The Condition

This is an important condition of outdoor reared poultry, particularly turkeys. The causative agent is the protozoan parasite Histomonas meleagridis, which invades the caecal mucosa and spreads, via blood, to the liver. Lesions occur in the caecum and liver (see www.poultrymed.com/files/Histomonas.html). Clinical signs include anorexia, depression and yellow droppings. Mortality rates may be very high, and reach a peak one week after observations of the first clinical signs.

The most important route of transmission is via the eggs of the caecal nematode Heterakis gallinarum, and also the earthworm. These ensure the safe passage of the delicate parasite and ensure persistence of the infection in soil. The blackhead organism is very susceptible to environmental conditions, but when encased in Heterakis eggs or earthworms it may remain viable for several years (Lund, 1969). Lund (1969) cites observations from the USA, where turkeys acquired blackhead when maintained on chicken yards that had been vacant for two years. It has been demonstrated that histomoniasis can spread rapidly in turkeys, but not chickens, by direct contact, probably involving the phenomenon of cloacal drinking (McDougald, 2005). To be virulent bacteria must be present, notably Escherichia coli, Bacillus subtilis, and Clostridium spp., (Doll and Franker, 1963; McDougald, 2005). 

A number of bird species may be a source of infection. Outdoor-reared turkeys are especially at risk from the disease. Left untreated, mortality rates of 90% have been recorded in turkey flocks. There is also increasing evidence that free-range chicken systems are becoming more prone to exposure and clinical disease, with some farms having repeated strikes in consecutive flocks. In chickens, the mortality may be 10%20% with high morbidity, although many outbreaks pass unnoticed (McDougald, 2005).

Histomonas meleagridis was held primarily responsible for an outbreak of 6% increased mortality and 11% decreased egg production between weeks 57 and 72 in a flock of free-range layer hens, concurrently infected with Brachyspira-like bacteria. This case was reported as an example of ancient diseases re-emerging in alternative housing systems (Esquenet et al., 2003). Histomoniasis diagnosed in a flock of 6-wk-old chickens was the first report of the presence of histomonads in the bursa of Fabricius in commercial chickens.


Methods of Control and Prevention

Control should be focused on the role of carrier chickens and earthworms.

Range rotation may not be a practical option in situations where turkeys and chickens are kept on the same premises, since the Heterakis eggs survive in the soil for such a long period. Survival may be reduced in paddocks that are well drained and situated in a sunny location.

There are breed differences in susceptibility to histomoniasis. The Rhode Island Red have been shown to be more resistant than other breeds (Lund, 1969). Resistant breeds provide a source of contamination for other less tolerant breeds.

Immunization is not an option for prevention, as birds do not reliably become resistant to re-infection after suffering a primary exposure (McDougald, 2005).

There is some evidence that some essential oils included in the feed can be effective. Herbal products with extracts from cinnamon, garlic, lemon, and rosemary may be effective preventive or curative treatments of Histomonas meleagridis (Zenner et al., 2003; Hafez and Hauck, 2006).

For further details, see control and prevention of Heterakis infestation.


Methods of Treatment

Although the Veterinary Formulary (1998) does not include a specific anti-protozoal drug for the treatment of histomoniasis in chickens, a water-soluble form in drinking water is the most widely used preparation (Jordan and Pattison, 1996).

Herbal products with extracts from cinnamon, garlic, lemon, and rosemary may be effective preventive or curative treatment of Histomonas meleagridis (Zenner et al., 2003; Hafez and Hauck, 2006).

For most recent information on different sector body requirements on withdrawal periods for livestock products following medicinal use please see

Withdrawal of Products following medication.


Good Practice based on Current Knowledge

See also Heterakis infestation.



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