Tuberculosis (Tb)

Causative Agent

Mammals:

Bacterial disease caused by infection with Mycobacterium bovis, previously found in domestic cattle, but also known to occur in specific wildlife populations. In BC, the species of most concern is wood bison.

Birds:

Bacterial disease caused by infection with Mycobacterium avium.

Images

Click on images to enlarge. Firm nodules within the liver associated with avian tuberculosis. Tuberculosis nodules lying against ribs within the chest cavity of a wood bison

Distribution

Mammals: Geographic At present, BC is free of Mycobacterium bovis. There is a potential for Tb infection in wild wood bison (Bison bison athabascae) populations in northeastern BC if infected bison disperse from Wood Buffalo National Park. Seasonality Infections with M. bovis are chronic. Birds: Geographic Seasonal trends of tuberculosis in wild birds have not been documented; infections may occur throughout the year owing to the chronic nature of this disease. Seasonality In free-ranging wild birds, avian tuberculosis is found most often in species that live in close association with domestic stock (house sparrows [Passer domesticus] and Eurasian starlings [Sturnus vulgaris]) and in scavengers (crows and gulls), and likely exists in small numbers of free-ranging wild birds wherever there are major bird concentrations. A study in BC found tuberculosis in 0.6 percent of more than 600 wild birds.

Hosts, Transmission and Life Cycle

Mammals: Hosts Bovine Tb is not a naturally occurring disease in wild animals and is believed to have been introduced into wildlife populations through contact with domestic animals. In Canada, Tb has been confirmed both in farmed and free-ranging cervids in some areas, in free ranging wood bison in Wood Buffalo National Park and in some of the surrounding bison populations. For more information on Tb in wild mammals from the Canadian Food Inspection Agency, click here. Carnivores may become infected with bovine Tb by eating infected carcasses. Life Cycle Survival of Mycobacterium bovis outside mammalian hosts is variable depending on ambient environmental conditions (cold, damp condition increases survival; bright, dry conditions decreases survival of the bacteria). Grazing patterns of different species of wildlife may influence prevalence of Tb with wild populations. Communally feeding animals (e.g., elk [Cervus canadensis]) tend to have higher rates of infection than those who tend to feed alone or in smaller groups (e.g., mule deer [Odocoileus hemionus]). This pattern could affect wildlife populations should Tb ever become established in wood bison or other wild ungulate populations in BC. Transmission Sick animals shed bacteria through breathing, coughing, or sneezing, and new hosts are infected when they inhale or ingest bacteria. Inhalation: infection establishes itself in the lungs and spreads to the nearest lymph nodes. Ingestion: infection establishes itself in pharynx or small intestine and spreads to the nearest lymph nodes. Bacteria are generally destroyed by white blood cells that the body uses to fight infection; however, some evade destruction and increase in number. These large clusters are often visible to the naked eye. They resemble abscesses and are called “tubercles”. Following infection, an immune response is mounted by the host; a weak response allows bacteria to increase in number producing further tubercles and lead to localized necrosis. Clinical signs depend on which organs are affected. With progression, generalized Tb ensues, leading to weakness, debilitation and ultimately death or predisposition to predation. Birds: Hosts All bird species are susceptible to avian Tb. Common in captive birds (poultry, quail, pigeons, raptors, waterfowl). Humans, livestock, and other mammals can become infected with M. avium. Life Cycle In free-ranging wild birds, avian tuberculosis is found most often in species that live in close association with domestic livestock (house sparrows [Passer domesticus] and Eurasian starlings [Sturnus vulgaris]) and in scavengers (crows and gulls). Transmission Avian Tb generally is transmitted by direct contact with infected birds, ingestion of contaminated feed and water, or contact with a contaminated environment. Inhalation of the bacteria can cause respiratory tract infections.

Signs and Symptoms

Mammals:

Some animals may show signs of disease within 6 months of infection while others may survive for several years without showing signs. Severely infected animals may appear normal if lesions are confined to internal lymph nodes or restricted areas of the lung. Abscesses often develop and may discharge pus through skin or mucosal surfaces. Lesions may mineralize with age and become visible on x-rays. General symptoms include weakness, loss of appetite, weight loss, and fluctuating fever. Lesions in wood bison have been found in the respiratory tract and associated lymph nodes. Tb lesions may be found in any organ or body cavity; early in disease onset, lesions may be difficult to find while in the latter stages, nodules or lumps may be found in lungs and in lymph nodes associated with lungs, head or intestinal tract. A Tb-infected animal may have multiple tan or yellow lumps on the inside of the ribcage. Lungs may contain pea-sized tan or yellow lumps or lesions typical of the disease.

Birds:

Infected birds are often emaciated, weak, and lethargic, and they exhibit wasting of the muscles. Other symptoms depend on which body system is affected and may include diarrhea, lameness, and unthrifty appearance. The location of lesions indicates the route of infection: intestinal lesions suggest contaminated feed or water; lesions in the lung suggest inhalation. Infected birds often have solid-to-soft or crumbly, yellow to-white or grey nodules that are deeply embedded in infected organs and tissues, < 1 mm to several cm in size. The liver most often contains such nodules, but the spleen, lung, and intestines may also contain similar nodules. Clusters of these nodules may appear as firm, fleshy, grape-like clusters. Abscesses and nodule-like growths have been reported around the eyes, at the wing joints, on the legs, side of the face, and base of the beak. Some infected birds have died without any obvious clinical signs or external lesions. Infected birds often have enlarged livers and spleens.

Meat Edible?

For Both Mammals and Birds:

Tb bacteria are very rarely found in meat (muscle tissue). As a precaution, however, all meats, including hunter-harvested deer, should be thoroughly cooked to an internal temperature of 74 ºC (165 ºF) to kill bacteria. If the lungs, ribcage or internal organs from wild ungulates look abnormal (multiple tan or yellow lumps), THe meat should not be eaten and the nearest Health Authority contacted (see contact list). It is unlikely that a person field-dressing or eating meat of an animal infected with bovine Tb could become infected. Nonetheless, hunters should practice sanitary precautions, including washing after field-dressing an animal. Hunters that suspect their kill of being infected SHOULD NOT EAT THE MEAT and should contact their provincial or federal agencies (see contact list). Dogs and cats should not be allowed to roam freely where they could feed on carcasses or gut piles of infected deer.

Human Health Concerns and Risk Reduction

Bovine Tb:

Human infection occurs either via the respiratory route or through cuts or abrasions in the skin after being in close contact with infected animals. Bovine Tb is one of the most infectious forms of Tb and is reportable under the Health of Animals Act. M. bovis in the environment can be killed easily by a weak solution of common household bleach (mix 1 part bleach to 9 parts of water) or other commercial disinfectants. They are also quite heat sensitive.

Avian Tb:

There are many authenticated cases of M. avium infection in people, although humans are considered highly resistant to this organism. Avian Tb is generally considered non-contagious and so it is unlikely that an infected person could transmit M. avium to an uninfected person. Infection is more likely to occur in persons with pre-existent diseases, especially those involving the lungs, and in persons whose immune systems are impaired by other illnesses.

Samples for Diagnosis

Post-mortem investigation and microscopic analysis of lesions by a veterinary pathologist is required. Definitive diagnosis is accomplished by culturing and identifying the bacteria. For birds, if carcasses or tissues cannot be submitted to a laboratory within a short time, tissues may be preserved in 10 % buffered formalin solution. The whole carcass of a bird is preferred; however, if this is not possible, remove the leg at the hip joint, wrap it in clean aluminum foil, place it in a plastic bag, and freeze it for shipment to a qualified disease diagnostic laboratory. Bovine tuberculosis is a reportable disease in Canada, and under the Health of Animals Act, all cases must be reported to the Canadian Food Inspection Agency (CFIA).

Similar Diseases

Birds:

The emaciated, weak and lethargic condition of birds resembles lead poisoning. Nodular lesions in internal organs are often similar in appearance to those of aspergillosis; laboratory tests are needed to differentiate between the two diseases. Abscesses and nodular growths on the skin around the eyes, at the wing joints, on the legs, side of the face, and base of the beak of birds resemble avian pox.

Further Reading

Canadian Food Inspection Agency – Bovine Tuberculosis Clifton-Hadley, R.S., Sauter-Louis C.M., Lugton I.W., Jackson R., Durr P.A., Wilesmith J.W.. 2001. Mycobacterial diseases: Mycobacterium bovis infections. Pp. 340-371 in E.S. Williams, I.K. Barker (eds.), Infectious Diseases of Wild Mammals. 3rd Ed. Iowa State University Press. Ames, IA. Friend M. 2001. Tuberculosis. Pp. 93-98 in Field Manual of Wildlife Diseases: General Field Procedures and Diseases of Birds. M. Friend, J.C. Franson (Tech. eds.), E.A. Ciganovich (ed.). Biological Resources Division Information and Technology Report 1999-001. U.S. Department of the Interior and U.S. Geological Survey. Washington, DC. (Chapter in PDF format)

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