TB (Bovine Tuberculosis)

Tuberculosis (TB) in cattle is caused by the bacterium Mycobacterium bovis. 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 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 colostrum.

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.

MAFF is currently conducting a large-scale investigation into the relationship between M. bovis in badgers and in cattle.

Diagnostics

Diagnosing bovine tuberculosis is complicated, and a diagnostic gold standard that can detect all infected animals is not currently available. Conventional diagnostic tools (i.e., detection of antibodies or antigens) can be used only in the late stages of the disease. Consequently, the most widely used first bovine tuberculosis diagnostics are based on the cell-mediated immune response, which is determined by either skin or blood testing (IFN-y test).

Differences exist among bovine tuberculosis tests with respect to the time point and the sensitivity for detection of the disease. The IFN-y test (i.e., BOVIGAM assay) allows for the earliest detection, followed closely by the skin test. Serology tests for antibody response or antigen detection and pathological examinations can be used in later stages of the disease.

PCR is a reliable diagnostic tool for confirmation of the presence of mycobacteria belonging to the tuberculosis complex. Results using the PCR approach can be returned much faster when compared to bacterial culture testing methods. While the results of a M. bovis culture can take up to six weeks, results using PCR – from sample preparation to testing – take just three hours. The information is then delivered to the farmer or veterinarian in two to three days.

The choice of tests and their applications is dependent on both the risk of bovine tuberculosis infection in a region and the goal of a bovine tuberculosis program. Optimal TB programs enable sanitary decisions to be made sooner, increase the speed of a test and cull program and helps minimize the duration of farm closures.

（June 20, 2022. From “ The Cattle Site”)

• [cf7ic]