Cause

Blackhead is caused by a protozoan parasite, Histomonas meleagridis

Transmission

Direct transmission through infected water, feed or droppings have been proven. Indirect transmission by infected eggs of the ceacal worm, Heterakis gallinarum is also a major factor. Raising turkeys and chickens on wire and indoors decrease the incidence of blackhead.

Species affected

Chickens, turkeys and peafowl are natural hosts to blackhead infection. Although all these species can be affected, the disease is most significant in turkeys.

As the parasite is damaging the liver, the fat metabolism of the bird is altered. This can lead to an alteration in the composition of the yolk and therefore have an effect on the vitality of the day old chick. Also the diarrehea can create more dirty egg shells, having a negative effect on the chick quality as well. 

Clinical signs

Affected birds are depressed, stand or sit with ruffled feathers, and have yellowish diarrhoea. Darkening of head parts, especially in turkeys, gave the name to the disease. Gross lesions include circular necrotic areas in livers, with a crater-like center and cheesy cores in the caeca. Blackhead can cause high mortality, particularly in young turkey poults, but the disease can also affect older birds. In chickens the mortality from blackhead infection is usually lower with young chickens being the most susceptible.

Treatment and control

Treatment with protozoan chemotherapeutics is usually effective. Such drugs can also be given at preventive levels in turkey starter and grower feeds. It is not necessary to add them standardly to chicken diets.
Growing birds (especially turkeys) on wire and indoors can reduce the incidence of blackhead to a large extent, but even so, strict hygiene and elimination of caecal worms are important control measures.