Tuberculosis is a slowly spreading, chronic, granulomatous bacterial infection characterized by gradual weight loss. All birds appear to be susceptible, although to variable degrees; pheasants seem to be highly susceptible, whereas the disease is uncommon in turkeys. Tuberculosis is more prevalent in captive than in free-living wild birds. It is unlikely to be seen in commercial poultry because of the short life span and husbandry practices used. (Also see Tuberculosis and other Mycobacterial Infections.)
Etiology and Epidemiology
Mycobacterium avium ss is the most common cause, although M genavense has been isolated from some psittacine birds. Seroagglutination tests of isolates are recommended to differentiate strains of M avium that cause disease in chickens and birds from those that fail to produce disease in these species. M tuberculosis, the cause of tuberculosis in people, has infrequently been isolated from parrots and canaries. M avium is very resistant; it can survive in soil for ≥4 yr, in 3% hydrochloric acid for ≥2 hr, and in 2% sodium hydroxide for ≥30 min.
Tuberculosis is found worldwide, most commonly in small, barnyard flocks and in zoo aviaries; it is rarely found in young flocks. Wild birds, such as cranes, sparrows, starlings, and raptors, have been found to be infected. Tuberculosis has been found in emus and other ratites. The movement of ratites through sales and the long life of these animals have made tuberculosis a major concern for ratite producers. Isolation of ratites purchased at sales is essential to prevent the introduction of tuberculosis into established flocks.
Infected birds with advanced lesions often excrete the organism in their feces; therefore, ingestion of contaminated feces is the most common route of transmission. Cadavers and offal may infect predators and cannibalistic flock mates. Rabbits, pigs, nonhuman primates, and mink are readily infected. Cattle exposed to contaminated feces may develop granulomatous lesions in lymph nodes associated with the GI tract and respond to M bovis purified protein derivative (PPD) tuberculin and to PPD of M avium. M avium serovar 1 is often isolated from tuberculous free-living wild birds and birds in captivity. Mycobacterium avium ss and M genavense have been isolated from immunocompromised people; however, these organisms only rarely cause disease in nonimmunocompromised individuals.
Clinical Findings and Diagnosis
Signs usually do not develop until late in the infection, when birds become emaciated and sluggish and lameness may be seen. In chickens, granulomatous nodules of varying size are often found in the liver, spleen, bone marrow, and intestine. Some exotic species may have lesions in the liver and spleen without intestinal or bone marrow involvement. Small mesenteric nodules may be found. Lesions are not mineralized.
Live birds may be tested with M avium PPD tuberculin, although these tests are of limited value in birds that do not have wattles (it is often difficult to inject tuberculin intradermally because of skin thickness at other sites). A test conducted in skin on the underside of the wing has been used in Japanese quail exposed to M avium. The injection site is observed at 24 and 48 hr for induration and swelling. A positive test indicates exposure to M avium; however, a negative test is of little or no significance. Large numbers of acid-fast bacteria in smears from lesions provide a tentative diagnosis. Mycobacteriologic examination is required to confirm a diagnosis. Isolates can be identified based on biochemical, drug susceptibility, and seroagglutination tests and by restriction length polymorphism analyses.
Prevention and Control
Addition of exotic birds to an exhibit should originate from a closed collection with no history of tuberculosis, ie, tuberculosis has not been reported previously or diagnosed on necropsy during the past 5 yr. If no history is available, the birds should be quarantined or maintained in a separate exhibit away from the bird aviary. Transmission is by ingestion of food or water contaminated with M avium ss, ie, feces from contagious birds. Therefore, it is recommended that endangered and valuable species be housed in individual exhibits.
Chemotherapy is ineffective, because most M avium complex organisms are resistant to antituberculosis drugs. In commercial poultry flocks, relatively rapid turnover of populations, together with improved general sanitation, has largely eliminated this once common infection. Infected poultry should be destroyed, and housing facilities thoroughly cleaned and disinfected using cresylic compounds. Quaternary ammonium compounds and halogens (eg, chlorine) do not effectively kill mycobacteria. In exhibits with dirt floors, several inches of the floor should be removed and replaced with dirt from a place where poultry have not been maintained. All openings should be screened against wild birds.
Avian tuberculosis in zoos is difficult to eradicate. New additions to the aviary should be quarantined for ≥6 mo. Isolation and quarantine of birds of unknown status purchased at sales is essential to prevent introduction of tuberculosis into established flocks.
Last full review/revision November 2013 by Charles O. Thoen, DVM, PhD