Tuberculosis (TB) in cattle is caused by the bacterium Mycobacterium bovis. The disease incidence is increasing and is highest in the southwest of England (Clifton-Hadley, 1993). 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 (Andrews, 1992). 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 (Proud and Davis, 1998) and the consensus of scientific opinion is that badgers are a significant source of TB in cattle (Clifton-Hadley et al., 1995; Denny and Wilesmith, 1999; Eves, 1999; Martin et al., 1997; Martin et al., 1998). However, there appears to be a relationship between the type of landscape (e.g. southwest England) and the risk posed by badgers (White et al., 1993). M. bovis also infects people (Chalmers et al., 1996; Hardie and Watson, 1992) 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 (Griffin et al., 1996). There is evidence that more intensive dairy farms also have a higher risk of infection (Griffin et al., 1993).
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 (Neill et al., 1991). 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 (Costello et al., 1998). Spread from cows to calves may occur via the milk or colostrum (Evangelista and Anda, 1996).
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 (Andrews, 1992; Cassidy et al., 1999).
Further informationFurther information on bovine tuberculosis (TB) in cattle can be found by following the links below. This information references cattle health and welfare in organic cattle production systems and is part of a compendium on animal health and welfare in organic livestock production provided by the University of Reading.
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