Disease factsheet: Bovine Tuberculosis
If you suspect signs of any notifiable disease, you must immediately notify a Defra Divisional Veterinary Manager.
- Defra's Bovine Tuberculosis
homepage.
- Definition
- Animals affected
- Clinical signs
- Post Mortem
- Control
- Treatment
Definition
Bovine TB is a infectious and contagious disease of cattle caused by the bacterium Mycobacterium bovis (M.bovis). It is characterised by the development of tubercles in any organ of the body.
Animals affected
Although the main reservoir and natural host of M.bovis is cattle, human beings and a wide range of mammals are susceptible to the bacterium. Infection of farmed deer, farmed wild boar, goats, llamas, alpacas, pigs, dogs and cats has been reported in GB, but these appear to be spillover hosts. A number of wild animals (European badger, the brush-tailed possum, buffalo, bison and several species of wild deer) can act as reservoirs of M.bovis for cattle and other mammals and may pose major impediments to eradicating the disease in cattle.
Clinical signs
Aerosol exposure to M.bovis is considered to be the main route of infection of cattle, but infection by ingestion of contaminated material also occurs.
Bovine TB is usually an insidious and chronic (slowly progressing) inflammatory condition. Once an animal is infected with M.bovis, the rate of progress of infection is dependent on a number of factors including infectious dose, genotype and immune status of the host and stress.
The disease varies in severity and signs depending on the affected organ, but often signs in cattle only appear in advanced cases and, if so, are few and non-specific. This makes clinical cases of bovine TB, if present at all, rather difficult to spot by a lay person. The signs include weakness, loss of condition and appetite, swelling of various lymph nodes, persistent cough and respiratory distress. Udder involvement these days is rare, but can result in progressive hardening of the affected quarter and enlargement of the supramammary (top of the udder) lymph nodes. In such cases the organism can be demonstrated in milk samples.
Bovine TB infection is usually diagnosed in the live animal on the basis of the tuberculin test. As a result of effective control programmes introduced in the 1950s (see below), clinical cases of bovine TB in GB are very rare nowadays.
Post-mortem
After death, a preliminary diagnosis of TB can be made by the presence of typical lesions (granulomas or tubercles) in various organs and their associated lymph nodes. Generally, lesions are confined to the lymph nodes of the head and to the lungs and their associated lymph nodes. Occasionally, TB granulomas are also found in the liver, spleen, lymph nodes of the gut and the lining of the thoracic and abdominal cavities. TB granulomas start as microscopic lesions, but can develop into large nodules containing a thick, yellow, cheese-like pus (caseation), sometimes with a gritty texture (calcification). The presumptive diagnosis of TB can be confirmed by histopathological examination and microbiological culture at specialised laboratories.
In deer, lesions caused by M.bovis tend to be thin-walled abscesses containing pus with little caseation or calcification.
Control
M bovis is naturally transmissible between animals and people by aerosols from infected animals or their carcases and by consumption of unpasteurised milk and dairy products from infected cows. TB in humans caused by M.bovis constitutes a public health problem in developing countries. In GB the risk to human health is considered low due to vaccination at childhood, the introduction of pasteurisation of milk, routine meat inspection and the relatively low prevalence of M.bovis infection in the national herd.
Suspect clinical signs and lesions of TB in cattle are notifiable to the Divisional Veterinary Manager (DVM) of the State Veterinary Service (SVS) under the Animal Health Act 1981 (sections 32 and 34) and the Tuberculosis Orders of 1984 (the latter may soon be amended by new legislation). The TB Orders enact in GB the European Directive regarding health problems affecting intra-community trade in bovine animals and swine (64/432/EEC, as amended). Local Authorities are responsible for the enforcement of the TB orders.
TB in cattle has been subject in GB to a compulsory eradication programme since 1950. There are two main elements of this programme:
- routine free tuberculin skin testing of cattle herds (except beef fattening units) carried out by the State Veterinary Service (SVS) every 1 to 4 years depending on the local incidence of TB
- routine post-mortem meat inspection by the Meat Hygiene Service of all cattle carcases, followed by tracing back to the farm of origin of those with TB lesions
Under the TB control programme, cattle failing the tuberculin test (reactors) and those considered to have been direct contacts exposed to TB are compulsorily slaughtered by the SVS. The DVM restricts movements of cattle on and off infected herds by service of a notice (TB2), a copy of which is sent to the relevant Local Authority. For a dairy herd, this entails suspension of its officially TB free status. The Chief Environmental Health Officer of the local food authority must then ensure that milk from the herd is heat-treated as per regulation 9(10) of the Dairy Product (Hygiene) Regulations (1995). The DVM may also request assistance from the Local Authority Trading Standards officers when a herd owner refuses to present animals for tuberculin testing or surrender reactors for slaughter.
There is no compulsory tuberculosis testing regime for farmed deer although, as for cattle, TB is a notifiable disease in wild, park and farmed deer under the Tuberculosis (Deer) Order 1989. TB is not notifiable in any other species, although the SVS encourages reports of isolates from other farm stock, non-agricultural species and in-contact humans.
Treatment
This is not usually undertaken in livestock because of the chronic, contagious nature of the disease and its potential public health hazards. In GB the TB Orders specifically prohibit the treatment of cattle and deer for tuberculosis.
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Page last modified:
August 24, 2006