Polymerase chain reaction (PCR) has shown that black/dar- kling beetles (Alphitobius diaperinus) may contain H. melea- gridis DNA, providing evidence of their role as a potential vec- tor. 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. melea- gridis has low persistency and a short survival time of only up to 9 hours in water or fecal material.
Clinical signs and diagnosis Clinical signs in chickens may be less clear than those in
turkeys, or even go unnoticed, but can result in high mortali- ty. Some signs of a blackhead challenge, such as blood in ce- cal 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 accumula- tions of clotted blood and tissue). Liver lesions are highly vari- able but typically manifest as circular depressed target-like areas up to 1cm in diameter. Liver lesions do not always de- velop in chickens despite the development of cecal cores. If blackhead is suspected, birds should be submitted to a di- agnostic 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 three entities can produce cecal cores). However, cecal lesions together
▶ POULTRY WORLD | No. 5/6, 2021
Ingestion of adult common cecal worms (Heterakis galli- narum) or their embryonated ova (eggs) in- fected with H. meleagridis is the main culprit for blackhead.
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