Histomoniasis (Blackhead)

Barry Whitworth, DVM Area Food/Animal Quality and Health Specialist for Eastern Oklahoma

group of poultry in front of white background

A question sometimes asked by backyard poultry enthusiasts is “Can turkeys and chickens be raised in the same area?” A simple answer is that many backyard poultry producers do; however, many poultry experts would caution poultry producers about comingling these two species. The reason for this is a small protozoan parasite called Histomonas meleagridis.

H. meleagridis is a flagellated ameboid protozoan. The survival of this protozoan is dependent on Heterakis gallinarum, commonly referred to as the cecal worm of poultry. The cecal worms eat the protozoans. Once inside the cecal worm, the female cecal worm incorporates the protozoan in her eggs. The contaminated cecal worm eggs are shed in the bird’s droppings. At this time, H. meleagridis can be transmitted to domestic birds by ingestion of the contaminated cecal worm eggs or intermediated host (earthworms) that has ingested the contaminated eggs. Also, the protozoan can be transmitted directly from one bird to another bird. Unless protected in the cecal worm eggs or earthworms, H. meleagridis only survives a short period of time in the environment.

Several different species of birds (turkeys, chickens, ducks, geese, game birds, zoo birds) have been infected with H. meleagridis. Turkeys are considered the most susceptible with high morbidity and mortality rates. Young turkeys seem more susceptible than older turkeys. Blackhead can be rapid with birds appearing healthy in the morning and sick and/or dead by the afternoon. This makes control difficult. Chickens are easily infected but unlike turkeys, they have a milder form of the disease. Although not as deadly as in turkeys, production losses can be substantial with chickens. The difference between the severity of disease between turkeys and chickens may be explained by the turkey’s poor immune response compared to chickens. Lastly, some studies indicate that the development of disease is dependent on the presence of bacteria (Clostridium perfringens, Escherichia coli) or other microorganisms such as coccidia (Eimeria tenella).

Once a bird is infected with H. meleagridis, the protozoan penetrates the intestinal cecal wall of the bird. They will multiply and enter the bloodstream. Eventually, the protozoans infect the liver. The protozoans can be found in other tissues as well. Clinical signs will develop in one to two weeks.

Infected turkeys will display clinical signs such as yellow stained feces, anorexia, drooping wings, drowsiness, and problems walking. Eventually, turkeys will become emaciated. Studies have found sickness rates greater than 80% and death rates up to 100% in turkeys. As mentioned earlier, chickens tend to have milder forms of the disease; however, mortality rates around 30% have occurred.

Diagnosis of blackhead disease should be based on laboratory testing; however, poultry producers can make a presumptive diagnosis based on clinical signs, morbidity/mortality rates, and viewing the internal tissues. The primary internal lesions seen are in the ceca and liver. The cecal lesions are thick cecal walls with a cheesy like core and inflamed surfaces with an occasional ulcerated area. The liver will have circular dead depressed areas surrounded by red raised areas (bulls-eye-like lesions). 

All previous approved drugs used to treat/prevent/control blackhead disease have been banned by the United States Food and Drug Administration. Several different prevention and control practices have been suggested with few being thoroughly researched. One key is to start with healthy chicks and turkey poults. Studies have shown that beginning with high quality young reduces issues with blackhead disease. Some recommended practices by poultry experts are confirming the diagnosis of blackhead disease, maintain a proper environmental (wet moist conditions favor the development of the protozoan), control coccidiosis, control internal parasites (cecal worm) through deworming, do not comingle chickens and turkeys, and follow biosecurity protocols. Development of resistant genetic lines of birds has the potential to reduce the incidence of the disease. Several different natural treatments/preventions (essential oils, oregano, garlic, rosemary, etc.) have shown promise in laboratory conditions, but have not been researched in the field situations.

Fortunately, blackhead disease has not been found to be a major cause of death in backyard poultry operations; however, backyard poultry enthusiast should be careful when commingling turkeys and chickens. If a producer would like more information about blackhead disease, they should contact their local veterinarian and/or their local Oklahoma State University Cooperative County Extension Agriculture Educator.

References

Cadmus, K. J., Mete, A., Harris, M., Anderson, D., Davison, S., Sato, Y., Helm, J., Boger, L., Odani, J., Ficken, M. D., & Pabilonia, K. L. (2019). Causes of mortality in backyard poultry in eight states in the United States. Journal of veterinary diagnostic investigation : official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc31(3), 318–326.

Clark, S., & Kimminau, E. (2017). Critical Review: Future Control of Blackhead Disease (Histomoniasis) in Poultry. Avian diseases61(3), 281–288.

Swayne, D.E. and Halvorson, D.A. 2003 Influenza. In Y. M. Saif (ed.). Diseases of Poultry, 11th ed. Iowa State Press: Ames, Iowa, 135-160.