Blackhead (Histomonas meleagridis)

Blackhead or has it’s proper name is Histomoniasis.

Blackhead is one of the diseases that can be passed on from bird to bird in peafowl. For this reason I have decided write this page and include in it what I know and other known facts about the disease blackhead and it’s prevention, control and treatment.

Large, pale areas in the liver of a bird infected with Blackhead (Histomonas meleagridis)
Large, pale areas in the liver of a bird infected with Blackhead (Histomonas meleagridis)

Blackhead disease continues to cause sporadic but severe disease losses in commercial flocks world wide. Both reared in confinement and on free range. Common game birds as well as peafowl and certain other gallinaceous birds also often fall victim to this disease.

While death losses often reach 80- 100 per cent in domestic turkeys and peafowl a different face of the disease is seen in chickens. Broiler breeder pullets may suffer 10% mortality, extensive culling losses, and poor uniformity at time of lay. In the absence of any highly effective treatment drugs, emphasis on control is placed on prevention and containment by management and quarantine. Fortunately, research has been initiated in several laboratories in the USA and Europe, which are yielding important new findings which will be of value in planning future control programs.

At one time, researchers and diagnosticians believed that infections in turkey flocks arose from the ingestion of embryonated eggs of the caecal worm Heterakis gallinarum, or ingestion of earthworms that were carrying larvae of the caecal worm. This mechanism did not explain the phenomenon of rapid spread of blackhead through a flock of turkeys or peafowl, and led some to question whether other intermediate hosts might be involved.

This question was addressed more recently in experiments where uninfected turkey poults were placed in pens alongside other directly inoculated poults, in the absence of any other possible carrier or host. The results were clear-cut and dramatic. The uninoculated birds readily contracted the infection, became sick and died. By the end of the study, all of the birds had died or were sick with blackhead. Direct transmission of blackhead disease from directly inoculated turkey poults to uninoculated poults in the absence of intermediate hosts or other carriers. Depending on the exposure level, all of the uninoculated birds died or became ill from histomoniasis.

Further work on this means of acquisition of infection showed that the oral route was not involved, and that birds probably became infected by intake of liquid faeces in a process known as cloacal drinking.

The implication of this finding was that if healthy birds could be separated from sick birds, it would be impossible for the infection to spread through the flock. This could be accomplished by means of migration barriers, to divide the house into smaller sections, thereby limiting risk. This concept has been used successfully in some instances.

Where do peafowl get infections?

It is common that veterinarians and other investigators are unable to find Heterakis worms associated with outbreaks in peafowl. If the above discussion is considered, then it would be logical that the infection comes from outside the flock, probably tracked in on the shoes. The source of such contamination is most likely chickens, which are often found not too distant from the turkey flock.

It is now known that chickens, among the domestic gallinaceous birds, are the best hosts for Heterakis worms, and that the eggs produced by these worms in chickens are the best for causing disease when inoculated into turkeys and peafowl. It has also been shown that young chickens were 16 times as effective as mature chickens in hosting caecal worms, and that young turkeys were almost negligible in this respect. In tests done on eight species of gallinaceous birds and found that the Chinese ring neck pheasant was the best host for caecal worms, followed by chickens and guinea fowl.

The Caecal worm (Heterakis gallinarum) eggs, which are the only known biological vector of the blackhead organism. (Earthworms can harbour caecal worms until they are eaten by chickens or turkeys, but this is only an ‘extra’ reservoir of infection and not a necessary part of the life cycle). If a blackhead outbreak a cares then that yard can remain infective to turkeys and peafowl for many years.

Game birds are usually overlooked as a source of infection, but obviously could be another reservoir of infection as they may be present in areas where peafowl are raised. The pheasant and chukar are far better hosts for Heterakis than even chickens, and suffer little from the effects of histomoniasis, making them ideal as reservoirs of the disease.

How is blackhead controlled by management?

Prevention of blackhead in peafowl by management is two-fold:

prevention of exposure by quarantine or isolation, especially avoiding any contact with chickens or game birds that are not treated for worms regularly and use of migration barriers to prevent commingling of infected birds with uninfected birds.

It is interesting that the contagious spread of blackhead by direct contact of birds is not as important in chickens as in turkeys and peafowl(Hu et al. 2006). Thus, it is likely that infections in chickens result only from ingestion of Heterakis ova. Also, farm owners should be aware of the hobbies of their workers and discourage the keeping of backyard chickens, pheasants, partridges or showing poultry.

Could a vaccinationan against histomoniasis be developed?

Previous investigators considered Immunization by vaccination is considered impractical in turkeys to control blackhead disease. How ever repeated infection and treatment with dimetridazole produced turkeys that were resistant to re-infection after three infection/treatment cycles. So if you work on the fact the a turkey is going to be culled at 4 to 5 months, but a peafowl could live into early teens then treatment with dimetridazole is worth the time and expense.

Recent experiments have been successful in demonstrating some protection when turkey poults were given two or more inoculations with an antigen consisting of freeze/thawed cultured H. meleagridis. A single inoculation failed to offer protection when birds were challenged within a few weeks.

More work is needed to identify the best vaccination regime, the amount of antigen needed, and the possible contribution of bacteria to the host immune response.

Histomonas does not respond to anticoccidials or antibiotics. Like some of its common relatives Trichomonas and Giardia, it is anaerobic and lacks mitochondria. These organisms make energy by an anaerobic process involving special organelles called hydrogenomes, This explains why histomonads do not respond to these other types of chemotherapeutic agents; they simply lack the metabolic machinery to be interfered with. Sometimes birds infected with blackhead, particularly chickens, will seem to respond to antibiotic treatment. However, this is probably because of secondary infections with bacteria that could be affected by the drug.

Antibiotics normally have little beneficial effects on peafowl during a blackhead outbreak. Early and frequent preventive use of wormers (benzimidazole) can be of benefit in peafowl because worms are known to be a source of infection. Once a flock member is infected it seems to spread easily from bird to bird, this is probably not of value after outbreaks start, but can help keep it out of a free range or a enclosed flock not on wire floor.

Discussion and Conclusions

Even though losses from blackhead disease continue to be highly significant in turkeys, some progress has been made in recommendations for control by management. Chickens and game birds are likely the most important source of infection for turkeys and other birds, as they are prolific in generation of infective cecal worm ova.

The life cycle of Histomonas has been reconsidered, after the discovery that turkeys could become infected from direct contact with other birds or contaminated faeces.

The unique structure and metabolism of Histomonas makes these organisms immune to treatment with anticoccidials and antibiotics, but antibiotics are usually considered beneficial to treat secondary bacterial infections.

The protozoa-causing blackhead may remain infective within the eggs of the cecal worms in the soil for nearly three years; therefore each flock of new turkeys should be raised on new uncontaminated ground. Young turkeys should never be reared near older turkeys or with chickens that may carry the infection. In addition to domestic chickens, various wild birds such as pheasant and grouse may serve as reservoirs of infection for domestic turkeys.

The periodic moving of feeders, waterers, and roosts will help prevent the local build up of infective organisms. Good sanitation and litter management will help prevent transmission of the cecal worm as well as the blackhead organism. Many histiostats or preventative drugs are available and they are commonly included in commercial turkey rations. Because of the very serious nature of blackhead in turkeys, it is advisable to develop a regular program of preventative drug treatment.


Nine drugs with known or suspected antiprotozoal activity were tested in vitro, and in vivo for activity against Histomonas meleagridis (the protozoa responsible for blackhead). The nitroimidazoles dimetridazole, metronidazole, ornidazole, and tinidazole suppressed growth of H. meleagridis in vitro at 10 ?g/ml or higher. Paromomycin sulfate, and carbadox were weakly effective at high levels. Quinolinol, mebendazole, diloxanide furoate, and albendazole had no demonstrable efficacy in vitro. Drugs showing some activity in vitro were tested in young chickens inoculated intracloacally with 2×105H. meleagridis/bird. Dimetridazole, metronidazole, ornidazole, and tinidazole were highly effective at 200 ppm in feed. Paromomycin sulfate, and carbadox were ineffective in vivo, with no improvement in liver or cecal lesion scores compared to that of infected controls. Thus, the only new entities with efficacy against blackhead disease in vivo were nitroimidazoles, related to the positive control dimetridazole.


The best treatment for the disease, emetryl, is no longer available on the market and is now illegal because it was found to be carcinogenic. Which was a pittie so far has peafowl were concerned because they were not being reared for meat like turkeys and chickens. So now the treatment seems to be hit or miss for some people. What I have found is treating with Metronidazole or dimetridazole seems to be the best treatment at this time and the paragraph above would seem to say the same. You can get metronidazole from your veterinarian, it is a very common small animal drug or you can try your local pet store that carries fish supplies. Look for a product called Fish Zole, the active ingredient should be metronidazole. I have also found both Metronidazole and dimetridazole online in 400mg tables.

The dose is 50mg/kg by mouth once daily for 5 days. The fish zole comes as a 250mg tablet, so to dose the birds you will need to weigh your birds and figure out the dose to determine how much to give. As a reference point a 10lb (4.5kg) bird will get 1 tablet.


Crush 400mg of tabs to 1 litre of drinking water and treat for 2 days. (Shake well.) Because the drug will settle in the water, stir it up with a stick every few hours.

To treat, use Metronidazole 400mg per litre of drinking water for 5 days


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