ANSES finds a link between boar exposure to Mycobacterium bovis and the distribution of tuberculosis outbreaks on cattle farms

ANSES's Maisons-Alfort Laboratory for Animal Health (LSAn), in its capacity as National Reference Laboratory for bovine tuberculosis, is tasked with developing and characterising analysis methods for the surveillance of the disease in different animal species, and transferring these methods to the network of accredited laboratories that it coordinates in France. The Nancy Laboratory for Rabies and Wildlife (LRFSN) is in charge of the surveillance and study of wild animals’ role in the circulation of diseases that can be transmitted to humans or to domestic animals. The collaboration between these two laboratories focused on studying the exposure of boars to Mycobacterium bovis, the bacterium responsible for bovine tuberculosis, in a context characterised by the persistence and regular occurrence of outbreaks of the disease in cattle farms.

A bacterium which can infect a wide range of domestic and wild species

France was officially declared bovine tuberculosis-free by the European Union in December 2000. Before that date, the Mycobacterium bovis bacterium, responsible for the disease and capable of infecting a large number of animal species, had never been isolated in a free-ranging wild animal. However, since 2001, the disease has been identified in wild ungulates in several French regions, including Normandy (2001), Corsica and Burgundy (2003), as well as Pyrénées-Atlantiques (2005), all regions where several outbreaks continue to be detected each year in cattle herds.

It was in 2001 that the first cases of bovine tuberculosis were detected in deer (Cervus elaphus) and wild boars (Sus scrofa) in the Forest of Brotonne (Normandy). In 2006, tuberculosis prevalence in that forest reached 24% in deer and 42% in boars. Both of these wild species are known to be capable of both spreading the pathogen and constituting reservoirs of the disease.

These discoveries have led scientists to ask whether wildlife might be at the origin of tuberculosis cases in domestic cattle farms, and also to direct research towards tools for diagnosing the bacterium in wild species.

An ANSES study conducted thanks to a new monitoring method

In 2008, a serological analysis method was developed in Spain which made it possible to detect boar exposure to bacteria of the tuberculosis complex to which M. Bovis belongs. This method is based on the antigen-antibody relationship. It makes it possible to detect whether Mycobacterium bovis antibodies have been produced by boars, indicating that the pathogen may be present in them, and proving that the bacterium is in circulation in the environment. Thus, by using this method and a collection of boar sera collected between 2000 and 2010, the ANSES laboratories and their national partners (ONCFS^[1] and the Ministry of Agriculture) and Spanish partners (IREC^[2]) wanted to retrospectively estimate boar exposure to the bacteriumin France, and then assess whether this exposure correlated with tuberculosis outbreaks detected in cattle farms, another indicator of M. bovis presence.

The analysis of 2080 boar sera from 58 French départements showed that 30 boars had been exposed to  Mycobacterium bovis, representing 2.2% of the analysed sample. These boars came from seven of the départements studied, all but one known for having had outbreaks of bovine tuberculosis in farms during the study period. 

A link found between boar exposure to M. bovis and tuberculosis outbreaks in cattle farms, but further research needed

These serological results combined with calculations of the average distance separating the boars from the infected cattle farms made it possible to show that boar exposure to Mycobacterium bovis is significantly correlated to bovine tuberculosis outbreaks observed in farms. The study therefore showed that the serological analysis method used could be an effective tool for the surveillance of bovine tuberculosis exposure in boar populations. Further studies are now needed to further refine assessment of the test’s characteristics (sensitivity and specificity) as compared to those currently used for diagnosing bovine tuberculosis in boars (bacterial culturing and molecular DNA detection).

According to the results of other studies, conducted by the ONCFS, ANSES and INRA in particular, wildlife (boars, deer, badgers) could be a reservoir for bovine tuberculosis in France; however, it has not yet been shown whether it is a factor in emergence of the disease in domestic animals. Continued research in collaboration with the national (ONCFS, INRA) and international (i.e. Spanish, see above) partners is also needed here.