Effect of season and lactation stage on the diagnostic sensitivity of direct real-time PCR assay for detection of fecal shedding of Mycobacterium avium subsp paratuberculosis

Abstract

Mycobacterium avium subsp paratuberculosis (MAP) is the causative organism of Johne's disease. Dairy Farmers of Canada lists this production-limiting disease as one of the top two animal health priorities for the Canadian dairy industry. Infection of newborn calves from ingestion of MAP-infected feces, colostrum, or milk is considered the main route of transmission and a major concern in a herd. Current diagnostics rely on identification of the bacterium in feces via culture and molecular tests or identification of MAP antibodies in milk or serum via enzyme-linked immunosorbent assays (ELISA). However, these tests are inadequate to meet industry needs for herd biosecurity and environmental-transmission control of MAP, especially when MAP-infected cows are in the subclinical stages of the disease. Milk ELISAs have poor predictive values due to imperfect sensitivity and specificity, especially when used in herds with a low prevalence of MAP-infected cows. Although culture of fecal samples for MAP is currently the gold standard diagnostic test for identification of MAP-infected cows, the long incubation times, costs, and intermittent shedding of MAP in feces hinder its use as an efficient screening tool. The goal of this study was to assess how shedding patterns of MAP in feces vary with lactation stage and season, as determined via direct real-time polymerase chain reaction (PCR) assay.