Histomonas meleagridis: life cycle and epidemiology

Maarten De Gussem
Janssen Animal Health
mdgussem@janbe.jnj.com

H. meleagridis is a member of the class Sarcomastigophora. This protozoal parasite of poultry bears characteristics both of both amoeba (Sarcodina) and flagellates (Mastigophora). H. meleagridis is a pleomorph organism, as morphology is adapted to the location in the final or intermediate host.

When present in caecal cavities or in cultures, one or two flagellae can be found. This form is also able to form pseudopodes, which are used for feeding with bacteria, cell debris and other small particles. Their size is more or less 10 to 20 µm. Reproduction is through simple binary division.

When present in tissues such as the caecal wall or the liver, only the intercellular amoebal form will be found. In the intermediate host Heterakis gallinarum another, smaller form is found. Fresh Histomonas is inactivated within hours when outside its final or intermediate hosts. Unlike Eimeria species for example, no cyst, that allows survival longer than approximately one day, is formed. The inability of Histomonas, whenever not protected by the eggs of the intermediate host Heterakis gallinarum, to pass the pH barrier of the stomach emphasizes the importance of Heterakis in the life-cycle of Histomonas.

Only when a turkey sits on the caecal dropping within hours after discharge, and cloacal drinking allows the re-introduction into a new bird’s caeca, direct transmission can be successful.

Histomonas meleagridis is an obligatory parasite. When introduced in the caeca of its host, either by transmission through the intermediate host or directly through cloacal drinking, they start dividing in the lumen.

When Heterakis is present in the caecum, the adult worms can again ingest the histomonads. An active invasion of the ovum causes the inclusion in the intermediate host..

After loosing its flagellum the histomonads invade the caecal wall, using pseudopodia. In this phase, the invasive forms release proteolytic enzymes, which aid in phagocytosis of partly digested host tissue. The histomonads are transported towards the liver and other organs, most probably via the portal vein system.

Pathogenicity largely depends on whether the histomonads reach the liver or not. This depends more on host factors than on strain factors, although pathogenicity differences between strains are evident. The host factors include species, breed, strain, age, intestinal flora and concurrent infections such as coccidiosis.