Download Lecture 14

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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.
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The causative agent is H. meleagridis, a
flagellated ameboid protozoan.
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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.
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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).
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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.
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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.
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Sulfur-colored droppings associated with
histomoniasis in turkeys are seldom found in
chickens, but bloody cecal discharges have
been observed.
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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.
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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.
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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
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Preventive chemotherapy
Presently, the only product available for the
prevention of histomoniasis in poultry is
nitarsone (Histostat7, Alpharma, Clifton, NJ).