The Condition

This is an important condition of outdoor reared poultry, particularly turkeys. The causative agent is the protozoan parasite Histomonas meleagridis, which invades the caecal mucosa and spreads, via blood, to the liver. Lesions occur in the caecum and liver. Clinical signs include anorexia, depression and yellow droppings. Mortality rates may be very high, and reach a peak one week after observations of the first clinical signs.

The most important route of transmission is via the eggs of the caecal nematode Heterakis gallinarum, and also the earthworm. These ensure the safe passage of the delicate parasite and ensure persistence of the infection in soil. The blackhead organism is very susceptible to environmental conditions, but when encased in Heterakis eggs or earthworms it may remain viable for several years (Lund, 1969). Lund (1969) cites observations from the USA, where turkeys acquired blackhead when maintained on chicken yards that had been vacant for two years.

A number of bird species may be a source of infection. Outdoor-reared turkeys are especially at risk from the disease. Left untreated, mortality rates of 90% have been recorded in turkey flocks. There is also increasing evidence that free-range chicken systems are becoming more prone to exposure and clinical disease, with some farms having repeated strikes in consecutive flocks.