U.S. Department of Agriculture
Food Safety and Inspection Service
Washington, DC 20250
What is Tuberculosis?
Tuberculosis is an infectious disease of humans and animals. The most common cause of human disease is the bacteria (mycobacteria) called Mycobacterium tuberculosis (M. tuberculosis). In 1995, almost 23,000 human cases were reported to the Centers for Disease Control and Prevention (CDC). In the past, another major cause of human tuberculosis was Mycobacterium bovis (M. bovis), the causative agent of tuberculosis in cattle. Humans got infected through drinking raw milk, but with the introduction of milk pasteurization this form of human tuberculosis has become very rare in the United States. Other mycobacteria that cause human diseases are very rare in people with healthy immune systems.
Tuberculosis in humans is commonly seen as a lung infection or pneumonia. While infection of other organs such as the brain, bones, intestines, and kidneys also occurs, these are rare. Individuals with weakened immune systems are more susceptible to these rarer presentations of human tuberculosis
How does one get infected?
Humans get infected by M. tuberculosis when they inhale the mycobacterium. This usually occurs when someone with active tuberculosis of the lungs coughs, causing the release of thousands of bacteria into the air around them. People breathing the same air for prolonged periods are likely to inhale the bacteria. Thus, most people who get infected have close contact with the infectious person, i.e., live in the same house. Tuberculosis is not transmitted by a foodborne route. Meat inspectors working with infected animals are not at increased risk for tuberculosis.
What are the symptoms of tuberculosis?
Patients with tuberculosis of the lungs have a prolonged illness with fever, cough, night-sweats, weight loss and may occasionally cough up blood. As the disease progresses, symptoms get worse and shortness of breath ensues. Untreated disease can be fatal.
How is tuberculosis diagnosed and treated?
Tuberculosis may be suspected on the basis of a chest X-ray picture or a positive skin test that indicates exposure to the bacteria. Sputum examination for presence of the organism is very strong evidence of disease. However, definitive diagnosis requires bacteriologic studies with isolation of the specific mycobacteria. Physicians ultimately diagnose disease based upon clinical symptoms in association with x-ray pictures and positive bacteriologic tests.
Tuberculosis is a treatable and curable disease. However, drug treatment requires that a number of antibiotics be taken for a prolonged period of time, usually six to nine months. Some cases of tuberculosis are caused by mycobacteria that are resistant to the commonly used anti-tuberculous drugs. Such resistant bacteria frequently arise as a result of incomplete or intermittent treatment. These cases are complicated by the need for multiple drugs for even more prolonged periods of time. Tuberculosis due to resistant mycobacteria can be incurable and fatal. Thus, it is essential that the prescribed treatment be followed accurately for the full course of therapy. Many treatment programs use "directly observed therapy" where a health worker directly observes patients taking their medications to ensure compliance and help minimize resistance to therapy.
What is the risk of disease once infected?
Infection with M. tuberculosis does not necessarily mean that one will get the disease. While the risk for getting sick with tuberculosis is highest in the first two years of infection, about 90 percent of healthy people who get exposed to the mycobacterium will never get tuberculosis. Individuals with weakened immune systems, however, are at higher risk of active disease. Thus, tuberculosis is a major concern among infants, the very old, and those with a compromised immune system such as human immunodeficiency virus (HIV) infection.
What should I do to prevent getting tuberculosis once exposed?
Not everyone exposed to tuberculosis will get infected. A skin test helps to identify infected individuals. Infected persons may benefit from drug therapy and this can prevent TB disease with its symptoms. This involves taking one or more of the anti-tuberculosis drugs for several months to cure the infection and to decrease or eliminate the risk from subsequent active disease. Individuals with positive skin tests but no active disease cannot infect others. A physician would be the best guide to your need for taking preventive drugs following exposure.
Tuberculosis in animals
Like humans, animals also are susceptible to tuberculosis infections. While M. tuberculosis is the principal bacteria that affects man, M. bovis can also cause disease. However, M. bovis is rare in US livestock and is close to eradication. There is a very low incidence in US farmed cervidae, such as captive deer. These are primarily deer that are being raised on commercial game farms to be sold for domestic venison consumption. There is one small focus of M. bovis infection in wild white tail deer in a state game preserve located in northeast Michigan.
In adult cattle 85% to 90% of infections are due to M. bovis and occur through the respiratory tract. The TB eradication program has virtually eliminated the disease in US cattle with the exception of a few well-defined areas. Swine "tuberculosis" is caused by a variety of mycobacteria, primarily M. avium and M. bovis. These various organisms may produce tuberculosis-type lesions in swine. There has never been a confirmed case of mycobacteriosis transmission from swine to a human in the US.
FSIS inspection and disposition
All livestock slaughtered in federally inspected facilities undergo routine antemortem and postmortem inspection. Carcasses identified as having lesions resembling tuberculosis undergo additional inspection procedures in an effort to determine the extent and distribution of the lesions. In 1995, FSIS condemned less than fifty cattle and 1500 swine at slaughter for tuberculosis lesions. There are two groups of cattle examined at slaughter for TB. These include animals with visible TB lesions and those with no visible lesions, but which are positive reactors to a TB test. Cattle with lesions of TB are either condemned or passed for cooking after removal of the affected tissue.
Tuberculin reactor cattle are identified at their origin (test site) and have a "T" brand high on the left hip near the tail head. The animals are transported to slaughter under quarantine and are slaughtered as "suspects." The "suspect" receives an expanded examination by the veterinarian at the slaughter establishment. The inspector looks for evidence of lesions before making a final disposition.
Where to get more information about tuberculosisPhysicians and health care providers at public health facilities are best qualified to answer questions about tuberculosis. Additional information may be obtained by writing:
American Public Health Association
1015 15th Street, NW
Washington, D.C. 20005
CDC, National Center for HIV, STD, and TB prevention
Division of Tuberculosis Elimination
Atlanta, GA 30333
Via Internet at http://www.cdc.gov
For information about animal TB or animal health write:
USDA, Animal and Plant Health Inspection Service, Veterinary Services National Animal Health Program
4700 River Road, Unit 43
Riverdale, MD 20737
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Last Updated On 10/1/97.