Clinical Findings and Transmission

The M. tb complex of bacteria contains the related species Mycobacterium tuberculosis, M. bovis, M. microti, and M. africanum. Mycobacterium tuberculosis complex has a wide host range, but historically, maintenance hosts of the disease have been primarily ruminants and primates. Cases of M. tb complex have been documented in captive and free-ranging carnivores of the families Felidae, Canidae, Procyonidae, Mustelidae, and others.^1,2,7,21,23 The majority of Mycobacterium isolates from the order Carnivora, including the family Felidae, have been Mycobacterium bovis.

The preponderance of M. bovis isolates in both captive and free-ranging carnivores is thought to result from ingestion of infected prey species and feeding on contaminated carrion or carcasses.^1,12,2,25 In some cases, however, no clear source of infection could be determined.^7,21

Pulmonary, gastrointestinal, and hepatic infections of M. bovis have been seen in free-ranging African lions (Panthera leo) and cheetahs (Acinonyx jubatus).¹² Captive tigers (Panthera tigris), African lions, and snow leopards (Uncia uncia) have presented clinically with respiratory distress, unthriftiness, weight loss, and lameness.^7,16,19,21 Pulmonary involvement has been the most consistently reported clinical finding in captive felids, with radiographic signs of bacterial pneumonia, cavitary lesions, multifocal nodules, pneumothorax, and partially collapsed lung; in one case, hypertrophic pulmonary osteoarthropathy of the bones of the forelimbs was noted.^7,19,21,24 Hemogram changes are generally nonspecific signs of inflammatory disease; moderate leukocytosis, neutrophilia, monocytosis, and anemia. Serum chemistry profile abnormalities consisted primarily of hypoalbuminemia and hyperglobulinemia with occasional hypercalcemia.^19,21 During clinical examination and at subsequent necropsy, mucopurulent exudate was seen in the trachea of several big cats.^16,19,21,24

Infected wild carnivores have been identified during epidemiologic surveys of live collected animals and from carcasses.^2,12,25 In wild carnivores, identification of M. bovis infection has coincided with M. bovis infections in prey species.^12,2,25

When typing of M. bovis isolates in free-ranging carnivores and a feral domestic cat has been performed, the isolates matched serovars found in the local maintenance host.^11,25

Carnivores are considered “spillover” rather than maintenance hosts for M. bovis, and long-term sustained infections in a carnivore population without continued reexposure to contaminated material from prey species has not been documented for most species.^1,2,12 Transmission from one carnivore to another is also poorly documented both in captive and free-ranging animals, primarily because of the inability to differentiate between infections from repeated contact with an infectious source or true contagious spread between conspecifics.^2,7,19,12

Exceptions to the “nonmaintenance host” rule exist; one is the European badger (Meles meles) in England and Ireland.³ Infection in this species is maintained in the population through social contact and effective aerosol and cutaneous transmission, making the badger a true maintenance host.³ Once established, M. bovis infection appears to be progressive in carnivores.^7,19,21

Free-ranging lions are thought to have a comparatively fast progression of the disease and may quickly succumb to pulmonary or hepatic infection with M. bovis.¹² Captive large felids have also succumbed rapidly after onset of clinical signs.^7,21

Clinical cases of M. bovis reported in zoo collection carnivores have been limited, with few contact animals becoming infected. Once a case is documented in a facility, however, every attempt should be made to identify and eliminate other infected animals and find the source of the infection. The disease has been invariably fatal in carnivores; treatment of known infected animals is not recommended because of safety concerns for staff and potential for spread to contact animals.

Diagnosis

Antemortem diagnosis of M. tb– complex infection may be difficult because clinical presentation and organ systems affected are variable. The progressive nature of the disease and the animal’s failure to show clinical signs until the disease becomes advanced often lead to cases being diagnosed at necropsy, by histologic examination, or through microbiologic methods.⁷ However, cultures may be negative despite gross and histologic evidence of the disease. In captive Panthera spp., antemortem diagnostics were successful in isolating the organism through bronchoscopic or transtracheal aspirates of tracheal exudates.^16,19,21

Indirect antemortem diagnosis may be worthwhile. However, several studies have shown the lack of value of the intradermal tuberculin skin test for M. tb complex in carnivores.^11,19,24 The use of polymerase chain reaction (PCR) has shown the presence of M. bovis genetic material in necropsy samples and may prove to be a useful adjunct to microbial culture attempts.⁷ A protein-A enzyme-linked immunosorbent assay (ELISA) has also shown some promise as a tool to screen M. bovis suspects and contact animals for exposure or potential disease.^11,21 Protein-A ELISA used to screen both domestic cats and captive African lions as contact and noncontact animals suggested that some contact animals did develop antibodies to both M. bovis and M. avium purified protein derivative (PPD) antigens.^11,21 Long-term follow-up with these animals revealed no evidence of clinical infection in the African lions and no histologic evidence at necropsy in the domestic cats.^11,21 Results were at least suggestive that such non–species-specific ELISA tests could be useful in antemortem diagnosis of M. bovis infection in felids.

Other non–species-specific serum antibody tests, such as the multiple-antigen ELISA, multi-antigen print immunoassay (MAPIA), and lateral-flow technology (Rapid Test) have been developed for diagnosis of M. tb– complex infection in other nondomestic animals.^17,20 MAPIA and Rapid Test have been adapted to assist in the detection of M. tb– complex disease in one species of carnivore, the European badger (M. meles), and may be attempted on sera from other carnivores and contact animals suspected of infection with M. tb complex. Unfortunately, these tests are not available commercially, although access may be possible through direct requests to research institutions working with these tests.

An antemortem test that is non–species specific and does not rely on host immune response is antigen 85(Ag85). The Ag85 test has shown promise in detecting Mycobacterium infections in other wildlife species and may be useful in detecting infections in free-ranging and captive carnivores. Antigen 85 is a major secretory protein produced by actively growing mycobacteria. This test has become an adjunct test in human medicine to detect mycobacterial infections and recrudescence.¹⁴ Access to these diagnostic tests may be available through human testing services or by direct research request in some cases.