Bovine tuberculosis

Bovine tuberculosis (bTB) is a chronic, insidious disease of animals caused by a bacterium called Mycobacterium bovis (M. bovis), which is closely related to the bacteria that cause human and avian tuberculosis and Johne’s disease.

bTB is a notifiable, zoonotic disease which means we have measures set out in our legislation as to how we control it.

A national bTB control plan is necessary to comply with World Organisation for Animal Health (WOAH) regulations and to allow access to international trade.

The threat of a bTB breakdown as well as the identification of reactors on farm has a massive impact on the mental wellbeing of the farm team, including those carrying out the bTB test.

The measures used to control bTB put immense strain on a farm's ability to be financially and environmentally sustainable.

This disease can affect most mammals, often causing no clinical signs, but it can progress to a general state of illness, coughing and eventual death.

Due to control measures in the UK, clinical signs of bTB are rarely reported, but tens of thousands of cattle are slaughtered as reactors each year, primarily in England, Wales and Northern Ireland.

Scotland is recognised as Officially TB Free (OTF), according to the World Organisation of Animal Health definition.

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, including camelids, pet cats and dogs, badgers and deer.

The disease is contagious and spread by direct and indirect 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 can be infected by ingesting infected colostrum and milk.

Indirect contact with infected animals can be a risk in bTB spread. Shared feed, water and even grazing could be a source of bTB infection if contaminated by an infected animal, such as another bovine or badger.

As the course of the disease is slow, taking months or years to render an animal sick or even cause death, a bTB-infected animal can spread bTB to many other animals.

Therefore, the movement of undetected infected domestic animals and contact with infected wild animals are the major ways of spreading the disease.

M. bovis is not the major cause of human tuberculosis, which is caused by M. tuberculosis, but humans are susceptible to bTB.

bTB is no longer a major public health risk in the UK, due to our national control plan which includes:

• Routine surveillance testing to identification and removed infecte cattle
• Post-mortem examinations supervised by Official Veterinarians in the slaughterhouse
• Strict criteria of when animals can enter the food chain
• Pasteurisation of national milk supplies
• Human vaccination scheme

Humans can be infected by drinking raw (unpasteurised) milk from infected cattle or inhaling infective droplets, so those most at risk are individuals working closely with infected animals, for example farmers, veterinary surgeons and slaughterhouse staff.

All areas of the UK have risk-based surveillance testing, using the Single Intradermal Comparative Cervical Tuberculin (SICCT) test or skin test.

Once even a single reactor is found, herds are placed under movement restrictions and animals can only be removed under licence to slaughter either directly or via an approved market or finishing unit.

The skin test is very specific (99.98%); this means there is a very high probability that the test-positive animal is infected with bTB.

All animals classed as bTB skin test reactors are compulsorily slaughtered, and compensation is paid to the farmer.

All reactors are examined at slaughter for visible bTB lesions, usually located in the lungs, head and neck lymph nodes and occasionally in udder tissue.

In most breakdowns, samples are taken from reactors for further testing to understand the extent of the infection and to extract genetic information that could help identify the source of the bTB infection.

The majority of bTB reactors are classed as NVL (no visible lesions); this means the animal was infected with bTB, but it was identified before the lesion had time to develop to a size that was visible at slaughter.

This is not an indication that the animal was incorrectly identified as a reactor.

In some breakdowns, supplementary tests are carried out to help identify and remove infected animals not identified by the skin test.

In the UK this would be the interferon gamma test (the blood test). More sensitive tests are used to identify more infected animals; those that would test false negative on the skin test have a greater chance of being correctly identified as test positive. 

Disease eradication programmes 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.

All devolved governments of the UK have their own bTB eradication or control strategy.

England and Wales have seen a significant decline in the number of new herd breakdowns over the last 15–20 years. The ambition for England is to be Officially TB Free by 2038 and for Wales by 2041.

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