1. Histomoniasis (Blackhead)

2. INTRODUCTION
Histomoniasis is a parasitic disorder of the ceca and liver of many gallinaceous birds. The disease (caused by the protozoan Histomonas meleagridis) is characterized by necrotic foci in the liver and ulceration of the ceca and has been called infectious enterohepatitis or blackhead.

3. ETIOLOGY AND CLASSIFICATION
The causative agent is H. meleagridis, a flagellated ameboid protozoan.

4. Vectors
The role of heterakids as vectors for histomonads is extremely important because they, too, are parasites of gallinaceous birds. Studies show that the cecal worms actually become infected by the histomonads and incorporate them into the eggs.

5. Incubation Period
Disease is caused when histomonads penetrate the cecal wall, multiply, enter the bloodstream, and eventually parasitize the liver. Overt signs of histomoniasis are apparent from 7—12 days and occur most commonly 11 days postinfection (PI).

6. Clinical Signs
Early signs of histomoniasis in turkeys include yellow feces, drowsiness, dropping of the wings, walking with a stilted gait, closed eyes, head down close to the body or tucked under a wing, and anorexia.

7. The head may or may not be cyanotic, a sign observed by those who gave the disease the name blackhead. About 12 days PI, turkeys become emaciated. Infections in chickens may be mild and go unnoticed or may be severe and cause high mortality.

8. A. (left) normal uninfected poult; (right) histomonas-infected poult of the same age. Sickly appearance occurs later in the course of infection and is not distinctive to histomoniasis

9. Sulfur-colored droppings associated with histomoniasis in turkeys are seldom found in chickens, but bloody cecal discharges have been observed.

10. Gross Lesions
The primary lesions of histomoniasis develop in the ceca and liver. Lesions are observed initially in the ceca about day 8. After tissue invasion by histomonads, cecal walls become thickened and hyperemic. Serous and hemorrhagic exudate from the mucosa fills the lumen of ceca and distends the walls with a caseous or cheesy core, and ulceration of the cecal wall may lead to perforation of the organ and cause generalized peritonitis.

11. Liver lesions in turkeys are often apparent on day 10 of infection and are highly variable in appearance. Often, the lesion is described as a circular depressed area of necrosis up to 1 cm in diameter and is circumscribed by a raised ring. The liver may be enlarged and discolored green or tan.

12. F. Discrete pathognomonic lesions with raised surface from a turkey infected with histomoniasis

13. DIAGNOSIS Gross appearance of lesions, laboratory confirmation
PREVENTION AND CONTROL
Reduction or exclusion of the eggs
Exclusion of chickens from any contact
Rearing turkeys indoors, eliminating access to earthworms

14. Preventive chemotherapy
Presently, the only product available for the prevention of histomoniasis in poultry is nitarsone (Histostat7, Alpharma, Clifton, NJ).