Histomoniasis: Introduction - The Merck Veterinary Manual

Histomoniasis: Introduction
(Blackhead, Infectious enterohepatitis)

Histomoniasis is caused by a protozoan that infects the ceca, and later the liver, of turkeys, chickens, and occasionally other galliform birds. In turkeys, most infections are fatal; in other birds, mortality is less common.

Etiology:

The protozoan parasite Histomonas meleagridis is transmitted most often in embryonated eggs of the cecal nematode Heterakis gallinarum , and sometimes directly by contact with infected birds. Outbreaks spread quickly through flocks by direct contact. A large percentage of chickens harbor this worm, and histomonads have been located in adult worms of both sexes. Three species of earthworms can harbor H gallinarum larvae containing H meleagridis , which are infective to both chickens and turkeys. H meleagridis survives for long periods within Heterakis eggs, which are resistant and may remain viable in the soil for years. Histomonads are released from Heterakis larvae in the ceca a few days after entry of the nematode and replicate rapidly in cecal tissues. The parasites migrate into the submucosa and muscularis mucosae and cause extensive and severe necrosis. Histomonads reach the liver either by the vascular system or via the peritoneal cavity, and rounded necrotic lesions quickly appear on the liver surface. Histomonads interact with other gut organisms, such as bacteria and coccidia, and depend on these for full virulence.

Traditionally, histomoniasis has been thought of as affecting turkeys, while doing little damage to chickens. However, outbreaks in chickens may cause high morbidity, moderate mortality, and extensive culling. Liver lesions tend to be less severe in chickens, but morbidity can be especially high in young layer or breeder pullets. Tissue responses to infection may resolve in 4 wk, but birds may be carriers for another 6 wk.

Clinical Findings:

Signs are apparent 7-12 days after infection and include listlessness, drooping wings, unkempt feathers, and yellow droppings. The origin of the name “blackhead” is obscure. Young birds have a more acute disease and die within a few days after signs appear. Older birds may be sick for some time and become emaciated before death.

Lesions:

The primary lesions are in the ceca, which exhibit marked inflammatory changes and ulcerations, causing a thickening of the cecal wall. Occasionally these ulcers erode the cecal wall, leading to peritonitis and involvement of other organs. The ceca contain a yellowish green, caseous exudate or, in later stages, a dry, cheesy core. Liver lesions are highly variable in appearance; in turkeys, they may be up to 4 cm in diameter and involve the entire organ. The liver and cecal lesions together are pathognomonic. However, the liver lesions must be differentiated from those of tuberculosis, leukosis, avian trichomoniasis, and mycosis. In some cases, especially in chickens, histopathologic examination is helpful. Histomonads are intercellular, although they may be so closely packed as to appear intracellular. The nuclei are much smaller than those of the host cells, and the cytoplasm less vacuolated. Scrapings from the liver lesions or ceca may be placed in isotonic saline solution for direct microscopic examination; Histomonas spp must be differentiated from other cecal flagellates.

Histomonas meleagridis, bulls-eye lesions, liver, turkey

Histomonas meleagridis, liver, turkey

Prevention and Treatment:

Because healthy chickens often carry infected cecal worms, any contact between chickens and turkeys should be avoided. Grouse and quail also may carry the infection to turkey yards. Because H gallinarum ova can survive in soil for many months or years, turkeys should not be put on ground contaminated by chickens. Once established in a flock, infection spreads rapidly without the use of a carrier.

The only drug used for the control (prophylaxis) of histomoniasis in the USA is nitarsone at 0.01875% of feed until 5 days before marketing. There is no effective treatment available commercially. Nitroimidazoles such as ronidazole, ipronidazole, and dimetridazole are effective for treatment or prevention but are not available in the USA. Frequent worming of flocks with benzimidazole anthelminthics helps reduce exposure to heterakid worms that carry the infection.