Blackhead disease (histomoniasis) is an important poultry disease that affects turkeys, chickens, and game birds such as partridges, pheasants, and quail. The disease is caused by the protozoa Histomonas meleagridis, tiny, single-celled organisms that are spread to the bird by the roundworm Heterakis gallinarum.

Lifecycle and Signs of Disease

The lifecycle of the protozoa H. meleagridis is complex:

• The protozoa multiply in an infected bird’s cecum, a part of its digestive tract;
• They move to the bird’s intestines where the roundworm H. gallinarum lives;
• The roundworm eats the protozoa;
• The roundworm’s eggs become infected with the protozoa;
• The bird sheds the protozoal-infected roundworm eggs in its droppings.

Healthy birds become infected when they eat food, invertebrates (such as earthworms), or bird droppings that are contaminated with the protozoa. Direct bird-to-bird transmission can also occur within a flock. Because chickens, partridges, and pheasants commonly have the roundworm in their intestines, they often are the source of the protozoal infection for other birds.

Birds with blackhead disease are usually listless and have drooping wings, unkempt feathers, and yellow droppings. Typically, the cecum and liver of an infected bird will become inflamed and develop ulcers. Young birds become sick quickly and usually die within a few days after signs appear. The disease develops more slowly in older birds and they often become emaciated and may eventually die.

Turkeys are highly susceptible to blackhead disease. Once a turkey flock has been infected, 70 to 100% of the birds may die. In one survey, U.S. turkey industry professionals reported at least 50 outbreaks of the disease each year since 2009.¹ Blackhead disease is less severe in chickens but can lead to poor health and reduced egg production