Histomoniasis, or blackhead, is a complex disease. Although primarily affecting turkeys with lesions in the ceca and liver, blackhead can also have a significant impact on chickens.
Blackhead is caused by the protozoan flagellate, Histomonas meleagridis, which has a broad host range, infecting gallinaceous birds including pheasants, partridges and bobwhite quails, in addition to chickens and turkeys.
With the ban on many of the medications used to fight the disease and changes in animal husbandry, such as re-using litter and increased stocking density, blackhead has re-emerged in many areas, including in North America and Europe. The focus for control of blackhead is on prevention by cleaning and disinfecting, as well as controlling intermediate host and carriers and the use of new diagnostic methods to better understand how to manage and eradicate the disease.
Ingestion of adult common cecal worms (Heterakis gallinarum) or their embryonated ova (eggs) infected with H. meleagridis is the main culprit for blackhead. Heterakis gallinarum is the only worm known to serve as an intermediate host for blackhead. After a series of divisions, a uniquely adapted, very small form of H. meleagridis actively invades the reproductive tract of the cecal worm and is subsequently shed within the infected worm egg.
Cecal worm eggs are extremely resistant to environmental conditions and may remain infective for 2-3 years. There is some anecdotal or circumstantial evidence of a link between ground works (e.g. disturbing the litter to de-cake or during clean out) and Histomonas outbreaks, as the ground works may re-circulate buried caecal worm eggs.
Transport of the infected cecal worm egg by one of several other potential disease carriers or transport hosts is also a risk. These include earthworms (which may ingest the eggs) or mechanical disease carriers, such as flies or rodents, which may simply transport the sticky eggs on their bodies. Polymerase chain reaction (PCR) has shown that black/darkling beetles (Alphitobius diaperinus) may contain H. meleagridis DNA, providing evidence of their role as a potential vector. People and equipment can also act as carriers and floodwater may trigger increased earthworm activity.
On top of vectors, direct transmission of H. meleagridis through cloacal drinking has been proven in turkeys only. This may be the reason why this disease appears to spread more rapidly in turkeys compared to chickens. Experimental direct oral inoculation of H. meleagridis (direct administration of the protozoa itself) has little success due to the parasites’ susceptibility to the acidic crop and gizzard.
However, when H. meleagridis is administered in turkeys intracloacally or via ‘cloacal drinking’, blackhead disease manifests. Outside the host (or intermediate host: Heterakis gallinarum), H. meleagridis has low persistency and a short survival time of only up to 9 hours in water or fecal material.
Clinical signs in chickens are not as clear as in turkeys, and may even go unnoticed, but can result in high mortality. Some signs of a blackhead challenge, such as blood in cecal discharges (droppings), poor body-weight uniformity in rearing and drops in egg production, can be mistaken for other disease challenges.
Clinical signs can develop 7-14 days after infection. Field observations suggest that a co-infection with coccidia, mainly E. tenella, may exacerbate clinical signs. Initially the ceca will become swollen with a thickened wall. In more advanced cases, cecal cores develop (hard accumulations of clotted blood and tissue). Liver lesions are highly variable but typically manifest as circular depressed target-like areas up to 1cm in diameter. Liver lesions do not always develop in chickens despite the development of cecal cores.
If blackhead is suspected, birds should be submitted to a diagnostic lab for post mortem examination. On a macroscopic level (via necropsy/autopsy) a differentiation can be made between infections with agents such as salmonellosis and coccidiosis, as the lesions created by these infections can be easily mistaken for Histomoniasis lesions (all 3 entities can produce cecal cores).
However, cecal lesions together with liver lesions are representative of a blackhead infection. On a microscopic level the protozoa can easily be found in affected ceca and livers. This may be confirmed with histopathology by taking tissue samples to confirm the presence of histomads, at least for the first case in an outbreak.
Due to the limited number of medications available to treat blackhead, prevention is the key. Good biosecurity between and within sheds is paramount, including dedicated clothing and footwear per shed, avoiding the movement of equipment between houses and on and off farms, along with consistent and effective rodent and insect control.
On top of the basics, the complete and effective removal of litter between flocks is recommended, especially after an outbreak. Clean out may require an extended period of time in addition to the use of an appropriate cleaning and disinfection protocol. Reducing primary disease carriers/intermediate hosts is one of the key steps in the strategy for blackhead control.
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Consistent, early and scheduled worming following veterinarian advice will help reduce exposure to cecal worms and eggs and the Histomonads they carry. The most effective solution may be to treat for more than a single day (3-5 days). Product rotation is also recommended every 3-4 flocks, or when a product appears to be ineffective. Deworming programmes need to take into account the parasite prepatent period (24-36 days) when establishing the frequency of the treatments.
Histomonas vaccination based upon an attenuated clonal strain of H. meleagridis has been shown to be highly effective in experimental trials. Further efforts are needed to standardise production and optimise administration of the vaccine in the field, plus there is no commercially available vaccine. Recent research has shown that allowing conditions that improve bird comfort and reduce stress may reduce the chance of the birds showing clinical disease, even in the presence of Histomonas.
Reducing vector and co-factors of blackhead
Besides cecal worms and earthworms, other organisms may serve as mechanical disease carriers.
• Control measures should include the reduction of darkling beetles, flies, and rodents and other pests. There should be zero tolerance for the presence of pests.
• Minimise the possibility of flooding that may increase the presence of earthworms.
• Disinfect any flooded areas to reduce the potentially increased presence of earthworms in dirt floor housing.
Blackhead is more likely to spread to the liver when coccidiosis is not under control.
• E. tenella, in particular, has been identified as an aggravating factor for blackhead.
• The number of birds with severe lesions increases when both Histomonas and E. tenella are present in the bird.
• In many instances there have also been blackhead cases identified right after active infections of E. necatrix, E. brunetti and E. maxima.
With the withdrawal of effective treatments, there is growing interest in the development and use of alternative intestinal health products to mitigate blackhead disease issues.
• Proper brooding management.
• Nutritional products include: prebiotics, probiotics, organic acids, plant extracts, essential oils, enzymes, and volatile fatty acids, among others.
• There is limited research currently available which supports the effectiveness of these products.
Control of E. coli
Although H. meleagridis is considered the causative agent of blackhead, it has been demonstrated that the parasite fails to cause clinical disease in the absence of bacteria. Several control strategies for E. coli have been used that appear to decrease the severity of blackhead or may help in the face of an outbreak.
• Live or inactivated E. coli vaccines.
• Organic acids via feed or water.
• Yeast cell-based products that trap the bacteria and minimise their replication in the ceca.
Jose J. Bruzual and Colin Adams, Aviagen