Detection of elevated levels of circulating antigen 85 by dot immunobinding assay in captive wild animals with tuberculosis

Antemortem diagnosis of tuberculosis in captive wild animals is often difficult. In addition to the variability of host cellular immune response, which does not always indicate current active infection, reactivity to saprophytic or other mycobacteria is common and may interfere with the interpretation of the intradermal tuberculin skin test. Furthermore the immobilization required for administering the test and evaluating skin reactions in these animals may result in unacceptable levels of morbidity and mortality, of particular concern in individuals of rare or endangered species. Proteins of the antigen 85 (Ag85) complex are major secretory products of actively metabolizing mycobacteria in vitro Production of these proteins by mycobacteria during growth in vivo could result in increases in circulating levels of Ag85 in hosts with active tuberculosis. A dot blot immunoassay has been used to detect and quantify circulating Ag85 in captive wild animals with tuberculosis. Elevated levels of Ag85 were observed in animals with active tuberculosis as compared with uninfected animals. Study populations included a herd of nyala (Tragelaphus angasi) (n = 9) with no history of exposure to Mycobacterium bovis. Serum Ag85 levels ranged from <5 to 15 μU/ ml (median 5 μU/ml). The other group included 11 animals from a mixed collection with a documented history of an M. bovis outbreak. Animals with pulmonary granulomatous lesions (n = 3) had serum Ag85 levels ranging from 320 to 1,280 μU/ml (median, 320 μU/ml). Animals with only chronic mediastinal or mesenteric lymphadenitis (n = 4) had serum Ag85 levels ranging from <5 to 320 μM/ml (median, 52.5 μM/ml). Animals with no lesions present on necropsy (n = 4) had serum Ag85 levels ranging from <5 to 80 μU/ml (median, <5 μU/ml). This assay could provide an important adjunct to intradermal skin testing for antemortem diagnosis of tuberculosis in nondomestic species.