunogenicity of the vaccine in the target animals. In vivo immunity tests (pathogenicity test) using guinea pigs were mainly used due to few and less sensitive serological diagnostic tools being available at that time. The need for serology in relation to anthrax only became apparent during the development of a human vaccine much after the introduction of the veterinary vaccine. Consequently, though anthrax is recognized as a primary disease of herbivores, there are no records of an in depth study on the immunogenicity and protectivity of the widely accepted strain 34F2 Sterne spore vaccine in any ruminant species. This study was undertaken to fill the present knowledge gap relating to the immunogenicity and duration of protectivity induced by vaccination with the Sterne live spore vaccine in a goat model, which are highly susceptible to the disease. Twenty-one age-matched Boer goats were procured and split into 3 groups of five animals each and vaccinated using different immunization regimens. Six goats served as negative controls utilized in determining the minimum infective dose (MID). Serum samples were collected at intervals for analysis in the laboratory. Bacillus anthracis virulent spore challenge was done not earlier than 3 weeks after the last vaccination and survival was monitored for 14 days. Our findings revealed the MID of virulent B. anthracis spores in naïve goats under experimental conditions to be below 36 spores using the subcutaneous route. Production of anti-anthrax immunoglobulins in the first month following Sterne vaccination was 400-fold higher than pre-vaccination levels with subsequent decline over time to a 50- fold difference, 14 months post vaccination. A similar trend was reflected in the toxin neutralizing antibody titres. There was a correlation between the toxin neutralizing antibody titres and protection against challenge with virulent anthrax spores (P = 0.01). Goats challenged 6 and 62 weeks after vaccination showed a survival rate of 60% and 80% respectively. Those revaccinated one year after the first vaccination were fully protected from virulent anthrax spore challenge. Early pre-validation data of a quantitative indirect ELISA for the detection of anti-PA antibodies was promising and should be further investigated as a tool for anthrax vaccine studies in goats.