Introduction.
Bacteriological culture of faeces is the most commonly used diagnostic test for paratuberculosis in Denmark. With the hypothesis of a shift in immune response from Th1 to Th2 preceding progression of paratuberculosis into the shedding stages, measures of cellular immunity is expected to decrease parallel to an increase in serological response prior to diagnosis by culture. The purpose of the present study was to compare cellular immune response with antibody response in the subclinical stages in cattle naturally infected with paratuberculosis before their first culture-positive faeces sample.
Materials and methods.
Cattle were tested simultaneously and repeatedly for cellular immune response, antibody response and faecal excretion. Twenty-nine cattle of 1 month to 49 months of age, selected from 9 dairy herds were sampled 3 to 8 times during a 2-year period. Whole blood samples were stimulated with johnin PPD (PPDj), and secreted interferon-gamma was assayed by the BOVIGAM ELISA. Serum samples were tested by an in-house absorbed indirect IgG ELISA using a commercially available antigen (Allied Monitor, USA), and decontaminated faeces samples were inoculated on modified Löwenstein-Jensen medium.
Results and discussion.
The measures of cellular immunity were decreasing in 19 (66%) of 29 cattle prior to diagnosis by faeces culture; of these, 14 (74%; 48% of total) cattle had simultaneously increasing serological response. Thus, the hypothesis of a shift from Th1 to Th2 response was in the present study only confirmed by approximately half of the culture-positive cattle. Circumstances not taken into account in this study could influence the results. Culture-positive results could occur because of passive excretion (false-positive), faeces samples contaminated with moulds inhibit in-vitro growth of the pathogen (false-negative results prior to a positive sample), or peripheral measures of cellular immune response may not accurately reflect local immune responses in the intestine more closely related to excretion of the pathogen.
