Comparison of 2 PCR assays on environmental samples cultured for Mycobacterium avium subsp. paratuberculosis

Mycobacterium avium subsp. paratuberculosis (MAP) is the causal agent of paratuberculosis, a chronic, contagious, and incurable enteric disease of ruminants. An in-house IS900 PCR assay validated for MAP detection in sheep has been shown to have a higher sensitivity than a commercial PCR and fecal culture. We have now compared the performance of this in-house IS900 PCR assay with a commercial ISMap02 PCR assay for the detection of MAP DNA in bovine dairy farm environmental samples. We purposefully selected 30 culture-positive, 62 culture-negative, and 62 non-interpretable environmental samples. We applied the IS900 PCR assay directly to the frozen inoculum of these samples. Inocula were incubated in an automated system, and growth was confirmed by an acid-fast bacilli stain and the IS900 PCR assay. Among culture-positive samples before incubation, the IS900 PCR assay yielded significantly more positive results than the ISMap02 PCR assay; however, among culture-negative samples, the IS900 PCR assay yielded positive results both before and after incubation. The ISMap02 PCR assay did not flag positively among the culture-negative samples either before or after incubation. The IS900 PCR assay is a sensitive method that can be used to detect MAP DNA in environmental samples before incubation. The ISMap02 PCR assay is a specific method used to detect MAP DNA in environmental samples both before and after incubation.

Bayesian accuracy estimates of environmental sampling for determining herd paratuberculosis infection status and its association with the within-herd individual fecal culture prevalence in Québec dairies

The objectives of this retrospective analysis were to: 1) estimate the diagnostic sensitivity (Se) and specificity (Sp) of bacterial culture of environmental samples for determining Mycobacterium avium subsp. paratuberculosis (MAP) infection status in Québec dairies, using a Bayesian Latent Class Model (BLCM); and 2) determine the association between the number of positive environmental samples and the individual fecal culture (IFC) apparent and true MAP within-herd prevalence. Environmental and individual fecal samples were collected from 87 commercial dairy herds participating in previous research projects. Environmental samples included two composite samples of 20 g collected from different locations within each of the following sites: an area where manure from the majority of adult cattle accumulates, a manure storage area and another site of manure accumulation chosen by the veterinarian. Samples were cultured using the MGIT Para TB culture liquid media and the BACTEC MGIT 960 system. The Se and Sp of environmental sampling were estimated using a one-test-one-population BLCM. Herds were considered positive for environmental sampling if at least one out of the six samples collected was positive. The apparent and true IFC within-herd MAP prevalence estimates for each herd were obtained using a two-stage cluster BLCM, then merged in a single dataset with the environmental sample results. The association between the within-herd MAP prevalence results (apparent and true), and the number of positive environmental samples was assessed using a zero-inflated negative binomial (ZINB) model. In all BLCMs, median posterior estimates and 95 % Bayesian credible intervals (BCI) were obtained with OpenBUGS statistical freeware. Se and Sp of environmental sampling were 43.7 % (95 % BCI: 32.5-55.5) and 96.2 % (95 % BCI: 84.2-99.8), respectively. Overall, the number of positive environmental samples increased with the apparent and true MAP within-herd prevalence. The true prevalence was higher than the apparent prevalence for a given number of positive environmental samples. The probability of not observing a positive environmental sample decreased with the prevalence. Despite its imperfect accuracy, environmental sampling is an inexpensive and non-invasive sampling method to determine MAP infection status in tie-stall herds that can be used as a proxy to estimate the true within-herd prevalence. The absence of positive environmental samples in a single sampling visit is likely an indicator of a very low within-herd prevalence rather than being MAP exempt.

Incidence of fecal excretion of Mycobacterium avium subsp. paratuberculosis in dairy cows before and after the enrolment in the Québec voluntary program

Paratuberculosis is a chronic and contagious enteric disease of ruminants caused by Mycobacterium avium subsp. paratuberculosis (MAP). This disease of worldwide distribution is responsible for significant economic losses and the bacteria itself has been linked to human Crohn's disease. Paratuberculosis control programs focus on reducing MAP transmission by implementing better management practices that target infection routes. In Québec, a Voluntary Paratuberculosis Prevention and Control Program (QVPPCP) was launched in 2007. The objectives of this prospective cohort study were threefold. The first was to describe the changes in the incidence of fecal excretion of MAP in cows born before and after farm enrolment in the QVPPCP. The second was to estimate the impact of the risk of within-herd transmission of MAP (measured by the risk assessment score (RAS)) on the incidence of fecal excretion of MAP. And the third was to evaluate the impact of calf rearing practices on the incidence of fecal excretion of MAP. Eighteen MAP-positive herds were visited annually from 2011 to 2015. At each visit, individual fecal samples from all adult cows were collected. MAP was cultured using liquid media and an automated system. A risk assessment questionnaire was completed upon enrolment in the QVPPCP and at each visit. The RAS of the farm was attributed to each cow according to its birthdate. Cox proportional hazards models were used to estimate the hazard ratios (HR) for the exposure variables. Herd clustering was taken into account using robust standard errors. A total of 2158 cows were included (cohort born before n=919; cohort born after n=1239). The incidence and hazard of fecal excretion were significantly lower for the cohort-after than the cohort-before (incidence rate ratio=0.38; 95% CI: 0.18-0.78 and HR=0.48; 95% CI: 0.23-0.98). The HR of fecal excretion for cows exposed to a high RAS was 2.20 times (95% CI: 1.21-3.99) that of cows exposed to a low RAS. Poor calving cow hygiene (HR=3.41; 95% CI: 1.40-8.31) and contact between pre-weaned heifers and adult cows or their feces were significantly associated with an increased hazard of fecal excretion of MAP (HR=2.66; 95% CI: 1.08-6.56). Our results suggest that enrolment in the QVPPCP reduces the risk of MAP fecal excretion. They support the hypothesis that contact between calves and adult cows or their feces increases MAP transmission. The incidence results also suggest that MAP prevalence could be reduced to low levels regardless of initial MAP prevalence.