Members of the genus are minute roundworms (pinworms), hardly 1 cm long, infecting different species of gallinaceous birds, including chickens, turkeys, ducks, geese, grouse, guineafowl, partridges, pheasants, and quail.

About 10 species are placed in the genus, but classification is often ambiguous due to their close resemblance, and a number of synonyms have arisen.

[1] The nematodes are whitish in colour, cylindrical, and with fine longitudinal striations throughout the body.

The anterior end is the mouth with distinct lips, and the anus opens towards the posterior.

[2] H. gallinarum is the most well-known species, and is most important as it transmits the protozoan parasite, Histomonas meleagridis in birds.