Bovine Tuberculosis (bTB) is a chronic infectious disease that presents one of the most complex animal health challenges in countries like the United Kingdom and Ireland. The Eurasian badger acts as a significant wildlife reservoir for the infection, confirming that badgers carry and spread the disease to cattle. This situation establishes a persistent wildlife-livestock interface issue, where the disease cycles between domestic animals and wild populations, making eradication exceptionally difficult. Understanding the specific nature of the pathogen and its transmission pathways is fundamental to developing effective management strategies.
The Specifics of Bovine Tuberculosis
Bovine Tuberculosis is caused by the bacterium Mycobacterium bovis, which is closely related to the agent that causes most human tuberculosis, M. tuberculosis. This slow-growing, aerobic bacterium can infect a wide range of mammalian species, including badgers, deer, goats, and pigs. In badgers, the disease often follows a slow, chronic course, meaning that many infected animals can appear outwardly healthy for long periods.
As the infection progresses, lesions typically develop, most frequently in the lungs and associated lymph nodes. Infected badgers begin to excrete the bacteria through various routes, including sputum, feces, and urine. This shedding of M. bovis into the environment is the mechanism that facilitates transmission to cattle.
Transmission Pathways Between Wildlife and Livestock
The movement of M. bovis from badgers to cattle primarily occurs through indirect contact with contaminated environmental material. Badgers excrete the bacteria in their bodily fluids, which then pollutes the shared environment, particularly pastureland and farm buildings. Cattle become infected when they ingest or inhale these infectious particles.
A major contamination route involves badger latrine sites, which are communal areas where badgers defecate. If these latrines are located in grazing fields, the cattle can ingest the bacteria while feeding on contaminated grass. Contamination also occurs when infected badgers access farm resources, such as by drinking from shared water troughs or eating from feed stores.
Studies using whole-genome sequencing of the bacteria have confirmed that the same specific strains of M. bovis are often present in local badger and cattle populations. Genomic analyses suggest that while transmission occurs in both directions, the rate of transmission from badgers to cattle is approximately twice as high as the reverse. However, the most frequent path for new infections is within species, meaning cattle-to-cattle transmission is also a major factor in outbreaks.
Strategies for Disease Management
Interventions to control bTB focus on three distinct areas: wildlife, livestock, and farm biosecurity.
Wildlife Management
Wildlife interventions have historically centered on badger culling, which has been part of control programs in both the UK and Ireland for decades. The Randomised Badger Culling Trial (RBCT) in Britain showed evidence that culling can reduce the incidence of bTB in cattle herds in the cull area. However, culling is highly controversial and can sometimes lead to a temporary increase in infection rates in surrounding areas due to the social disruption of badger groups, known as the perturbation effect.
An alternative approach for managing the wildlife reservoir is badger vaccination, now progressively implemented in both the UK and Ireland. Vaccination programs use the Bacillus Calmette-Guérin (BCG) vaccine. This vaccine does not prevent infection entirely but can reduce the severity of the disease and the amount of bacteria shed by badgers. Some small-scale studies have shown promising results, with the percentage of infected badgers falling significantly in vaccinated areas. The long-term, large-scale effectiveness of vaccination compared to culling remains a subject of ongoing research and debate.
Livestock Control
Livestock interventions are a mandatory part of the disease control strategy, relying heavily on rigorous testing and removal. Cattle herds are routinely tested using the tuberculin skin test. Any animal that tests positive is compulsorily slaughtered to prevent further spread. This measure is paired with strict movement controls, which restrict the transport of cattle from affected farms to limit the geographic spread of the disease. Development of an effective cattle vaccine is also a high priority, with research being accelerated to create a tool that could be used alongside the current test-and-slaughter regime.
Farm Biosecurity
Biosecurity measures on farms are implemented to physically separate cattle from badgers and their excretions. Farmers are encouraged to secure feed stores, mineral licks, and water sources to prevent contamination by badgers. Exclusion measures, such as badger-proof fencing around farm buildings and pastures, are used to minimize the opportunity for contact with infected wildlife. These practical steps are effective in reducing the risk of transmission at the farm level.
Implications for Human and Animal Health
The persistence of bTB has profound consequences for both the agricultural economy and the well-being of farmers. The economic costs associated with the disease are substantial, exceeding £100 million annually in the UK for testing, compensation, and eradication programs. Farms that experience an outbreak face significant financial losses from the compulsory slaughter of cattle, reduced productivity, and the disruption caused by prolonged movement restrictions. In addition to the financial burden, the emotional toll on farming families is considerable, with outbreaks causing severe stress and mental health issues for those who must cull their herds.
From a human health perspective, M. bovis is a zoonotic pathogen, meaning it can be transmitted to people. Historically, the main route of human infection was through the consumption of raw, unpasteurized milk from infected cows. In developed countries, the risk of contracting M. bovis is now very low. Routine milk pasteurization effectively kills the bacterium, and meat products are inspected. The few cases diagnosed in humans typically involve occupational exposure, such as among farmers or abattoir workers.