Research Report – OPEN ACCESS
Nora Navarro-Gonzalez and 12 colleagues from BIOEPAR, INRA, Oniris, 44307, Nantes, France just published an article describing diagnostic testing patterns for paratuberculosis in naturally infected dairy cattle. The publication, titled “Longitudinal study of Mycobacterium avium ssp. paratuberculosis fecal shedding patterns and concurrent serological patterns in naturally infected dairy cattle“, appears in the Journal of Dairy Science October issue (vol 102, issue 10, pages 9117-9137.
Mycobacterium avium ssp. paratuberculosis (MAP) is the etiological agent of paratuberculosis, a disease that affects ruminants worldwide. Despite global interest in the control of this disease, gaps exist in our knowledge of fecal shedding patterns and concurrent serological patterns. This longitudinal study in dairy cattle herds with high MAP seroprevalence in France aimed at accurately describing fecal shedding patterns over 1 year; relating those shedding patterns to individual animal characteristics (age, breed, parity); and exploring the association between fecal shedding patterns and serological patterns. To describe temporal fecal shedding patterns and continuity of shedding, along with the standard quantitative PCR (qPCR) threshold cycle we used a cutoff value that related to low or nonculturable fecal shedding. We also defined a threshold cycle indicative of shedding in high quantities to describe infection progression patterns. Twenty-one herds completed the study, and 782 cows were tested 4 times each. We obtained 4 sets of paired fecal qPCR and serum ELISA results from 757 cows. Although we targeted highly likely infectious animals, we found a large diversity of shedding patterns, as well as high variability between herds in the proportion of animals showing a given pattern. The fecal qPCR results of almost 20% of the final study sample were positioned at least once in the range that indicated low or nonculturable fecal shedding (between the adjusted and the standard cutoff value). Although these animals would typically be classified as non-shedders, they could be important to infection dynamics on the farm. Animals that shed at least twice consecutively and animals that shed in high quantities rarely reverted to negativity. Repeated fecal qPCR can be used to detect temporal fecal shedding traits, and the decision to cull an animal could practically be based on temporal, semiquantitative results. Overall, we found a mismatch between fecal shedding and ELISA seropositivity (637 animals were ELISA-negative 4 times, but only 13% of those animals were qPCR-negative 4 times). We found that having more than 2 ELISA-positive samples was strongly related to persistent and continuous shedding. We suggest that although serological testing is much less sensitive than qPCR, it can also be used, particularly over the course of multiple testing events, to identify animals that are most likely to contribute to the contamination of the farm environment.
Comment: This is another important contribution that highlights the importance of a regular testing program in dairy herds and the higher sensitivity of qPCR over ELISA methods for MAP infection detection. Sample pooling makes the cost of qPCR testing almost the same as that of ELISA testing (depending on the laboratory used) and offers far higher diagnostic accuracy, i.e. “more bang for the buck”! However, samples must be pooled by the testing lab, and be pooled according to animal age (animals of similar birth dates pooled together). Laboratories should be contacted to discuss how samples should be submitted to achieve this age-based pooling.
For more about strategic polling of fecal samples form dairy cattle see the article published in 2000 by Dr. Kees Kalis in the Journal of Veterinary Diagnostic Investigation 12:547-551.