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.

This disease can affect practically all mammals, causing a general state of illness, coughing and eventual death. Bovine tuberculosis is an important disease of cattle and is a significant zoonosis (a disease of animls which can also infect humans).

Where is the disease found?

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

Many developed countries have reduced or eliminated bovine tuberculosis from their cattle population; however significant pockets of infection remain in wildlife in Canada, the United Kingdom, the United States and New Zealand. Although cattle are considered to be the true hosts of M. bovis, the disease has been reported in many other domesticated and non-domesticated animals.

How is the disease transmitted and spread?

The disease is contagious and spread by contact with infected domestic and wild animals.

The usual route of infection is by inhaling infected droplets which are expelled from the lungs by coughing. Calves and humans can also become infected by ingesting raw milk from infected cows.

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

Is there a public health risk?

Mycobacterium bovis is not the major cause of humantuberculosis, 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 infective droplets. It is estimated in some countries that up to ten percent of human tuberculosis is due to Bovine TB.

What are the clinical signs?

Bovine tuberculosis usually has a prolonged course, and symptoms take months or years to appear. The usual clinical signs include:

• weakness
• loss of appetite
• weight loss
• fluctuating fever
• intermittent hacking cough
• diarrhoea
• large prominent lymph nodes

How is the disease diagnosed?

The standard method for detection of TB is the tuberculin test, where a small amount of antigen is injected into the skin, and the immune reaction is measured. Definitive diagnosis is made by growing the bacteria in the laboratory, a process that takes at least eight weeks.

Prevention and control of the disease

The standard control measure applied to TB is test and slaughter.

Disease eradication programs consisting of post mortem meat inspection, 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.

Post mortem meat inspection of animals looks for tubercles in the lungs and lymph nodes. Detecting these 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.

Source: World Organisation for Animal Health