Gross and microscopic findings and investigation of the aetiopathogenesis of mycobacteriosis in a captive population of white-winged ducks (Cairina scutulata).

The white-winged duck (Cairina scutulata) is critically endangered. Breeding collections of this duck are established in the United Kingdom and the USA. Infection with Mycobacterium avium avium serotype 1 is a major cause of mortality in the UK collection. In this study, the aetiopathogenesis of deaths occurring in the US collection was studied. All ducks (n=21) that died over a 21-month period were examined. Mycobacteriosis was diagnosed in 20 ducks, killing 19 of them. Multifocal to diffuse granulomatous lesions, often with abundant intralesional organisms, were seen in all 20 ducks. Unusual manifestations of this disease were the extensive involvement of the respiratory system and the absence of multinucleated giant cells. Sequence analysis showed that the ducks were infected with a sequevar of M. a. avium that contains serotypes 2, 3, 4, and 9. Given that the long-term ingestion of metals affects immune function, we measured an array of such elements in the liver of six ducks. Concentrations were undetectable or low. The disseminated nature of the disease, high concentration of mycobacteria and absence of multinucleated giant cells within lesions suggest that these ducks were unable to effectively kill the mycobacteria and point to a possible defect or inhibition in cell mediated immunity. Taken together with previously reported UK data, these results suggest that captive white-winged ducks are highly susceptible to at least two sequevars of M. a. avium and that mycobacteriosis is a major threat to ex situ breeding. We hypothesize that the minimal heterozygosis previously shown in these ducks could be contributing to an apparently ineffective immune response.