Histomoniasis is a parasitic protozoan infection of turkeys, chickens, peafowl and several game bird species. Most infections are fatal in turkeys, but mortality is less common in other birds. Although chickens are relatively resistant to the condition, significant disease has been observed in breeding chickens and free-range layers. Clinical signs include depression, inappetence, poor growth, increased thirst, mustard-yellow diarrhoea, listlessness and dry, ruffled feathers. The common name for this disease is ‘blackhead’, which stems from the clinical sign where the bird’s head may become cyanotic (blue-black in colour). Chronically sick birds will become dehydrated and emaciated before dying and young poults may die within a few days of signs appearing.
Histomoniasis is caused by the protozoan Histomonas meleagridis which acts with existing intestinal bacteria, such as Escherichia coli, to cause the condition. After the protozoan is ingested it first infects the caeca then migrates through the blood stream to the liver. It is then passed out in the droppings and also within the eggs of the caecal worm Heterakis gallinae, if that is also present in the bird. 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, and currently no treatments have been approved for use in Australia. As control primarily relies on management and control of H. gallinae, frequent worming of flocks with benzimidazole anthelminthics helps reduce exposure to the caecal worms that carry the infection. Good management, biosecurity, and sanitation and housing birds on wire or concrete floors, or intensive re-littering for floor-housed birds, can reduce the level of infection.