Microfilariae are commonly found in the blood of some wild bird species but are rare to absent in poultry except in southeast Asia, where asymptomatic infections in chickens and waterfowl occur. Prevalence of microfilariae in wild birds varies from 3% to 6% but can be as high as 20% in some species such as ptarmigan, swans, and geese. When psittaciformes were commonly imported, it was not unusual to observe microfilariae in their peripheral blood, especially in imported cockatoos.
At least 16 genera of filarids are found in avian species. All have an indirect life cycle, with bloodsucking insects (eg, lice, mosquitoes, midges) serving as intermediate hosts. Adults mature in body cavities, including the eye and ventricles of the brain, respiratory system, cardiovascular system, or connective tissues; some produce characteristic subcutaneous nodules.
Microfilariae may be numerous in circulation, especially in the skin vasculature. Microfilariae can be seen in blood smears. However, a buffy coat smear obtained from a microhematocrit tube is a more sensitive method of diagnosis. Increased numbers of microfilariae have been seen in stressed individuals, but they rarely cause clinical disease or mortality. A possible exception is infection of emus with Chandlerella quiscali, a common filarid of the brain of free-living grackles. Affected emus show signs of CNS disease (eg, torticollis, ataxia) and have encephalitis with necrosis and adult C quiscali in the lesion. C quiscali apparently does not produce microfilariae in emus. Treatment with ivermectin, fenbendazole, or levamisole and surgical removal of adult parasites has been used.