Facteurs de risque associés au statut de troupeau positif à Mycobacterium avium ssp. paratuberculosis

Paratuberculosis is a chronic and contagious enteric disease of ruminants caused by Mycobacterium avium subsp. paratuberculosis (MAP). Control of paratuberculosis is justified given the associated economic losses and the potential role of MAP in Crohn’s disease in humans. Cattle usually become infected at a young age, primarily by the fecal-oral route. After a long incubation period (up to several years), infected cows may start shedding MAP without showing clinical signs, thus perpetuating MAP infections on the farm. Management procedures that limit exposure of susceptible animals to MAP are more effective at reducing disease prevalence than simply testing and culling MAP infected cows. Any management practices that expose (directly or indirectly) susceptible animals to fecal material from MAP shedders can be considered a risk factor for infection. The objectives of this master’s thesis are: 1) critically review the available scientific literature that evaluates the association between management practices (measured by a risk assessment questionnaire (RAQ)) and MAP herd status, et 2) identify the association between management practices and MAP herd status (determined using bacteriological culture of environmental samples) of dairy herds in Québec, Canada. A systematic review was performed to answer the first objective. We included studies that assessed PTB risk factors using a RAQ and measured the association between risks factors and MAP herd status. For the second objective, a case-control study was designed. A total of 26 case herds in which MAP had been isolated from at least 1 environmental sample in each herd and 91 control herds (no clinical cases of paratuberculosis and negative on 2 consecutive yearly environmental samplings) were selected. Multivariable logistic regression was used to evaluate the association between selected risk factors and MAP herd status. Twenty-one cross-sectional, 5 case control and 3 longitudinal studies met the inclusion criteria. Herd size was significantly associated with MAP herd status in  12 (4 with low RoB) out of 18 studies, introduction of new animals was significantly associated with MAP herd status in 10 (4 with low RoB) out of 24, history of PTB was significantly associated with MAP herd status in 6 (4 with low RoB) out of 13, and management of colostrum and milk were significantly associated with MAP herd status in 5 (none with low RoB) out of 18. For Québec dairy herds, herd size (OR=1.17; 95% CI: 1.02-1.33) and proportion of cows purchased per year in the last 5 years (OR=5.44; 95% CI: 1.23-23.98) were significantly associated with a positive MAP herd status. The results of this master’s thesis provide a large compilation of available information about risk factors associated with MAP herd status evaluated using a RAQ. Some risk factors are apparently more consistent across studies. However, results should be interpreted in the light of the quality and risk of bias of each study. Management practices aiming to prevent the introduction of new animals into the herd and to reduce the contact of newborn calves with adult animals or their feces are key elements to minimize MAP introduction and transmission into a herd. These elements should be prioritized in control programs.

Keyphrases: Mycobacterium avium ssp. paratuberculosis, Mycobacterium avium subsp. paratuberculosis, Dairy herds, Risk factors, Management practices, Environmental sampling, Herd status