Histomoniasis or blackhead disease is a worldwide disease of gallinaceous birds caused by a protozoan parasite called Histomonas meleagridis. It was first discovered in 1893 in Rhode Island, reported across the continent and was soon found in many other countries.
Infections sometimes cause a dark or blackish appearance of the skin of the head in some birds due to an excessive concentration of reduced haemoglobin in the blood or cyanosis. It has always been thought to most seriously affect turkeys, with high mortality nearing 100 percent of a whole flock, but recent reports have shown that it may also affect chickens and other game birds, causing high morbidity, moderate mortality and extensive culling.
Like most other parasites, it has a complex life cycle that involves an intermediate host, the common caecal worm, Heterakis gallinae which is synonymous with Heterakis gallinarum. H. meleagridis is most often transmitted to poultry through the eggs of this second parasite, which is commonly found in both chickens and turkeys.
The eggs of the caecal worm may remain infective in the soil for a period of three years or longer and are able to transmit the protozoa responsible for the disease throughout this period.
H. meleagridis may also be transmitted by earthworms that accidentally eat the caecal worm eggs. The caecal worm larvae released from the egg and the parasite within that larva may remain in the earthworm for a year or more. When chickens or turkeys eat the infected earthworms, the caecal worm larvae containing the H. meleagridis parasites are released, which may result in infection with Blackhead Disease.
Poultry may also acquire the organism directly from the droppings of infected birds. However, H. meleagridis found free in the droppings and not protected by a nematode egg or the earthworm will die quickly, particularly during warm and dry weather.
Article made possible through the contribution of Meriden Animal Health.