Mycobacterium Bovis

Also known as

Bovine tuberculosis

Description

Mycobacterium bovis is a slow-growing aerobic bacterium and the causative agent of tuberculosis in cattle (known as bovine TB). This bacterium is related to Mycobacterium tuberculosis that causes tuberculosis in humans. M. bovis can cross species and cause tuberculosis-like infection in humans and other mammals. Even though it primarily affects cattle, it can be transmitted to any warm-blooded animal.

Bovine tuberculosis is a chronic and slowly progressive infectious disease affecting humans, cattle, deer, llamas, pigs, domestic cats, wild carnivores (foxes, coyotes) and omnivores (common brushtail possum, mustelids—weasels, badgers, etc.--and rodents) but rarely affects equines or sheep.

The disease can be transmitted in multiple ways; it can be spread in exhaled air, saliva and sputum (mucous coughed up from the respiratory tract), urine, feces, and pus. Thus the disease can be transmitted by direct contact, contact with the excreta or discharges from an infected animal, or inhalation of aerosol droplets from the lungs.

Because M. bovis is excreted in milk and is a threat to human health, milk for human consumption is usually pasteurized (a process that effectively kills this pathogen). Most countries have undergone eradication programs to get rid of bovine tuberculosis, but it still emerges periodically, due to movement of infected animals into a country, or by contact with infected wildlife.

In the early 1900s, M. bovis caused an estimated 5 to 30% of all cases of human TB in the U.S. and United Kingdom. At one time it was a significant cause of human deaths in both countries. Most human infections were acquired by children through consumption of infected raw milk, causing in an intestinal or generalized form of TB as opposed to the respiratory form of the disease typical of human tuberculosis.

In the late 1890’s and early 1900’s, about 15,000 Americans, mostly children, died annually from bovine TB and many more suffered pain and disfigurement.

By 1917, 5% of U.S. cattle were infected with bovine tuberculosis, including 10% of dairy animals and up to 2% of range cattle and the infection rates were increasing.

Some states initiated control programs. For instance, when threatened by a milk sales cutoff ordered by urban public health officials, the Vermont state government launched an innovative eradication campaign against bovine tuberculosis on farms—a program that was in effect from 1877 to 1936.

Infections in humans are rare today in developed countries, mainly due to pasteurization of milk. In the UK, cattle are tested for the disease as part of an eradication program and culled if they test positive. The culled cattle can still enter the human food chain, but only after a meat inspector or a government veterinary surgeon has inspected the carcass and certified it is fit for human consumption.

In countries where milk pasteurization is not routine, M. bovis is a relatively common cause of human tuberculosis.

M. bovis is killed by sunlight and heat, but is resistant to drying. It can survive in a wide range of acids and alkalis and can also remain viable for long periods in moist and warm soil. In cattle feces, it will survive 1 to 8 weeks.

Incubation time ranges from months to years. Infected cattle usually don’t show outward signs of infection. When clinical signs are present, they are often vague, such as weight loss, depression, and sluggishness. Transmission of tuberculosis between animals occurs when susceptible animals are in close contact with respiratory secretions or aerosols from infected animals, such as in enclosed areas or barns with poor ventilation.

Close contact is necessary for transmission; the infection is not easily spread between cattle that are separated by any distance, but can be spread via feed or watering sites contaminated with saliva and other bodily discharges (urine, manure).

Various body systems can be affected, but signs are usually confined to the respiratory tract. A soft, chronic cough occurs sporadically. In advanced cases, there is noticeable increase in the depth and rate of respiration as well as difficult breathing.

When listening to the chest with a stethoscope there may be areas of dullness, and some animals’ breathing may squeak, whistle or have a snoring sound.

Mycobacterium bovis infection commonly involves the lungs, but may spread to other organs. Animals often don’t show signs until the infection has reached an advanced stage.

Cases of bovine tuberculosis are usually diagnosed on the basis of tell-tale abnormalities found on slaughter inspection. Tuberculosis in live cattle is identified using the caudal fold test (CFT), in which a small dose of tuberculin is injected into the skin around the tail and then checked 72 hours later to see if there is a skin reaction.

When infected animals are identified, state and federal officials trace them back to their herd of origin and work to identify other infected herds.

Bovine TB is difficult to diagnose with clinical signs alone. In the early stages of TB, clinical signs are not visible. In later stages, clinical signs may include emaciation, lethargy, weakness, anorexia, low-grade fever, and pneumonia with a chronic, moist cough. Lymph nodes may also be enlarged.

Signs

• Weight loss
• Off feed
• Low-grade fever
• Chronic cough

Cause

There are three strains of bacteria that cause tuberculosis in animals and humans.
Mycobacterium bovis affects cattle and many other species, including people and wildlife. Mycobacterium avium affects mostly poultry but can cross-infect cattle.
Mycobacterium tuberculosis affects humans only.

M. bovis can infect wild mammals. High rates of infection have been found in badgers in the UK; badgers have been a significant source of TB in cattle in southwest England.

Animals are probably more likely to be infected by M. bovis when poorly nourished or under stress. Growing heifers and younger cows are most at risk, and intensive dairy operations have a higher risk of infection.

M. bovis is spread infected animals via their breath, milk, discharging lesions, saliva, urine or feces. In cattle, excretion of M. bovis begins about 87 days after infection occurs. Entry is usually by inhalation (especially in close confinement with other cattle) or ingestion. Once in a herd, infection probably spreads from cow to cow by inhalation. Transmission from cows to their calves may occur via milk or colostrum.

M. bovis can be transmitted to wildlife, such as deer, but close contact is necessary. In Michigan, white-tailed deer are a major factor in the persistence of tuberculosis. Hunters in several states are encouraged to look for abscesses or other suspicious lesions in the deer they harvest, and to report unusual findings to the state game department.

The vast majority of human TB cases are caused by M. tuberculosis, not M. bovis. Humans, not cattle, are the most common source of human infection, but it is possible for cattle to become infected with M. tuberculosis from humans, and transmit the disease back to other humans, though such instances are rare.

Countries can eliminate the public health problem of M. bovis by making pasteurization of milk and fresh cheese compulsory, or eradicating bovine TB from cattle herds, or both.

Some countries try to control infected wildlife species that spread the disease to cattle. In New Zealand, the introduced common brushtail possum is responsible for spread of M. bovis. The Biosecurity Act of 1993 established a national pest-management strategy in New Zealand, with the Animal Health Board operating a nationwide program of cattle testing and possum control.

Their goal is to eradicateM. bovis from wild species across one-quarter of New Zealand’s at-risk areas by 2026 and, eventually, eradicating the disease entirely.

The New Zealand program successfully reduced cattle and deer herd infection rates from more than 1700 herds in 1994 to fewer than 100 herds in 2011. Possums are controlled through a combination of trapping, ground-baiting, and aerial poison.

In the 1930s, 40% of cattle in the UK were infected with M. bovis and 50,000 new cases of human M. bovis infection were reported every year. Badgers were identified as carriers in 1971. In the UK, many other mammals were also found to be infected, but frequency is generally less than in cattle and badgers.

In some areas of southwest England, fallow deer (due to their gregarious behavior), have been implicated as maintenance hosts for bovine TB and the risk of transmission to cattle from fallow deer is thought to be greater than from badgers.

One of the reasons the UK requires infected or suspected cattle to be culled is to meet EU regulations for export of meat and dairy products to other member states. Meat and dairy products can still be sold in the UK into the human food chain, if carcass inspections and milk pasteurization have been applied.

Bovine tuberculosis is probably a negligible public-health problem in the UK, if milk is pasteurized, since TB is rarely spread by aerosol from cattle to humans.

In the U.S., by 2013, the USDA stated that cattle herds were free of bovine TB in all states except Michigan (where the disease exists in deer) and California (where infected cattle sometimes come in from Mexico).

M. bovis is endemic in white-tailed deer in northeastern Michigan and northern Minnesota, and sporadically imported into the U.S. from Mexico. White-tailed deer have been confirmed as a maintenance host in the Michigan outbreak of bovine tuberculosis, although other mammals such as raccoons, possums, and coyotes can serve as spill-over and dead-end hosts.

The fact that white-tailed deer are a maintenance host for M.bovis remains a significant barrier to the U.S. eradication of the disease in livestock.

The disease is found in cattle throughout the world, but some countries have been able to reduce or limit incidence of the disease through a process of test and cull. Most of Europe and several Caribbean countries (including Cuba) are virtually free of M. bovis.

Australia is officially free of the disease after a successful control program, but residual infections may still exist in feral water buffalo in isolated parts of the Northern Territory.

In Canada, affected wild elk and white-tailed deer are found in and around Riding Mountain National Park in Manitoba. The Canadian Food Inspection Agency split Manitoba into two management areas: The Riding Mountain TB eradication area, where the disease has been found, and the Manitoba TB Eradication Area (the rest of the province where the disease has not been found).

The disease has also been found in African buffalo in South Africa. In Mexico, the disease is prevalent and rising among humans.

Infection of humans with M. bovis is referred to as zoonotic tuberculosis. The main route of transmission is through consumption of unpasteurized milk or other dairy products, although transmission via inhalation and consumption of poorly cooked meat has also been reported.

In 2018, based on the Global Tuberculosis Report, an estimated 142,000 new cases of zoonotic tuberculosis, and 12,500 deaths due to the disease occurred in Africa, the Americas, Europe, the Eastern Mediterranean, and the Western Pacific. Regions without adequate disease control measures and/or disease surveillance are at higher risk.

It is difficult to clinically distinguish zoonotic tuberculosis from tuberculosis caused by Mycobacterium tuberculosis in people, and the most commonly used diagnostics cannot effectively distinguish between M. bovis and M. tuberculosis, so there is probably an underestimation of total M. bovis cases worldwide.

In the U.S. the two most common methods of introduction of this disease into a cattle herd are purchase of or exposure to infected cattle, or exposure to infected wildlife.

Prevention

Cattle can be tested for TB. If possible, producers should keep a closed herd and raise their own replacement stock. Buy animals from an accredited TB-free herd, test new animals prior to purchase, and isolate them for 60 days and retest before putting them with your herd.

Restrict or eliminate all contact between your herd and other herds.     Disinfect any trailers or facilities that have housed newly purchased animals or animals that did not originate from your herd. Keep visitors away from your herd whenever possible. This includes milk haulers, feed delivery people, and anyone who may have contact with other herds.

Make sure fences are in good condition to keep wildlife away from cattle if wildlife in your area are infected with TB, and contact the USDA Wildlife Services office in your State for advice on reducing wildlife contact with your herd.

Most dairy operations have greater chance for exposure to bovine TB than beef ranches since dairy cows spend more time in enclosed areas or crowded conditions, where there is higher risk of exposure, but wildlife can spread infection to beef cattle at pasture.

Wildlife and cattle with TB can pass infection back and forth. In areas where free-ranging deer are known to be infected with TB, deer that have direct contact with cattle pose a risk, and livestock can become infected if they share common watering or feeding places contaminated with saliva and other bodily discharges from infected wildlife.

Biosecurity measures are important, keeping cattle from coming into contact with wildlife.

Cattle feed should be stored in areas where wildlife cannot access it. If wildlife get into stored feed, it should be discarded immediately, and not fed to cattle.

Treatment

Active Bovine TB can be treated with antibiotics for 6 to 9 months. Latent Bovine TB is also treated with antibiotics to reduce the risk of developing active disease, but most cattle with TB are culled rather than treated.