Longitudinal study of Mycobacterium avium ssp. paratuberculosis fecal shedding patterns and concurrent serological patterns in naturally infected dairy cattle.

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.

Sensitivities of a bulk-tank milk ELISA and composite fecal qPCR to detect various seroprevalence levels of paratuberculosis in cattle herds in Normandy, France.

This study evaluated an ELISA on bulk tank milk (BTM) samples and a qPCR on a single composite fecal sample to detect paratuberculosis seropositive cattle dairy herds. Individual serum (n = 15 372), BTM and composite fecal samples were obtained from 192 herds. The within-herd apparent seroprevalence was categorized and compared with BTM ELISA and fecal qPCR results. The BTM ELISA had poor overall sensitivity (16%) to detect seropositive herds but higher sensitivity (53%) in the higher apparent seroprevalence group of > 9%. Using an optimized cut-off point (5.0% S/P), sensitivities overall and in the high apparent seroprevalence group were 53% and 88%, respectively. The BTM ELISA gave 5% positive results in seronegative herds and 25% using the optimized cut-off. Fecal qPCR had 72% sensitivity to detect seropositive herds and 88% in the higher apparent seroprevalence group, but gave 25% positive results in fully seronegative herds. The combination of BTM ELISA and composite fecal qPCR improved the sensitivity to detect seropositive herds.

Sensibilités d'un test ELISA sur lait de réservoir et d'une qPCR sur prélèvement composite de fèces pour le dépistage de cheptels bovins à différents niveaux de séroprévalence en paratuberculose en Normandie, France. L'étude a été entreprise pour évaluer les performances diagnostiques d'un test ELISA, effectué sur un échantillon de lait de réservoir (BTM), et d'une qPCR, réalisée sur un échantillon composite de fèces (CF), pour détecter les troupeaux de bovins séropositifs pour la paratuberculose. Les sérums individuels (n = 15 372), les échantillons de BTM et de CF ont été collectés dans 192 troupeaux. La séroprévalence apparente intra-troupeau a été calculée puis catégorisée avant d'être comparée aux résultats de l'ELISA sur BTM et de la qPCR sur CF. Le test ELISA sur BTM a montré une faible sensibilité globale (16 %) mais celle-ci était plus élevée dans les élevages les plus fortement séropositifs > 9 % (53 %). En utilisant un seuil optimisé (E/P 5,0 %), les sensibilités étaient de 53 % et 88 %, respectivement. Le test ELISA sur BTM a donné 5 % de résultats positifs sur des troupeaux entièrement séronégatifs et 25 % en utilisant le seuil optimisé. La qPCR sur CF avait une bonne sensibilité (72 %), en particulier dans les élevages fortement séropositifs (88 %), et a donné 25 % de résultats positifs sur des cheptels entièrement séronégatifs. L'utilisation combinée de BTM ELISA et CF qPCR a permis d'améliorer la sensibilité à dépister des cheptels séropositifs.(Traduit par Docteur Serge Messier).