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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. Control of bovine Tb (bTB) is important, not only because it is a potential zoonosis, but also because of the severe economic effects of slaughter and movement restrictions.

Bovine tuberculosis results from infection by Mycobacterium bovis, a Gram positive, acid-fast bacterium in the Mycobacterium tuberculosis complex of the family Mycobacteriaceae.

 

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 and may survive in drinking water for up to 18 days.

 

Cattle are the primary hosts for M. bovis, but other domesticated and wild mammals can also be infected. Known maintenance hosts include brush–tailed opossums (and possibly ferrets) in New Zealand, badgers in the United Kingdom and Ireland, bison and elk in Canada, and kudu and African buffalo in southern Africa. White-tailed deer in the United States (Michigan) have been classified as maintenance hosts; however, some authors now believe they may be spillover hosts. Species reported to be spillover hosts include sheep, goats, horses, pigs, dogs, cats, ferrets, camels, llamas, many species of wild ruminants including deer and elk; elephants, rhinoceroses, foxes, coyotes, mink, primates, opossums, otters, seals, sea lions, hares, raccoons, bears, warthogs, large cats (including lions, tigers, leopards, cheetahs and lynx) and several species of rodents. Most mammals may be susceptible.

 

M. bovis can infect people and was in the past a major cause of death in humans mainly through the consumption of raw, unpasteurised milk. Currently, M. tuberculosis causes the majority of the human cases of TB reported, which disease is acquired directly from an infected person.

 

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. The strongest factors associated with increased bTB breakdown risk are movement of cattle onto farms from markets or sales, operating a farm over multiple premises and the use of close contact housing.

 

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 90 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.

Tuberculosis is usually a chronic debilitating disease in cattle, but it can occasionally be acute and rapidly progressive. Early infections are often asymptomatic. In countries with eradication programs, most infected cattle are identified early and symptomatic infections are uncommon.

 

In the late stages, common symptoms include progressive emaciation, a low–grade fluctuating fever, weakness and inappetence. Animals with pulmonary involvement usually have a moist cough that is worse in the morning, during cold weather or exercise, and may have dyspnea or tachypnea. In the terminal stages, animals may become extremely emaciated and develop acute respiratory distress. In some animals, the retropharyngeal or other lymph nodes enlarge and may rupture and drain. Greatly enlarged lymph nodes can also obstruct blood vessels, airways, or the digestive tract.

 

If the digestive tract is involved, intermittent diarrhea and constipation may be seen. In cervids, bovine tuberculosis may be a subacute or chronic disease, and the rate of progression is variable. In some animals, the only symptom may be abscesses of unknown origin in isolated lymph nodes, and symptoms may not develop for several years. In other cases, the disease may be disseminated, with a rapid, fulminating course.

BTB is a notifiable disease and any suspect cases should be notified to the local State Veterinary Service office.

 

Cattle can be tested for tuberculosis using the comparative intradermal tuberculin skin test, with testing frequency determined by the risk of disease in the local area, ranging between yearly to 4 yearly frequency. Animals which react to the test are often compulsorily slaughtered, and restrictions are placed so that animals may not be moved on or off the affected farm (except direct to slaughter under license) until further tests show that the herd is clear.

 

As the disease is zoonotic (can spread to humans), the use of raw milk on an affected farm is also restricted. The sale of raw (green top) milk and the production of any milk products from unpasteurised milk on the affected farm or using milk from the farm is prohibited until the farm is officially cleared of infection.

 

A closed herd system and breeding own replacements should help to keep the disease away from the farm. Bought-in animals should be quarantined away from the main herd as part of the herd’s normal Biosecurity control measures.

 

If possible, avoid common grazing. If common grazing is inevitable then skin tests should be synchronised on all farms using the common grazing.

 

Keep wildlife out of buildings, especially feed stores.

Vaccination against TB in cattle is possible.

 

The National Farmers Union in the UK has a whole section of its website dedicated to Bovine TB with excellent information.