Bovine Tuberculosis Facts for Manitoba Livestock Producers

Bovine Tuberculosis (TB) is a contagious disease, which can affect most warm-blooded animals, including man. Cattle, goats and pigs are the domestic livestock most susceptible to infection, while horses are relatively resistant. The disease is prevented and controlled in most developed countries by regulations, due to its ability to infect humans and cause significant livestock production losses. For a brief description of the disease in livestock and what happens in Canada when it is diagnosed, please see the following: - 

Bovine TB is caused by the bacteria Mycobacterium bovis, and it probably has been causing disease in cattle even before they were domesticated. Another bacteria within this family, Mycobacterium tuberculosis is the cause of what is commonly called human tuberculosis. M. tuberculosis is a species more adapted to humans and may have initially developed from a strain of M. bovis, many thousands of years ago, that humans were exposed to after they started herding cattle.

The regulatory efforts for controlling TB in Canadian domestic livestock are under the federal jurisdiction of the Canadian Food Inspection Agency (CFIA). It is one of the "Reportable" diseases under their Health of Animals Act, making it an offense not to report the disease to CFIA if it is suspected or identified in an animal. Once confirmed, CFIA initiates and manages a strict testing and eradication program. The program includes the destruction of all infected animals as well as all the susceptible animals in contact with the diseased animals.

Before control programs were developed in Canada, TB was a common disease. Due to the scope of the problem many thought the programs would fail. With the hard work and co-operation of many people, including livestock producers and veterinarians, the programs, however, have been effective. For further information on regulatory efforts please see TB Timeline in Canada and Manitoba on the Manitoba Agriculture website.

Signs of Bovine TB

Bovine TB is usually a very slow disease to develop. Infected animals may not show any outward signs of illness, but many eventually exhibit weight loss and a gradual decline in general health. TB lesions may be found in any organ or body cavity of diseased animals. What signs an animal shows may depend on what organs are most affected. If the lungs are affected there may be a chronic intermittent cough and labored breathing. The lesions usually show up as tubercles (nodules or knobby swellings) which is how tuberculosis received its name.

In the early stages the lesions may be hard to find while in later stages they are easier to detect and often found in the lungs and lymph nodes in the chest, along the digestive tract, and within the head and upper neck. Lymph nodes are roundish bodies that help fight infection by supplying a type of white blood cell (lymphocytes) and filtering lymph fluid for disease. If one of these lymph nodes is near the surface of the body, such as around the head, it may show up as a firm swelling.

The tubercles are caused by the body trying to wall off the infection. They may be firm and tannish and sometimes gritty; in some types of animals such as deer the tubercles are softer and look like more typical abscesses with a creamy center.

The site of initial infection usually does not heal and the disease slowly progresses by bacteria spreading through the blood and lymph.

How Long Does Bovine TB Last in the Environment?

The bacteria that causes bovine TB is harmed by direct sunlight, high temperatures and dry conditions. Out in the field on a dry summer day, in direct sunlight, it may last only a few days or less. If the bacteria is within a patty of manure in hot dry conditions, however, it is more protected and could last approximately a week. If the bacteria get into a stagnant pool of water it could last approximately 18 days. If it gets dropped onto a bale in the middle of winter it could last several months.

Experimentally under hot conditions, M. bovis was isolated for up to 4 weeks from shaded soil but could not be re-isolated from soil in direct sunlight. Other reports on the length of survival of M. bovis vary from 18-332 days at temperatures ranging from 12-24°C (54-75°F). Under laboratory conditions, M. bovis has been isolated for up to 8 weeks from various feeds kept at 24°C (75°F) and 14 weeks from various feeds kept at 0°C (32°F). Under field conditions, however, it is generally difficult to isolate M. bovis from pastures grazed by animals known to be infected with bovine TB.

Unfortunately these studies often do not reflect how infective contaminated material is for animals. The actual length of time the bacteria can be isolated is longer than the material is infective for animals. This is because only a few bacteria need to be present to isolate the organism while it usually takes many thousands of bacteria to infect an animal by ingestion.

How Bovine TB is Spread Between Animals

Bovine TB is not a highly infectious disease. Spread usually requires frequent and extended exposure. The greatest risk of spread is through respiration (breathing). Invisible droplets (aerosols) containing TB bacteria may be exhaled or coughed out by infected animals and then inhaled by other animals including humans. Animals who are in close contact with infected animals, especially within confined areas such as barns, are at greatest risk for contracting TB. This is why in the early days, dairy TB was more of problem than beef TB.

Ingesting water or feed that has been contaminated with excretions (saliva, manure, etc.) or discharges from infected animals can also transmit the disease. This is more difficult to do, however, as the digestive tract has more defense mechanisms and has a thicker lining then the airways. It may take approximately 5,000 to 10,000 times more M. bovis bacteria to infect an animal by ingestion than it does by respiration. Animals and humans can also get bovine TB from drinking unpasteurized milk from infected cows or consuming raw undercooked meat from infected animals.

Where TB is present in wild deer and elk, spillover into domestic livestock is thought to occur primarily during periods of cooler moister weather at concentrated sites of feeding (hay stacks, round bales, etc.) or possibly at stagnant pools. Normal grazing at pasture and free flowing water is not thought to pose a significant risk.

How is Bovine TB Diagnosed?

Animals can be diagnosed after death when suspicious changes such as tubercles, abscesses and/or enlarged lymph nodes are found that suggest TB may be present. Samples of the lesions are taken and tested at veterinary diagnostic laboratories to determine if TB is present. All slaughtered cattle in Canadian federally inspected plants (>95% of cattle) are examined for this disease and it is the major method for surveillance in the country.

Animals can also be diagnosed before death by taking biopsies and culturing suspicious lesions. Most commonly, however, they are tested by measuring their immune response to the bacteria. This is done in Canada by injecting a small amount of purified killed TB bacteria under the skin of the tail fold (caudal fold test or CFT) and checking the site after 72 hours for any swelling. If swelling does occur the animal is labeled "suspicious", the herd is put under quarantine and further tests are done on the animal to determine if it is infected.

There are a few drawbacks with the CFT test. A small number of infected cattle in the very early or late stages of the disease and cows that have recently calved may test negative. Also approximately 5-7% of uninfected cattle will falsely test suspicious for TB and require further testing.

More accurate and faster tests for TB are being investigated and may be available in a few years or less.

How Can I Prevent Bovine TB from Infecting My Livestock?

There are no effective vaccines to prevent the infection or economical medications to treat livestock after they become infected.

In general, cattle brought onto the farm should come from herds established as free from TB. This can be done by bringing in animals only from a recognized monitored TB free herd or from animals that have only been in "TB free" status areas. Individual TB testing of cattle can be done before they are introduced into the herd but results are not as accurate as on whole herd tests.

Sick animals should be separated from healthy ones. If you have an animal that does not respond to routine treatments you should contact your veterinarian. If an animal dies it should be examined by a veterinarian if the cause is uncertain. As TB is a "Reportable" disease in Canada, the local CFIA district office must be notified by the owner, veterinarian, transporter or other person in charge of the animal when TB is suspected.

If you live in an area where bovine tuberculosis has been found in wild elk and deer, steps to limit direct livestock-to-elk/deer contact include: A. Moving harvested forage or other feed into storage sites before winter; B. Putting barrier fencing around feed storage sites and feeding areas; C. Preventing access to stagnant water sources frequented by wild elk and deer

Steps should also be taken to decrease and eliminate the spread of TB between deer and elk and include: A. Banning supplemental feeding of deer and elk including its use for recreational and hunting purposes; B. Maintaining adequate natural habitat for wild elk and deer; C. Maintaining wild elk and deer populations at levels that should limit spread when TB has been diagnosed in the wild

How Can I Avoid Becoming Infected by Bovine TB Myself?

Humans can reduce their personal risk by drinking only pasteurized milk and buying meat that has been federally inspected (e.g. supermarkets).

Meat from wild deer and elk and from on-farm slaughter of livestock should only be used if the animal is healthy, in good condition, and has no changes that would make one suspect disease was present.

High temperatures will kill M. bovis; therefore cooking will destroy the organism. Please remember, in general, it is recommended meat should be well cooked for a number of health reasons.

When animals are treated for any disease and after animals are slaughtered (including wild), hands and exposed clothing should be well washed. Re-usable materials should be disinfected.

If exposure to bovine TB may have occurred, contact your physician.