Evidence of birth seasonality and clustering of Mycobacterium avium subspecies paratuberculosis infection in US dairy herds.

Paratuberculosis (Johne's disease) is a contagious intestinal infection of ruminants caused by Mycobacterium avium subspecies paratuberculosis (MAP). In cattle, young calves are at the highest risk for acquiring the infection which occurs mainly through ingestion of MAP from contaminated milk, colostrum and feces or environmental contacts. Data consisted of birth dates and ELISA results of 8000 mature cows from 24 Jersey herds from throughout the US and 4 Wisconsin Holstein herds. Some herds also had complete fecal culture (FC) results. The first infection (case) definition (CD1) relied on only ELISA results. A second case definition (CD2) was used in which results of both ELISA and FC tests were considered: animals testing positive to either test were considered "test-positives" and cows testing negative to ELISA or to both ELISA and FC were regarded as "test-negatives". Objective one was to assess seasonality in birth of MAP-infected animals. The effects of age, breed, herd and season of birth (expressed as the sine and cosine functions of birth days within year) were examined using logistic regression. Age was significantly associated with the MAP infection status of dairy cows for both CDs (OR=1.11; 95% CI 1.09, 1.14; P<0.0001 for CD1; OR=1.16; 95% CI 1.08, 1.24; P<0.0001 for CD2). Season of birth had a significant effect on the risk of MAP infection based on CD1 (OR=0.79; 95% CI 0.71, 0.89; P<0.001 for cosine of birth days) with a peak in summer and a trough in winter based on the fitted model. Objective two was to assess whether test-positive animals were randomly distributed or were clustered by date of birth within herds. A temporal cluster analysis approach (scan statistic) implemented in SaTScan software was used for each case definition to detect clusters of birth cohorts using birthdates. Results identified significant clustering of MAP infection cases for CD1 in multiple herds (P<0.05). These results necessitate matching cases and controls of MAP infection on their birth dates to control for non-uniform exposure to MAP in paratuberculosis case-control genome wide association studies, candidate gene studies or in on-farm disease intervention trials.