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.