Presentation of the disease and ANSES's work
Published on 11/10/2019
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, but a few outbreaks persist in certain départements and are currently covered by specific surveillance and management measures, making it possible for this status to be maintained. Below you will find 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 caused by the bacterium Mycobacterium bovis (M. bovis). This bacterium can infect several domestic and wild species, primarily cattle and deer but also wild boars, badgers and foxes.
The infection is often latent and generally slowly progressing in cattle, with clinical symptoms only appearing at a late stage. However, despite presenting with no or few symptoms, infected animals can have reduced productivity.
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, the disease has been steadily on the rise since 2004.
The criteria determining disease-free status for bovine tuberculosis:
- The annual prevalence of infected herds remains below 0.1% for six years,
- The rate of officially tuberculosis-free herds has remained above 99.9% for six years as of 31 December of each year,
- Compliance with the European regulations (Directive 64/432/EEC) on tuberculosis.
How is the disease transmitted?
It is usually transmitted by the respiratory route, although the digestive tract is also a route of infection. Domestic cattle represent the 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.
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.
The eradication of bovine tuberculosis mainly relies on infection being brought under control on farms.
However, wild animals, in particular wild boars, badgers and deer, can also contract the infection. They can thus be spillover hosts of the disease, likely to contaminate livestock. Without a source of contamination from outside the population, spillover hosts are incapable of maintaining infection within their own population indefinitely, although they can transmit it to another population.
The discovery of the role of wildlife role in the disease’s transmission cycle has led to increased efforts to screen for the infection in wildlife in the vicinity of domestic outbreaks.
Therefore, as part of the Sylvatub surveillance network, wildlife surveillance is in progress to keep the disease from becoming entrenched in these populations and causing them to take on the role of maintenance hosts, as has been the case in other countries (for example, in Great Britain for badgers and in Spain for wild boars).
Learn more about the disease and how it is transmitted
ANSES's laboratories, in particular the Nancy Laboratory for Rabies and Wildlife and the Maisons-Alfort Laboratory for Animal Health (home to the National Reference Laboratory (NRL) for tuberculosis) are undertaking extensive research work on bovine tuberculosis. Among other things, the NRL is responsible for developing analytical methods dedicated to the surveillance of this disease and transferring these methods to the network of accredited laboratories that it coordinates across the country.
The main research work undertaken by the Agency aims to improve knowledge of M. bovis infection in domestic populations and in wildlife, in order to understand the networks of transmission between these two types of populations:
- role of various wildlife species in the maintenance of infection and its transmission, whether direct between animals or indirect through environmental contamination,
- modelling of interactions between various infected compartments, to improve disease control in animals,
- characterisation of Mycobacterium bovis strains and of possible differential virulence traits in affected regions.
ANSES is also involved in research work to develop an oral vaccine. Early trials for preventive vaccination, as a possible way of controlling infection in wildlife under certain conditions, are being set up in collaboration with foreign teams in the United Kingdom and Spain.
Assessing risks and surveillance & control measures
For the last few years, the Agency has also been conducting risk assessment work and providing regulatory support to the public authorities with regard to this disease. Since 2005, more than 20 opinions and reports have been published on the topic of bovine tuberculosis.
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 (in French); among other things, it addressed procedures for animal trapping and/or the destruction of badger setts (burrows). 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 of the available data, relating both to 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. Measures relating to badgers can therefore only be implemented over a defined period, in a specifically demarcated area that is determined on the basis of surveillance data and regularly reassessed based on surveillance results. Moreover, operations must be preceded by an inventory of badger setts in the area. These control measures only apply to at-risk areas in a few French départements, which accounted for less than 4% of metropolitan France in 2018. They do not threaten the badger species in France.
ANSES also confirmed that only coordinated actions can effectively bring bovine tuberculosis under control. These measures should target cattle in infected herds, wild animals and the surrounding environment. Lastly, ANSES's experts have reiterated their recommendations from 2011, which stress that the preventive culling of badgers and other wild species in disease-free areas is in no way justified in the fight against tuberculosis.
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