Listed Disease

Bovine tuberculosis

  • Bovine tuberculosis is a chronic bacterial disease of animals caused by members of the Mycobacterium tuberculosis complex, primarily by M. bovis. It is a major zoonotic disease, and cattle are the main source of infection for humans. It also affects other domesticated animals such as sheep, goats, equines, pigs, dogs and cats, and wildlife species such as wild boars, deer, and antelopes. The name ‘tuberculosis’ comes from the nodules called ‘tubercles’, which form in the lymph nodes and other tissues of affected animals and humans. Although the infection in cattle herds has been controlled in most countries, complete elimination of the disease is complicated by persistent infection of wild animals, such as European badgers in the United Kingdom, white tailed deer in parts of the United States of America and brushtail possums in New Zealand. Bovine tuberculosis remains a serious problem for animal and human health in many developing countries.

Links to Code and Manual

What is bovine tuberculosis?

Bovine tuberculosis (bTB) is a chronic bacterial disease of animals caused by members of the Mycobacterium tuberculosis complex primarily by M. bovis, but also by M. caprae and to a lesser extent M. tuberculosis.  It is a major infectious disease among cattle, and also affects other domesticated animals and certain wildlife populations, causing a general state of illness, pneumonia, weight loss, and eventual death.

The name Tuberculosis comes from the nodules, called ‘tubercles’, which form in the lymph nodes and other affected tissues of affected animals.

Cattle are considered to be the major reservoir M. bovis, and are the main source of infection for humans. Nevertheless, the disease has been reported in many other domesticated and non-domesticated animals.

Mycobacterium bovis has been isolated from numerous wildlife species, including African buffalo, domestic Asian buffalo, bison, sheep, goats, equines, camels, pigs, wild boars, deer, antelopes, dogs, cats, foxes, mink, badgers, ferrets, rats, primates, llamas, kudus, elands, tapirs, elks, elephants, sitatungas, oryxes, addaxes, rhinoceroses, possums, ground squirrels, otters, seals, hares, moles, raccoons, coyotes and several predatory felines including lions, tigers, leopards and lynx.

Bovine tuberculosis is an WOAH-listed disease and must be reported to the WOAH as indicated in its Terrestrial Animal Health Code.

‘Most cases of human TB are caused by the bacterial species, Mycobacterium tuberculosis. Zoonotic TB is a form of TB in people predominantly caused by a closely related species, M. bovis, which belongs to the M. tuberculosis complex.‘

Geographical distribution

Bovine tuberculosis is found throughout the world, but some countries have never detected TB, and many developed countries have reduced or eliminated bovine TB from their cattle population and kept the disease limited to one or more zones. However, significant pockets of infection remain in wildlife. The highest prevalence of bovine tuberculosis is in Africa and parts of Asia, but the disease is also found in countries in Europe and the Americas.

Transmission and spread

The disease is contagious and can be transmitted directly by contact with infected domestic and wild animals or indirectly by ingestion of contaminated material.

The usual route of infection within cattle herds is by inhalation of infected aerosol, which are expelled from the lungs (by coughing). Calves can be infected by ingesting colostrum or milk from infected cows.

Humans can become infected by ingesting raw milk from infected cows, or through contact with infected tissues at abattoirs or butcheries.

The course of disease is slow and takes months or years to reach the fatal stage. Consequently, an infected animal can shed the bacteria within the herd before the appearance of clinical signs. Therefore, movement of undetected infected domestic animals is a major way of spreading the disease..

Clinical signs

Bovine tuberculosis may be subacute or chronic, with a variable rate of progression. A small number of animals may become severely affected within a few months of infection, while others may take several years to develop clinical signs. The bacteria can also lie dormant in the host without causing disease for a long periods.

The usual clinical signs include:

  • weakness
  • loss of appetite and weight
  • fluctuating fever
  • dyspnoea and intermittent hacking cough
  • signs of low-grade pneumonia
  • diarrhoea
  • enlarged, prominent lymph nodes.

Diagnosis

Bovine TB clinical signs are not specifically distinctive and, therefore, do not enable veterinarians to make a definitive diagnosis based on clinical signs alone.

The tuberculin skin test is the standard method of TB diagnosis in live domestic animals. It consists of injecting bovine tuberculin (a purified protein extract derived from M. bovis) intradermally and then measuring skin thickness at the site of injection 72 hours later to detect any subsequent swelling at the injection site (sign of delayed hypersensitivity associated with infection).

Blood-based in vitro tests that detect bacteria, antibodies, or cell-mediated immunity are also currently available, or under development. The most widely used blood-based test is a gamma interferon release assay which detects a cell-mediated immune response to infection with M. bovis. This test is based on the principle that bovine blood cells that have previously been exposed to M. bovis through an infection are known to produce elevated levels of gamma interferon following in vitro incubation with M. bovis antigens.

Meanwhile, the definitive diagnosis is confirmed by bacterial culture and identification in the laboratory, a process that can take eight weeks or more.

The recommended diagnostic methods, including the procedures for manufacturing and administering bovine tuberculin, are described in the WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.

Public health risk

The most common form of TB in people is caused by M. tuberculosis. However, it is not possible to clinically differentiate infections caused by M. tuberculosis from those caused by M. bovis, which is estimated to account for up to 10% of human tuberculosis cases in some countries. Diagnosis may be further complicated by the tendency of M. bovis infections to be located in tissues other than the lungs (i.e. extrapulmonary infection) and the fact that M. bovis is naturally resistant to one of the antimicrobials that is commonly used to treat human tuberculosis, pyrazinamide.

The WOAH Terrestrial Animal Health Code and WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals provide technical standards and recommendations  that are intended to manage the human and animal health risks associated with infection of animals with a member of the Mycobacterium tuberculosis complex, including M. bovis.

Roadmap for zoonotic tuberculosis

Human tuberculosis is a major cause of illness and mortality worldwide. It is primarily caused by M. tuberculosis and is usually transmitted through the respiratory route by close contact and inhalation of infected aerosols.  Zoonotic tuberculosis is a less common form of human tuberculosis that is caused by a related member of the Mycobacterium tuberculosis complex (M. bovis). The zoonotic form is primarily transmitted indirectly, through the consumption of contaminated milk, dairy products, or meat containing infected material. In regions where food hygiene is consistently applied, the risk to the general public has been reduced, however zoonotic tuberculosis infection remains an occupational hazard for farmers, abattoir workers, and butchers.

WOAH, the World Health Organization (WHO), the Food and Agriculture Organization of the UN (FAO) and the International Union Against Tuberculosis and Lung Disease (The Union) jointly launched the first-ever roadmap to tackle zoonotic TB in October 2017. It is based on a One Health approach recognising the interdependence of human and animal health sectors for addressing the major health and economic impacts of this disease.

This roadmap calls for concerted action from government agencies, donors, academia, non-governmental organizations and private stakeholders across political, financial and technical levels. It defines ten priorities for tackling zoonotic TB in people and bovine TB in animals. These fall under three core themes:

  • Improve the scientific evidence base
  • Reduce transmission at the animal-human interface
  • Strengthen intersectoral and collaborative approaches

Prevention and control

National control and eradication programs based on test and slaughter of infected animals have been successfully implemented in many countries, as the preferred approach to managing bovine tuberculosis. However, this approach remains impractical in some heavily infected countries because it could necessitate slaughtering large numbers of cattle, and this may not be feasible, due to human resource or financial limitations within the animal health program, or for cultural reasons. Therefore, countries use varying forms of test and segregation in early stages, and then switch to test-and-slaughter methods in the final stage.

Several disease eradication programmes have been very successful in reducing or eliminating the disease in cattle, by employing a multi-faceted approach that includes:

  • post mortem meat inspection (looking for tubercles in the lungs, lymph nodes, intestines, liver, spleen, pleura, and peritoneum), for detection of infected animals and herds
  • intensive surveillance including on-farm visits
  • systematic individual testing of cattle
  • removal of infected and in-contact animals
  • adequate local legislation
  • effective movement controls
  • individual animal identification
  • effective traceability.

Detecting infected animals prevents unsafe meat from entering the food chain and allows Veterinary Services to trace-back to the herd of origin of the infected animal which can then be tested and eliminated if needed.

Pasteurisation or heat treatment of milk from potentially infected animals to a temperature sufficient to kill the bacteria has proven effective for preventing the spread of disease to humans. 

Antimicrobial treatment of infected animals is rarely attempted because of the doses and duration of treatment that would be required, high cost of medications, and interference with the primary goal of eliminating the disease, and potential risk of developing resistance.

Vaccination is practiced in human medicine, but it is, so far, not used as a preventive measure in animals, due to the lack of availability of safe and effective vaccines, and potential interference with bovine tuberculosis surveillance and diagnostic tests, due to false positive reactions in vaccinated animals. Researchers are actively investigating potential new or improved bovine tuberculosis vaccines and alternate routes of vaccine delivery for use in domestic animals and wildlife reservoirs, as well as new diagnostic tests to reliably differentiate vaccinated animals from infected animals.

Responsible: Antimicrobial Resistance and Veterinary Products Department

References