TB (Bovine Tuberculosis)

Tuberculosis (TB) in cattle is caused by the bacterium Mycobacterium bovis. The disease incidence is increasing and is highest in the southwest of England. M. bovis is killed by sunlight, but is resistant to desiccation and can survive in a wide range of acids and alkalis. It is also able to remain viable for long periods in moist and warm soil. In cattle faeces it will survive 1 – 8 weeks. Bovine tuberculosis is a zoonotic disease and causes tuberculosis in human. The disease can be transmitted in raw milk but pasteurisation effectively prevents the spread via milk.

M. bovis has been found in several wild mammal species. High rates of infection have been found in badgers and the consensus of scientific opinion is that badgers are a significant source of TB in cattle. However, there appears to be a relationship between the type of landscape (e.g. southwest England) and the risk posed by badgers. M. bovis also infects people and was in the past a major cause of death in humans in the United Kingdom.

Animals are probably more likely to be infected by M. bovis when they are poorly nourished or under stress. Growing heifers and younger cows are most at risk. There is evidence that more intensive dairy farms also have a higher risk of infection.

M. bovis is spread in a number of ways by infectious animals – in their breath, milk, discharging lesions, saliva, urine or droppings. In cattle, excretion of M. bovis begins around 87 days after infection occurs. Entry is usually by inhalation (especially if housed) or ingestion (when badgers are the source of infection). Once in a herd, infection probably spreads from cow to cow by inhalation. Spread from cows to calves may occur via the milk or colostrums.

Various body systems can be affected, but signs are usually confined to the respiratory tract. A soft, chronic cough occurs once or twice at a time. In more advanced cases, there is a marked increase in the depth and rate of respiration as well as dyspnoea. Areas of dullness can be heard in the chest on auscultation or percussion. Some cases may squeak, whistle or have a snoring respiration.


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