Objective

To evaluate the impact of sub-clinical Mycobacterium avium subsp. paratuberculosis (MAP) infection on fertility of dairy sheep and goats, defined as lambing or kidding in the year before testing.

Materials and methods

One hundred sera and faecal samples were collected from each of four dairy sheep and goat flocks with a history of clinical paratuberculosis. At sampling, individual animal data were recorded. Faecal samples were cultured in Herrold's Egg Yolk medium supplemented with mycobactin J and antibiotics. Sera were tested by a commercially available serum ELISA. An animal was considered sub-clinically infected when found either seropositive or culture-positive. Rogan–Gladen estimates of the true prevalence (TP) of sub-clinically infected animals were calculated separately for sheep and goats. To assess the association between fertility and MAP infection, a multi-level logistic regression model was fitted to the data using the GLIMMIX macro in SAS®.

Results

TP was 10% (95%C.I: 1–25%) and 36% (9–47%) for sheep and goats, respectively. The association between MAP infection and fertility did not differ between species and was modified by parity. Low parity (<4) animals were more likely to be fertile when infected than non-infected. In animals of medium parity (4≤parity<7) this association was not significant. High parity (>7) animals were more likely to be infertile in the year before sampling when infected than non-infected.

Conclusions

High milk production stress and immune regulation during pregnancy may favour proliferation of sub-clinical infection in low parity animals that were probably infected with high numbers of bacilli early in their life. The farmers usually culled these animals when paratuberculosis became clinically evident in subsequent lactations. The detection of a negative association between sub-clinical infection and fertility in high parity stock was probably due to the long incubation period and subsequent induction of negative energy balance secondary to slow proliferation of enteric lesions.