Further information on bovine tuberculosis.
Bovine tuberculosis (bTB) is caused by Mycobacterium bovis, and the disease is characterised by the development of "tuberculous" lesions in any organ of the body. The contents of this page give information on the animals that can be affected by bTB, what to look out for in terms of clinical signs, and ways to prevent its introduction and spread.
Contents
Animals Affected
All mammals are susceptible to bTB. Cattle are the most commonly affected species, however, farmed deer, wild boar, goats, llamas, alpacas, pigs, dogs, cats and humans can all be infected and transmit the disease. A number of wild mammals, not all indigenous to the UK, such as badgers, deer, buffalo and bison can also act as reservoirs for the disease.
The wild species found to have the highest rate of infection so far is the badger. Badgers may be a significant source of bTB infection for cattle, however, there is no evidence yet to suggest that badgers are a reservoir for the disease in Scotland. In parts of GB where there is confirmed bTB in the local badger population, there are many other factors which might influence whether or not cattle become infected.
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Clinical Signs
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 the infectious dose and the immune status of the host.
The signs and severity of the disease vary depending on the body system most affected, but clinical signs of disease in cattle often only appear in advanced cases and may be non-specific. This makes clinical cases of bTB difficult to spot. The signs include weakness, loss of condition & appetite, swelling of various lymph nodes, persistent cough and respiratory distress. Udder involvement these days is rare, but the disease 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 detected in milk samples.
Humans and animals with TB develop an immune response which can be detected by the Tuberculin skin test. Tuberculin is a sterile laboratory product made by growing TB bacteria, killing them with heat, removing them from the substance on which they were grown, then properly diluting & preserving the remaining mixture. About 72 hours after tuberculin is injected into the animals affected with TB, a characteristic swelling reaction appears at the point of injection. This reaction is a positive result, indicating exposure to one type of Mycobacteria.
A variety of diagnostic methods are used to help confirm the presence of bovine TB. These may include the comparative cervical tuberculin test, serological tests, post-mortem examinations and other laboratory procedures are used.
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Control Methods
Bovine TB can be transmitted from animals to humans and vice versa. Although young animals and humans can contract the disease by drinking raw milk, the most common means of transmission is through inhalation. Invisible droplets (aerosols) containing bTB bacteria may be exhaled or coughed out by infected animals and then inhaled by susceptible animals. The risk of exposure is greatest in enclosed areas, such as barns. Livestock are also likely to infect each other when they share a common watering place contaminated with saliva and other discharges from infected animals.
Animals are probably more likely to be infected by M.bovis when they are poorly nourished or under stress. M.bovis can survive in the environment for several months, especially in moist, mild places, like ill-ventilated cattle sheds.
bTB spreads between cattle slowly compared to some infections, but it will spread. That is why the government regularly tests cattle for bTB using the skin test, with testing intervals from 1-4 years depending on the local incidence of disease. All of Scotland is currently on a four yearly testing regime. The Meat Hygiene Service also routinely inspects the meat of all cattle carcasses, and the State Veterinary Service (SVS) traces back to the farm of origin those with any tuberculous lesions.
bTB is a notifiable disease and any suspicious clinical signs or lesions should be notified to the local DVM. Herds with reactor cattle are placed under movement restrictions and reactor cattle are removed and compulsorily slaughtered, as are those considered to have been direct contacts. The DVM will serve a notice (TB 2) restricting the movements of cattle on and off the infected herd and will copy the notice to the Local Authority. 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.
Bovine tuberculosis seriously disrupts farm businesses, and can be very expensive. Even farmers in areas with little history of bTB breakdowns should therefore consider how they can keep bTB out of their herds.
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. The SVS encourages reports of isolates from other farm stock, non-agricultural species and in-contact humans.
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