Even if France is officially free from tuberculose bovine Since 2001 (less than 0.1% of cattle herds are infected with the disease), cases have been regularly declared, mainly in the South-West. Infections can lead to the culling of all animals on a farm, in order to stem the spread of the disease. Most infections are from cattle to cattle, but the wildlifenotably badgers and wild boars, can also serve as a relay. What exactly is the role of wildlife in transmission? To find out, scientists from the Epidemiology Unit and the National Reference Laboratory for Bovine Tuberculosis, within ANSES’s Animal Health Laboratory, set out to trace the transmission dynamics of Mycobacterium bovis, the bacterium that causes the disease. The first results have appeared in the journal Veterinary Research.
A tool to go back in time
To begin, the team studied strains of bacteria taken from the Landes and Pyrénées-Atlantiques on 146 cattle and 21 badgers between 2002 and 2017. By determining the genetic proximity of these strains, they reconstituted a phylogenetic tree, a sort of inverted genealogical tree, retracing the evolution of bacteria. This allowed them to estimate the probability that the ancestors of the bacteria strains circulated in badgers or cattle: “ If two bacteria with close sequences are isolated in cattle, there is a high probability that their common ancestor also circulated in cattle, the same for badgers “, explains Laetitia Canini, epidemiologist and co-author of the study.
This reconstruction of the transmission dynamics showed that during the evolution of the strains studied, there was 52 times more likely that a badger infects a cattle than the reverse. Once the bacterium had been transmitted to a bovine by a badger, its spread has been amplified by transmissions between cattle.
Identification of the common ancestor of the bacterium
The scientists also determined that the strains of bacteria they studied probably all came from a same bacteria carried by a badger in the 80s. « The bovine tuberculosis control system was set up in the 1960s in cattle. From this period, the prevalence of the disease fell very sharply in cattle. It is possible that the bacterium continued to circulate among badgers in certain areas, at the time there was no wildlife surveillance. “recalls the scientist.
It is now planned to refine the results, by including data from other regions and samples from wild boars. This is the objective of Hélène Duault’s thesis: ” We are going to trace the transmissions more precisely, until we determine “who transmits to whom”, that is to say which group of individuals (breeding or social group) within a species transmitted the bacterium “, she specifies. It will thus be possible, for example, to determine whether wild animals served as an intermediary to transmit the bacterium between two geographically distant farms. The results will make it possible to better adapt and target the bovine tuberculosis surveillance and prevention measures put in place.