Tracing the source of infections by the bacterium responsible for bovine tuberculosis
Cases of bovine tuberculosis are rare in France, but infected cattle herds are still regularly reported. In order to determine the exact role of wildlife, especially badgers, in transmission, scientists from the Laboratory for Animal Health have been reconstructing the evolution of the bacterium that causes the disease.
Although France has been officially free of bovine tuberculosis since 2001 (less than 0.1% of cattle herds are infected with the disease), cases are regularly reported, mainly in the South West. Infections can lead to the slaughtering of all the animals on a farm in order to contain the spread of the disease. Most transmission is from cattle to cattle, but wildlife, especially badgers and wild boar, can also act as vectors. What exactly is the role of wildlife in transmission? To answer this question, scientists from the Epidemiology Unit and the National Reference Laboratory for bovine tuberculosis, within the ANSES Laboratory for Animal Health, set out to trace the transmission dynamics of Mycobacterium bovis, the bacterium that causes the disease. The first results have now been published in the journal Veterinary Research.
A tool for travelling back in time
To begin with, the team studied bacterial strains collected from 146 cattle and 21 badgers in the Landes and Pyrénées-Atlantiques départements between 2002 and 2017. By determining the genetic proximity of these strains, they were able to reconstruct a phylogenetic tree, a sort of inverted family tree, which traces the bacterium's evolution. They could then estimate the likelihood that the ancestors of these bacterial strains had circulated in badgers or cattle: "If two bacteria with similar sequences are isolated from cattle, then there is a high probability that their common ancestor also circulated in cattle, and the same is true for badgers," explains Laetitia Canini, epidemiologist and co-author of the study.
This reconstruction of the transmission dynamics showed that over the course of the evolution of the studied strains, the risk of badger-to-cattle infection was 52 times higher than the cattle-to-badger infection. Once the bacterium was transmitted from a badger to a bovine, its spread was amplified through transmission among other cattle.
Identification of the common ancestor of the bacterium
The scientists also determined that the strains of bacteria they studied were all likely to have originated from the same bacterium carried by a badger in the 1980s. "The bovine tuberculosis control system for cattle was set up in the 1960s. Since then, the disease's prevalence in cattle has declined sharply. It is possible that the bacterium continued to circulate in badgers in certain areas, because at the time there was no wildlife surveillance," recalls the scientist.
The plan now is to refine the results, and to include data from other regions and from wild boar samples. This is the objective of Hélène Duault's thesis: "We are going to trace the transmissions in more detail until we can determine 'who is passing it on to whom', i.e. which group of individuals (livestock or social group) within a species has transmitted the bacterium," she explains. This will make it possible, for example, to determine whether wild animals served as intermediate hosts to transmit the bacteria between two geographically separate herds. The results will help us to better adapt and target the surveillance and prevention measures in place for bovine tuberculosis.