The parasitic blackhead disease, also known as histomoniasis, has returned in turkeys and chickens because of the loss of effective medicines, and new approaches to prophylaxis and therapy are urgently needed, according to the Veterinary Medicine University (Vetmeduni) of Vienna, Austria.

Vetmeduni announced Feb. 5 that its Christian Doppler Laboratory for Innovative Poultry Vaccines has developed a new vaccine that can successfully protect animals, but there are still several technical questions to be answered before it can be used in the field. Vetmeduni said one concern is the interaction between the causative single-celled parasite Histomonas meleagridis and the bacteria present in the intestine, which is of fundamental importance for the success of a vaccination.

Vetmeduni said a special feature of the parasite is its intensive interaction with bacteria both in vitro and in vivo, which is an important influencing factor summarized by Michael Hess, head of the University Clinic for Poultry & Fish at Vetmeduni Vienna, and his staff in two recent studies. Based on current analyses, the interaction of parasites and bacteria could represent a symbiosis with fatal consequences for the host and, thus, a unique alliance in medicine, Hess said, noting that the underlying functional mechanisms must be clarified in further studies.

“Future research should focus more on clarifying the interaction between bacteria and H. meleagridis, especially to find out whether targeted manipulation of the gut microbiome could minimize clinical consequences. Similarly, this knowledge could also be used to optimize the vaccine strategy developed. More detailed studies should also help explain the enormous differences in mortality and the manifestation of the parasite in certain companies,” Hess said.

Vetmeduni said another important influencing factor is Heterakis gallinarum, a roundworm that lives in the ceca, especially in domesticated chickens and turkeys. Although it causes only a slightly pathogenic infection, it is often the carrier of H. meleagridis. The importance of H. gallinarum cannot be overestimated, because H. meleagridis can survive in the eggs of the roundworm for up to three years, Vetmeduni said.

Limited research so far

The complexity of the pathogen, its epidemiology and the various influences on the pathogenesis of the disease require considerable efforts to develop a sophisticated protection strategy, Vetmeduni said. However, the current research results on the parasite H. meleagridis and blackhead disease are very limited, possibly due to various factors, such as difficulties in handling the parasite in vitro, which is also reflected in the large variation within the experimental studies, the university said.

Also, the interaction between H. meleagridis and the microbiome has hardly been investigated, so the true nature of the connection between the parasite and certain bacteria has not yet been established, Vetmeduni added.

The article "Spotlight on Histomonosis (Blackhead Disease): A Re-emerging Disease in Turkeys & Chickens" by Dieter Liebhart and Michael Hess was published in Avian Pathology. The article "Interplay between Histomonas Meleagridis & Bacteria: Mutualistic or Predator-Prey?" by Ivana Bilic and Hess was published in Trends in Parasitology.