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Home > Animal health in the World > Information on aquatic and terrestrial animal diseases

KEY FACTS

  • In 1881 Robert Koch discovered the tubercle bacillus as the cause of tuberculosis, and in 1898 M. bovis was identified.
  • During 2015 to 2016, 179 countries and territories reported their status with regards to bovine TB to OIE. Almost half of these countries reported the presence of the disease in animals.
  • Globally in 2014, only 12% of 2·7 million new bacteriologically confirmed tuberculosis cases were tested for drug resistance.
  • Although the infection has been controlled in most developed countries, the complete elimination is complicated by persistent infection of wild animals, such as badgers in the United Kingdom, white tailed deer in parts of the United States of America and brush-tail possum in New Zealand.
  • Bovine TB remains a serious problem for animal and human health in many developing countries.

 

 

What is bovine tuberculosis?

Bovine tuberculosis (TB) is a chronic disease of animals caused by a bacteria called Mycobacterium bovis (M. bovis) which is closely related to the bacteria that cause human and avian tuberculosis. It is a major infectious disease among cattle, other domesticated animals, and certain wildlife populations, causing a general state of illness, coughing and eventual death.

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

Cattle are considered to be the major reservoir M. bovis. Nevertheless, the disease has been reported in many other domesticated and non-domesticated animals. Isolations have been made from buffaloes, 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.

TB is an OIE-listed disease and must be reported to the OIE 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.”

 

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 is by inhaling infected aerosol, which are expelled from the lungs by coughing, but infection by through breaks in the skin also occurs. Calves and humans can also become infected by ingesting raw milk from infected cows.

The course of disease is slow and takes months or years to kill an infected animal. Consequently, an animal can spread the disease to many other herd mates before it begins to show clinical signs. Therefore, movement of undetected infected domestic animals and contact with infected wild animals are the major ways of spreading the disease.

 

Public health risk

Mycobacterium bovis is not the major cause of human tuberculosis, which is caused by M. tuberculosis, but humans are susceptible to bovine TB. Humans can be infected both by drinking raw milk from infected cattle, or by inhaling infected aerosols. It is estimated in some countries that up to 10% of human tuberculosis is due to Bovine TB.

The OIE Terrestrial Animal Health Code defines the recommendations, which are intended to manage the human and animal health risks associated with infection of animals with a member of the Mycobacterium tuberculosis (M. tuberculosis) complex, including M. bovis.

 

Clinical signs

TB 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. However, the bacteria can also lie dormant in the host without causing disease.

The usual clinical signs include:

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

 

Diagnostic

TB clinical signs are not specifically distinctive and do not allow to establish a diagnosis with certainty.

The standard method for detection of TB is the tuberculin test, where a small amount of TB protein (antigens) is injected intradermally, and the immune reaction is evaluated by measuring the skin thickness after 72 hours. Definitive diagnosis is made by growing the bacteria in the laboratory, a process that takes at least eight weeks.

At present, there are several quick and reliable diagnostic methods of the bovine tuberculosis, detailed in the guidelines for tuberculin manufacture and culture of M. bovis are detailed in the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.

The tuberculin test (a delayed hypersensitivity test) is the standard method of TB diagnosis in domestic animals. It consists in injecting bovine tuberculin intradermally and then measuring skin thickness of the site of injection 72 hours later to detect any subsequent swelling (sign of delayed hypersensitivity).
This test is prescribed for international trade.

 

Prevention and control

Test and slaughter is only control measure that guarantees the eradication of the disease from domestic animals. Meanwhile, it remains not practical in most of countries regarding that it can be used only in the final stages of eradication. Therefore, some countries use varying forms of test and segregation in early stages and then switch to test-and-slaughter methods in the final stage.

Disease eradication programmes consisting of post mortem meat inspection (looking for tubercles in the lungs, lymph nodes, intestines, liver, spleen, pleura, and peritoneum), intensive surveillance including on-farm visits, systematic individual testing of cattle and removal of infected and in-contact animals as well as movement controls have been very successful in reducing or eliminating the disease.

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 of milk of infected animals to a temperature sufficient to kill the bacteria has prevented the spread of disease in humans.

Treatment of infected animals is rarely attempted because of the high cost, lengthy time and the larger goal of eliminating the disease.

Vaccination is practiced in human medicine, but it is not widely used as a preventive measure in animals.

 

Roadmap for zoonotic tuberculosis

Zoonotic Tuberculosis is a form of tuberculosis affecting humans. It is mostly caused by Mycobacterium bovis, which belongs to the M. tuberculosis complex as Mycobacterium tuberculosis which affects only humans. It is a major cause of deaths worldwide. It is indirectly transmitted, through the consumption of contaminated milk and other dairy products.

The OIE, 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 collabortive approaches

Zoonotic TB Roadmap

Zoonotic TB factsheet

Zoonotic TB in human beings
caused by M. bovis - a call for
action (The Lancet)

 

Geographical distribution

TB is found throughout the world. The disease is more prevalent in most of Africa, parts of Asia and of the Americas.

TB never occurred in several countries 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.

 

 

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