Blackhead is a parasitic disease of gallinaceous birds (ground-feeding, chicken-like birds) caused by the protozoan named Histomonas meleagridis.
Blackhead occasionally affects wild turkeys, quail, and grouse, but it does not currently have a major impact on wild populations. It was once a significant disease of domestic turkeys, but improved husbandry and biosecurity greatly reduced the number of birds lost due to blackhead. Blackhead does not infect humans.
Blackhead has been reported in several species of gallinaceous birds including domestic and wild turkeys, domestic chickens, grouse, quail, and pheasants. The wild species most commonly infected in North America are wild turkeys and bobwhite quail. Blackhead causes high rates of flock mortality (80%-100%) in turkeys, while in chickens, the flock mortality is usually 10%-20%. The disease is relatively common in captive raised game birds.
Blackhead has been known to occur worldwide where chickens, turkeys, and other gallinaceous birds are raised in captivity, though the disease is more common in warmer regions. Most reports of blackhead in wild birds are from North America, though this may be due to lack of information from other parts of the world. Blackhead first came to New England in 1892 in Rhode Island and until World War II, it was the leading cause of death in turkeys in New England. Since Pennsylvania stopped farming turkeys for release this disease has ceased to be a problem in wild birds.
Histomonas transmission is complicated because the protozoan is carried by another parasite, the common poultry cecal worm (Heterakis gallinarum). The Histomonas protozoan infects the cecal worm and becomes incorporated into the worm’s eggs. This allows the protozoa to be protected from harsh environmental conditions in which it would not normally survive. Both parasites are transmitted when a bird ingests these eggs. Earthworms can act as an intermediate host for cecal worm larvae that are carrying Histomonas. Birds become infected with both parasites when they ingest these infected earthworms. The protozoa cannot survive very long on their own in the environment, but they are shed in the feces of infected birds and can be transmitted to new birds if eaten in a short period of time.
Clinical signs usually appear 1-3 weeks following ingestion of the protozoa. Birds infected with blackhead often exhibit non-specific signs such as lethargy, depression, loss of appetite, ruffled feathers, closed eyes, and drooped wings. The feces are often watery and sulfur-yellow in color. The disease is called blackhead because birds will sometimes develop a bluish or blackish coloration of the head, but this is not a reliable clinical sign. At necropsy the liver will contain large (0.5 inch or more in diameter) pale gray or yellow circles that are characteristic of blackhead. The ceca (a small pouch at the beginning of the large intestine) are usually thickened, ulcerated, and hemorrhagic.Some species are more susceptible to blackhead than others. Species such as the domestic chicken and ring-necked pheasant rarely develop clinical disease but readily transmit Histomonas as well as the cecal worm. These species act as reservoirs for both parasites and can introduce them to more susceptible species like turkeys.
Sulfur-yellow colored rings on the liver at necropsy are specific to blackhead; however, they may not be present in all affected birds. When rings are not present on the liver, the disease is diagnosed by laboratory identification of the protozoa. It is a common mistake to confuse other diseases of wild turkeys, such as avian pox, with blackhead. Please refer to the Avian Pox disease description for more information on clinical signs and diagnosis of this disease.
Medications are available to treat domestic and captive birds. A reliable vaccine does not currently exist.
Proper husbandry and biosecurity are necessary to protect domestic turkeys and captive game birds from blackhead. Reservoir species, such as chickens, should not be housed with susceptible species, such as turkeys. Reservoir hosts, such as ring-necked pheasants, should not be introduced to areas occupied by highly susceptible wild turkeys. Chicken manure should not be spread in areas inhabited by wild turkeys.