Comparative lymphocyte stimulation studies on whole blood from vaccinated and nonvaccinated cattle with paratuberculosis.

The lymphocyte stimulation test was evaluated in the diagnosis of bovine paratuberculosis, using whole blood samples from cattle. These samples were incubated with a purified protein derivative of Mycobacterium paratuberculosis. Samples from vaccinated and nonvaccinated cattle from paratuberculosis-free herds and from paratuberculous herds were used. Vaccinated and nonvaccinated groups from the paratuberculous herds each included fecal culture-positive cattle, fecal culture-negative cattle, and calves. All groups included some cattle with a stimulation index (SI) greater than or equal to 2, and all groups except 2 of the 4 vaccinated groups had some cattle with a SI less than or equal to 2. In paratuberculosis-free herds, 16% of the nonvaccinated cattle had a SI greater than or equal to 2, and 0.7% had a SI greater than or equal to 4. In paratuberculous herds, 42% of the nonvaccinated cattle that were negative on fecal culture had a SI greater than or equal to 2 and 24% had a SI greater than or equal to 4. Vaccinated cattle greater than 1 year old could not be distinguished from infected cattle by means of the lymphocyte stimulation test. If a SI of 2 or 4 had been used as a basis to cull individual animals, some paratuberculous cattle would have been identified incorrectly as noninfected, and some paratuberculosis-free cattle would have been identified incorrectly as infected.