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(Histomoniasis, infectious enterohepatitis)
Blackhead is a disease caused by a protozoan parasite, Histomonas meleagridis. It is classified as a flagellate but is one of a few protozoa that likewise have an amoeboid stage. The parasite is carried by the common poultry cecal worm, Heterakis gallinarum, found in the ceca (blind pouches of the large intestine) of a high percentage of chickens. This, plus the fact that chickens are not, as a rule, highly susceptible to blackhead, explains the frequent transmission of the disease from apparently healthy chickens to turkeys.
The parasites can live for long periods in the cecal worm and its eggs. It has also been determined that earthworms carry Heterakis and are thus of major importance in transmitting blackhead.
Although primarily a disease of domestic turkeys, it has been found in chickens, wild turkeys, grouse, quail, pheasants, and other gallinaceous birds. Its geographic distribution is limited mainly to the eastern and midwestern United States.
Outbreaks are most common in the spring and fall and are usually more serious in wet seasons than in dry ones. It is thought that wild turkeys acquire the disease by ranging on abandoned chicken or domestic turkey yards.
Transmission and Development
Blackhead occurs when the parasites gain access to the ceca of the bird and are able to multiply in the cecal wall and cavity.
Occasionally, a bird will ingest the naked organisms in contaminated feed or water, or while picking gravel or preening itself. In most instances a second parasite, the cecal worm (Heterakis gallinarum) is involved. This worm, which is one-third to one-half inch long and as thick as a thread, lives in the ceca of chickens, turkeys, and several other birds. The worm itself, or its eggs, which are microscopic, can harbor the blackhead organisms and carry them from one bird to another. The blackhead organisms are fragile and cannot live alone outside the bird host for more than a few hours. However, in the eggs of highly resistant cecal worms, they may remain viable and infective for over four years.
The organisms are eliminated from infected birds in their feces, alone or within the cecal worm and its eggs. These are then ingested by susceptible birds, resulting in infection.
The symptoms of blackhead are quite distinctive, but the name is misleading in that the head of the bird does not always turn dark.
The first symptoms are not specific but are suggestive of blackhead. The birds stand with their heads tilted downward or drawn to the body. Their feathers are ruffled, and their wings droop. Their eyes are partly closed. At first the birds are alert when they are disturbed but they quickly become indifferent if they are seriously ill. Young birds may die within two or three days after the first signs of illness, but older birds may suffer for several days before dying or starting a slow recovery. In wild turkeys in Michigan, the disease does not appear as an epidemic, but rather is diagnosed in occasional individuals. The passage of thin, sulphur-colored droppings is characteristic of blackhead, but the disease is well advanced in turkeys before this is conspicuous; this does not often appear as a symptom in chickens. The period of incubation after being infected is 15 to 21 days.
When birds that have just died from blackhead are opened, fairly characteristic symptoms may be expected. The ceca are inflamed and ulcerated; they may be filled with greenish-white material as thick as curdled milk or consolidated into cores. If the birds have been ill a long time, the cores will become a foul-smelling brown residue of a creamy consistency. The ceca need not be affected equally.
The affected liver presents a characteristic appearance, with areas of necrotic and degenerated tissues on the surface. These appear as yellow or sulfur-colored rings, one-half inch or more in diameter; large lesions are often marked by concentric rings. Small lesions are elevated, but as they increase in size they may appear as a depressed area (see illustration).
Upon opening a bird at necropsy, the presence of the sulfur-colored rings on the liver is sufficient for a diagnosis of blackhead; these lesions are specific (pathognomonic) for this disease. When such lesions are not evident, diagnosis must be made by histopathological identification of the causative agent. Other diseases such as tuberculosis, tumors, and mycotic infections have similarities to blackhead.
Treatment and Control
Employment of proper husbandry practices can be utilized by domestic turkey farmers to control blackhead and cecal worms in their flocks. Also, drugs are available for treatment and control. The current low level of infection in wild birds does not warrant attempts at treatment. However, if the disease should become significant, it is conceivable that medicated feed and water could be made available for wild flocks.
There is no threat to human health from blackhead. It does not infect humans. Wild birds, particularly turkeys, and to a lesser degree, pheasants, quail, grouse, and others are susceptible to blackhead. Because they are free-ranging, the disease does not affect whole populations as it may with confined domestic flocks. Blackhead, although occasionally found in wild turkeys in Michigan, does not appear to be a significant factor in limiting the population. A survey of 131 wild turkeys from four areas of the state showed no evidence of blackhead.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.