Histomoniasis is a parasitic protozoan infection of turkeys, chickens peafowl and several game bird species. In turkeys, most infections are fatal while in other birds, mortality is less common. Although chickens are relatively resistant to the condition, significant disease has been seen in breeding chickens and free-range layers. Symptoms include depression, reduced appetite, poor growth, increased thirst, sulphur-yellow diarrhoea, listlessness and dry, ruffled feathers. The head may become cyanotic (bluish in colour) and hence the common name of the disease, blackhead. Chronically sick birds will become dehydrated and emaciated before dying. Young poults may die within a few days of signs appearing. There are no current therapeutic drugs allowed for use against Histomoniasis in Australia.
Histomoniasis is caused by the protozoan Histomonas meleagridis. The protozoa acts together with existing intestinal bacteria such as Escherichia coli to cause the condition known as blackhead. The protozoa is ingested, infects the caeca then migrates through the blood stream to the liver. The protozoa is passed out in the droppings and, if the bird is also infected with the caecal worm Heterakis gallinae, within the eggs of H. gallinae. Most infections are caused by birds ingesting infected caecal worm eggs and sometimes directly by contact with infected birds. Outbreaks spread quickly through flocks by direct contact. Earthworms also feed on caecal worm eggs and birds can be infected by ingesting infected earthworms. The protozoa is very resistant when inside worm eggs, larvae or earthworms and can be perpetuated from flock to flock in this way. It has recently been demonstrated that infection occurs readily via the cloaca when turkeys are on contaminated litter.
Drugs have been produced that will control the disease but there may be restrictions on their use in commercial flocks. Frequent worming of flocks with benzimidazole anthelminthics helps reduce exposure to the caecal worms that carry the infection. Good management and biosecurity principles are important control factors. Good sanitation, housing birds on wire or concrete floors or intensive re-littering for floor-housed birds can reduce the level of infection. Chickens and turkeys should be kept separated. Control relies on good management and control of the caecal worm (H. gallinae).
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