Screening and diagnosis of paratuberculosis in young and adult dairy cattle

Abstract

A gamma interferon (γ-IFN) test, a skin test, and a commercial ELISA were evaluated for the detection of Paratuberculosis (PTB) exposure in cohort of calves at 2, 4, 6, and 8 months of age. The cohort was selected from two dairy farms with 11% (farm A) and 2% (farm B) sero-prevalence of PTB based on the serological status of the dams. The association between the PTB status of the dams and the cell-mediated immune response in the calves was explored. Results from this study indicated that: [a] the humoral response is not relevant in calves younger than 8 months; [b] the γ-IFN levels in plasma are high in 2-month old calves but decrease steadily with age; [c] positive skin test reactions are observed more frequently in calves older than 4 months of age in the cervical site; [d] poor agreement was observed between the γ-IFN and the skin test; and [e] no agreement was observed between CMI-based tests and fecal shedding in 8-month old heifers. Although Map was detected in 1.5% of the calves from herd A (1.5%) by culture and PCR, these two tests did not agree in their positive results. Due to the low number of potentially infected calves, the proposed tests could not be fully characterized. Reference tests with improved accuracy would be required to further evaluate these tests in young cattle. The association between a positive γ-IFN or skin test and risk factors related to the dams and to the calves was explored via logist regression analysis. This study showed that heifers born from dams shedding Mycobacterium avium subsp paratuberculosis (Map) in feces at 60 days in milk are 2.7 times more likely to have a positive γ-IFN test at 8 months of age than those born from non-shedders. Methods for detection of Map in feces were evaluated in detail. Conclusions from these studies indicate that the accuracy for detection low shedders increases with the parallel use of either a One-Tube Semi Nested PCR and conventional culture, or the nonradiometric and the conventional fecal culture.