Avian mycobacteriosis, also known as avian tuberculosis, is a contagious, slow-developing, chronicgranulomatous disease primarily caused by Mycobacterium avium subsp. avium
(MAA) and less commonly, Mycobacterium genavense
. The disease occurs in all avian species (domestic and wild) and sporadically in mammals. Chickens are more susceptible than other animal species.
Avian tuberculosis can have several different clinical presentations, depending on which organs are involved. In chickens, the intestinal tract and liver are the primary organs affected, with dissemination to other organs such as the spleen, bone marrow, air sacs, lungs, and skin. The infection results in the development of large tubercles or granulomas on the organ(s).
The most common form of avian tuberculosis, the intestinal form, typically causes progressive loss in body condition in the bird. Loss of body fat will cause the chicken’s breast muscles to atrophy, resulting in a prominent keel bone. Their face may appear smaller than normal, due to loss in body fat. Other signs observed include weight loss, white diarrhea sticking to vent feathers (“pasty butt”), dull and ruffled feathers, increased thirst, lethargy, and depression. The chicken’s comb, wattles, and earlobes often become very dry and pale in color.
If granulomas develop in the bone marrow of the bird’s leg bones or joints, the clinical signs often include an abnormally stiff and jerky hopping gait which sometimes leads to complete paralysis. This form is called tuberculous arthritis.
Avian tuberculosis has a long incubation period, and during the early stage of infection, clinical signs are subtle and less obvious. Once the disease has advanced, more obvious clinical signs develop. Clinical signs of avian tuberculosis are generally not specific and differ from bird to bird, and depend on the organs involved.
Most frequently observed clinical signs include:
- Loss in condition: The chicken may progressively experience a loss in body condition. Resulting in moderate to marked loss of pectoral muscle mass and develop a prominent keel. Loss of body fat causes sinking of the eyes within the orbits, making the chicken’s face appear smaller than normal. They will have a low body condition score. However, due to the weight of the granulomas that form inside their bodies, body weight alone is not a reliable form of distinguishing whether a bird has lost weight.
- Poor quality feathers: Affected chickens may have a delayed molt and preen less often, resulting in dull and poor-quality feathers.
- Diarrhea: When the intestines are involved, chickens may have persistent diarrhea and frequently stain their vent feathers with feces (pasty butt).
- Pale comb/wattles: The chicken's face, comb, wattles, and earlobes often become dry, pale, thinner, and occasionally have bluish discoloration.
- Abdominal enlargement: Chickens may present with marked caudal coelomic distension, with a very firm mass or fluid (ascites).
- Abnormal gait: Some chickens may develop bone lesions, causing lameness or stiffness, and walk with a peculiar jerky hopping gait.
- Difficulty breathing: If chickens develop granulomas and lesions in their lungs, or abdominal enlargement, it can interfere with their breathing.
- Subcutaneous lesions: Localized infections are generally the result of traumatic injuries that inoculate environmental mycobacteria into lesions. They are rare, and often present as nodular, ulcerated, fistulous draining tracts or nonhealing granulomatous or pyogranulomatous wound infections which do not respond to conventional drainage and antimicrobial therapy.
Susceptible chickens are primarily infected by ingesting or inhaling aerosolized Mycobacterium from a contaminated ground surface. Infected birds shed the organism in their droppings, where it can survive in the soil for months to years. There will be higher concentrations of the bacteria in areas which previously or currently house high populations of birds, living in unhygienic conditions. Wild birds, pigs, and certain mammals can act as significant reservoirs. Rodents can also be carriers of the bacteria, transmitting it through their feces to the environment.
If the chicken lives in an enclosure where avian tuberculosis was detected or suspected in other birds or previous flocks who lived there.
A CBC and blood chemistry may be helpful, depending on where the mycobacterial granulomas develop in the bird. Mild anemia with leukocytosis and a heterophilia and monocytosis may be seen in infected birds. Blood chemistry revealing an elevated AST, bile acids, or CK.
Radiographs (whole body VD and lateral views) may be useful for evaluating the musculoskeletal system and relative size and position of the viscera. Mycobacterial granulomas on the long bones may appear as osteolytic lesions in the radiographs.
Intradermal skin test:
Also referred to as the tuberculin test. The test involves giving an intradermal injection of 0.03-0.05 mL of purified protein derivative tuberculin into one of the chicken's wattles or their comb. A positive reaction is indicated by the presence of the inflammation and swelling of the comb or wattle after a 48 hour duration. In the United States, it must be obtained through the State Animal Health or USDA APHIS –VS District Office
. The reliability of this test to detect Mycobacterium
infected birds decreases in flocks with high infection rates.
There are several serological tests available at diagnostic veterinary laboratories which can detect the presence of Mycobacterium
antibodies. These include ELISA, rapid agglutination tests (RAT), complement fixation tests (CFT), and haemoagglutination tests (HAT). The samples required are serum extracted from blood. The results from these tests are often complicated by the occurrence of false negative reactions in healthy birds.
Fecal test: Mycobacterium
can sometimes be detected from fecal samples from infected birds. However, repeat culture examinations are required over a course of several consecutive days. The amount of Mycobacterium
shed in the feces of infected birds also depends on the stage of infection and organs affected.
Gross necropsy findings and histopathology may reveal characteristic lesions, and tissue cultures which confirm the presence of Mycobacterium
. Granulomas (multifocal to miliary nodules, ranging in size from microscopic up to about 4 cm in diameter) in affected organs are the most common finding in chickens with avian tuberculosis. The organs most frequently affected include the spleen, liver, intestines, lung, and bone marrow. Samples for culture should be stored and shipped at 4 degrees Celsius (39 degrees Fahrenheit). The laboratory should be informed that mycobacteria are suspected because they require specialized media and prolonged incubation for successful isolation.