Studies on the molecular epidemiology, pathogenesis, diagnosis and treatment of avian mycobacteriosis
We investigated the molecular epidemiology, differential susceptibility to infection and disease, pathogenesis, diagnosis and treatment of avian mycobacteriosis in captive ringneck doves (Streptopelia risoria) and in the endangered white-winged duck (Cairina scutulata), both naturally infected with Mycobacterium a. avium. Our studies in doves demonstrated lower susceptibility to infection and less severity of lesions in the white color morph compared with the non-white. Genetic mechanisms of immunity to mycobacteriosis may be contributing or determining these differences. Given that the genes that code for white coloration are sex linked in birds, it is very likely that the gene or genes modulating this different immune response to M. a. avium infection in these doves could be associated to these loci or at least located in the same sexual (Z) chromosome, as the association with white color suggest. In the same birds, spleen biopsies followed by liver biopsies had the greatest potential for the diagnosis of mycobacteriosis by the demonstration of acid-fast organisms. Additional culturing of spleen or liver biopsies significantly increased the diagnosis of mycobacteriosis. The use of polymerase chain reaction (PCR) was the less sensitive techniques. Uneven distribution and low number of organisms in the liver, spleen and bone marrow may have contributed with the low diagnostic value of PCR. In a second group of sixteen doves with mycobacteriosis from the same flock, the combination of azithromycine, ethambutol and rifampin for 180 days was well tolerated but failed to cure them. Furthermore, this study demonstrated the inefficacy of liver biopsy to evaluate treatment as well the presence of antibiotic resistance in two isolates. These results highlight that erradication of mycobacteriosis in birds is not easy to achieve. Together with the possible emergence of antibiotic resistance in potentially zoonotic mycobacteria our results suggest that the treatment of mycobacteriosis in birds should not be recommended. Finally, the last study shows that white-winged ducks are highly susceptible to at least two sequevars of M. a. avium and that mycobacteriosis is a major threat to the ex situ conservation program. The minimal heterozygosis previously shown in these ducks could be contributing to this apparently ineffective immune response.