Lymphatic filariasis is a human disease caused by parasitic worms known as filarial worms. This presents a controversial argument for the use of doxycycline as treatment of filarial infections. Eosinophilia is often prominent in filarial infections. It is a vector-borne disease, caused by filarial worm infected blackflies. Recent research has also focused on coinfection of "M. perstans" and other filarial parasites. Infections with parasitic filarial worms cause disease conditions generically known as filariasis. Diethylcarbamazine (DEC) also has been used in treating filarial infections. "M. perstans" is one of the most difficult human filarial infections to treat. Eight known filarial worms have humans as a definitive host. Filarial parasites have been reported from this species. "Mansonella streptocerca" is one of three filarial nematodes that cause subcutaneous filariasis in humans. "L. loa" is one of three parasitic filarial nematodes that cause subcutaneous filariasis in humans. "O. volvulus", along with most filarial nematodes, share an endosymbiotic relationship with the bacterium "Wolbachia". In cases of onchocerciasis, another common filarial parasite, the drug is effective. This allows third-stage filarial larvae to enter the bite wound.