Filarioidea all are specialised parasites and the definitive host is always a vertebrate, a mammal, bird, reptile or amphibian, but not a fish.

[4] Most of Filarioidea parasitise wild species, birds in particular, but some, especially in the family Onchocercidae, attack mammals, including humans and some domestic animals.

[4] The Filarioidea include several families: In and about endemic regions filarial diseases have been public health concerns for as long as recorded history.

The ones that mainly occupy lymph vessels and cause conditions such as adenolymphangitis, elephantiasis, and filarial fever are: Three other medically important parasitic species are: The other two are less seriously pathogenic but commonly parasitise humans.

Wherever established, they may survive for years, the fertilized females continuously producing motile embryos called microfilariae rather than eggs.

It must succeed in invading its vector organism fairly soon, because, unlike adult filarial worms, microfilariae only survive for a few months to a year or two depending on the species and they develop no further unless they are ingested by a suitable blood-feeding female insect.

In the new definitive host the microfilaria complete the final stage of development into sexual maturity; the process takes a few months to a year or more depending on species.