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Updated on 24/03/2021

Bovine tuberculosis, a disease monitored in both domestic and wild animals

Keywords : Bovine tuberculosis, Zoonosis, Animal diseases

Bovine tuberculosis is an animal disease transmissible to humans that primarily affects cattle herds. France has enjoyed disease-free status since 2001. A few outbreaks persist in certain départements but these are currently covered by specific surveillance and management measures, enabling this status to be maintained. Here is an overview of this disease, its situation in France and the role played by the Agency.

What is bovine tuberculosis?

Bovine tuberculosis is an infectious disease transmissible to humans (zoonosis) that is mainly caused by the bacterium Mycobacterium bovis (M. bovis). This bacterium can infect several domestic and wild species, primarily cattle and deer but also wild boar, badgers and foxes.
The infection is often latent and generally slowly progressing in cattle, with clinical symptoms only appearing at a late stage. The indirect losses caused by this disease have a major economic impact on the sector (inability to sell live animals, raw milk, semen, etc.).
Since 2001, France has been regarded as officially free of bovine tuberculosis by the European Union, despite the persistence each year of around a hundred outbreaks in livestock. In certain regions, especially Nouvelle Aquitaine, there has been a steady increase in the number of outbreaks since 2004. 

The criteria determining disease-free status for bovine tuberculosis

  • To be recognised as officially free of the disease, the annual prevalence of infected herds in the country must be less than 0.1% for six years.
  • To maintain this status, the level of officially disease-free herds must be above 99.9% on 31 December of each year, and the country must demonstrate effective surveillance abilities while complying with the European regulations on tuberculosis (Directive 64/432/EEC).

How is the disease transmitted?

Bovine tuberculosis is usually transmitted by the respiratory route, although the digestive tract is also a route of infection. Domestic cattle represent the main maintenance host species for M. bovis (except in rare cases), which means that the disease is first and foremost transmitted from infected to healthy cattle. Maintenance hosts are capable of maintaining infections between individuals from the same species in the same population, without any external source, and transmitting it to other receptive species.Eradication of bovine tuberculosis mainly relies on this infection being brought under control in livestock. 

In some cases, cattle can shed the bacterium and contaminate humans as well as other wildlife (deer, swine, some carnivores, etc.), whether directly or indirectly, since the bacterium can resist in the environment for several months under certain conditions.

Wild animals – particularly wild boar, badgers and deer – can therefore also become infected and could in turn infect livestock. However, apart from exceptional cases, these wild populations are considered to act as spillover hosts, unable to maintain the infection on a lasting basis without an external source of contamination, but capable of transmitting it to another domestic or wild population.

Recognition of the importance of wildlife in disease transmission has led to increased screening for infection in these populations around domestic outbreaks. Wildlife surveillance is carried out through the Sylvatub surveillance network, in order to implement control measures to keep the disease from becoming entrenched in these populations, and prevent certain animals acting as maintenance hosts, as has been seen in other countries (for example, badger populations in Great Britain and wild boar in certain parts of Spain).

ANSES's work on bovine tuberculosis

Learn more about the disease and how it is transmitted

ANSES conducts extensive research on bovine tuberculosis. This work, most of which is carried out with partner organisations, mainly takes place at the Nancy Laboratory for Rabies and Wildlife and the Maisons-Alfort Laboratory for Animal Health, which hosts the National Reference Laboratory for tuberculosis (NRL). Among other things, the NRL is responsible for developing analytical methods dedicated to the surveillance of this disease in different animal species, and transferring these methods to the network of approved laboratories for bovine tuberculosis detection across the country. 
The Agency's main research work is aimed at improving knowledge of M. bovis infection in domestic and wild animals, in order to understand the transmission networks between populations:

•    Studies of the roles of various wildlife species in the maintenance of infection and transmission, whether direct between animals or indirect through environmental contamination,
•    Modelling of interactions between the various infected species and their environment, to improve disease control in animals,
•    Characterisation of possible differences in virulence between Mycobacterium bovis strains in the affected regions. 

ANSES is also contributing to research on the development of an oral vaccine for badgers as a preventive measure to control wildlife infection under certain conditions. This work is being conducted in partnership with teams in the UK, Spain and the USA. 
 

Assessing risks and surveillance and control measures

For the last few years, the Agency has also been conducting risk assessment work and providing regulatory support to the public authorities on this disease. Since 2005, more than 20 opinions and reports on bovine tuberculosis have been published. The Agency's expert appraisal work has included a report published in 2011 on bovine tuberculosis and wildlife, in which it issued recommendations for managing wild animals infected with bovine tuberculosis; among other things, it addressed procedures for animal trapping and/or the destruction of badger setts.

In 2019, the Agency received a request from four nature conservation associations regarding the management of badger populations in the fight against bovine tuberculosis. After analysing all the available data, on badger populations and their health status, the Agency underlined that current laws closely regulate surveillance and control measures leading to the culling of wild animals, according to the area and level of infection. Control measures relating to badgers can therefore only be implemented in a specifically demarcated area determined on the basis of surveillance data, over a defined period, and are regularly reassessed based on surveillance results. These control measures only apply to at-risk areas in a few French départements, which account for less than 4% of metropolitan France. These measures do not threaten the badger species in France as a whole.  

Following a formal request from the DGAL, ANSES issued an opinion in February 2021 on the role of foxes in the transmission of bovine tuberculosis. The Agency concluded that this animal species can indeed contribute to transmission of the disease, but that due to its lifestyle and rapid population turnover, its role is probably less important than that of other wild species, such as badgers or wild boar. Management measures should be taken on a case-by-case basis and locally, in the immediate vicinity of contaminated farms. The opinion stated that the preventive culling of foxes, or badgers or other wildlife species, cannot be justified under any circumstances on the grounds of tuberculosis control.