Histomonosis (blackhead) Veterinary Information Note Histomonosis (blackhead) is caused by the protozoan parasite Histomonas meleagridis. Clinical signs Control Infection results in lesions in the caecum followed shortly afterwards by the development of lesions of variable appearance in the liver. Lesions are occasionally seen in other organs such as the spleen and bursa. No medications are currently licensed in Great Britain for the control or treatment of histomonosis. The risk of disease can be reduced with anthelmintic treatment to control Heterakis worms. The effects of histomonosis are known to be exacerbated in chickens by caecal coccidiosis. The clinical signs are non-specific and may include drowsiness, a stilted gait when walking, bright yellow droppings (particularly in turkeys), anorexia and death. Cyanosis of the skin and wattles of the head may occasionally be seen and was the origin of the name ‘Blackhead’. Diagnosis of the disease is usually on the basis of gross pathology, supported if appropriate by histopathology. The use of PCR testing has been reported for diagnosing disease in the early stages, but no PCR test is currently available in this country. Pathogenicity Turkeys are particularly susceptible to the disease, but blackhead is also encountered in other gallinaceous birds including chickens (particularly free-range layers), pheasants and partridges. Peafowl also appear to be particularly susceptible to the disease. Histomonas meleagridis is mainly transmitted by the caecal worm Heterakis gallinarum, which protects the organism within its eggs during transmission from bird to bird. The organism can also be harboured and transmitted by earthworms. Infection can also be spread by mechanical vectors, including people particularly as a result of contamination of footwear with Heterakis eggs. Infection can be transmitted directly between turkeys within an infected flock via the cloaca, in the absence of Heterakis worms, but this has not been demonstrated in chickens. There is no evidence that humans are susceptible to the disease. Bacteria such as Clostridium perfringens also play a role in the development of disease, which may explain an apparent response to antibiotic treatment in some outbreaks. Tiamulin has been reported to reduce mortality in affected turkey flocks and activity of tiamulin against Histomonas has been demonstrated in vitro. Histomonosis is endemic in the UK although relatively small numbers of incidents are diagnosed each year, mainly in small turkey flocks. The risk of disease in turkeys is increased if they are reared on the same site as chickens (or other gallinaceous birds) as these birds may harbour Heterakis worms, which can act as a reservoir for the organism. Infection in chickens may be asymptomatic or may be severe and cause mortality, and the outcome may be affected by the presence of concurrent and/or immunosuppressive diseases. Turkeys should not be reared on ground that has held chickens in previous years because Histomonas remains infective in its nematode and earthworm hosts for long periods of time, although it is short lived outside these hosts. It is also preferable not to rear turkeys on earth floors. Good biosecurity is important in excluding infection from turkey flocks. For further advice on general biosecurity measures in poultry, see the leaflet ‘Biosecurity and preventing disease (PB 11380) available on the Defra website. Revised April 2011 Page 1 of 1 AHVLA Avian Expert Group www.defra.gov.uk/ahvla/