Aims: To evaluate the survival of Mycobacterium avium subsp. paratuberculosis (MAP) during anaerobic digestion (AD), we studied two different biogas plants loaded with manure and slurry from paratuberculosis-infected dairy herds. Methods and Results: Both plants were operating under mesophilic conditions, the first with a single digester and the second with a double digester. Mycobacterium avium subsp. paratuberculosis detection was performed by sampling each stage of the process, specifically the prefermenter, fermenter, liquid digestate and solid digestate stages, for 11 months. In both plants, MAP was isolated from the prefermenter stage. Only the final products, the solid and liquid digestates, of the one-stage plant showed viable MAP, while no viable MAP was detected in the digestates of the two-stage plant. Conclusions: Mycobacterium avium subsp. paratuberculosis showed a significant decrease during subsequent steps of the AD process, particularly in the two-stage plant. We suggest that the second digester maintained the digestate under anaerobic conditions for a longer period of time, thus reducing MAP survival and MAP load under the culture detection limit. Significance and Impact of the Study: Our data are unable to exclude the presence of MAP in the final products of the biogas plants, particularly those products from the single digester; therefore, the use of digestates as fertilizers is a real concern related to the possible environmental contamination with MAP.
IntroductionMycobacterium avium subsp. paratuberculosis (MAP) is the aetiological agent of a chronic proliferative enteritis in ruminants known as paratuberculosis (PTB) or Johne's disease.In recent decades, PTB has been frequently linked with Crohn's disease in humans due to certain clinical similarities between these two enteric pathologies (Hermon-Taylor and Bull 2002); however, the role of MAP in Crohn's disease has not been definitively demonstrated (Chiodini et al. 2012;Sechi and Dow 2015). Furthermore, MAP has also been linked with other human diseases, such as type I diabetes, Hashimoto's thyroiditis, multiple sclerosis and others (Scanu et al. 2007;Sisto et al. 2010; Dow 2012;Frau et al. 2013;Waddell et al. 2015).Paratuberculosis is distributed worldwide in ruminants, and the prevalence of infected farms in countries with advanced animal husbandry is growing rapidly. In this regard, numerous investigations have reported its wide distribution in Europe (Nielsen and Toft 2009) and Italy (Pozzato et al. 2011), with herd prevalence estimates over 50% (Nielsen and Toft 2009).Animals are most frequently infected at a young age through the ingestion of faeces or contaminated feed, fodder, milk and colostrum (Sweeney 2011). The major 36