a group of infectious disorders caused by threadlike nematodes of the superfamily Filarioidea, that invade the subcutaneous tissues and lymphatics (lymphatic system) of mammals, producing reactions varying from acute inflammation to chronic scarring. In the form of heartworm, it may be fatal to dogs and other mammals.

In the human body the female nematode gives birth to elongated embryos, the microfilariae, which migrate through the peripheral blood and skin, from which they are taken by bloodsucking insects. Within the insect carrier, the microfilariae grow into motile, infective larvae that, at the insect's next blood meal, are introduced into the human host, where they reach maturity in about a year. The term filariasis is commonly used to designate bancroftian filariasis, caused by Wuchereria bancrofti, organisms that are widely distributed in tropical and subtropical regions of the world and are transmitted to man by mosquitoes, usually Culex fatigans. The nematode lives principally in the lymph nodes and lymph vessels, notably those draining the legs and genital area, where the adult worms induce allergic reactions in the sensitized tissues.

The initial inflammatory stage is characterized by granulomatous lesions, swelling, and impaired circulation; this stage is followed by enlargement of the lymph nodes and dilation of the lymph channels, which, over the years, harden and become infiltrated and clogged with fibrous tissue elements, resulting in some of the untreated cases in the condition known as elephantiasis, which is typically associated with the gross expansion of the tissues of the legs and scrotum. The most effective therapeutic drugs are diethylcarbamazine and sodium caparsolate, which kill the adult worms and microfilariae.

The form of filariasis known as filariasis malayi closely resembles bancroftian filariasis in its symptoms and pathological changes; it is caused by Brugia malayi, found chiefly in the Far East. onchocerciasis (river blindness) is caused by Onchocerca volvulus, which is transmitted to man by flies of the genus Simulium, which breed along fast-moving streams; the condition is widespread in southern Mexico and Guatemala and is common in Central Africa. Characteristic lesions are nodules beneath the skin, usually in the head region; the infection may also invade the eyes, causing blindness in about 5 percent of the infected individuals. Treatment consists of the surgical excision of the nodules and the administration of chemotherapeutics. Loiasis, prevalent in West and Central Africa, especially along the Congo River, is caused by Loa loa and transmitted by flies of the genus Chrysops. It is characterized by transient areas of allergic inflammation in the tissues beneath the skin, called calabar swellings; adult worms may sometimes be visible beneath the conjunctiva (the delicate membrane lining the eyelids and covering the exposed surface of the eyeball). Loiasis produces irritation but seldom permanent damage. Treatment includes surgical removal of the worms from the conjunctiva and drug therapy. Other forms of filariasis are caused by Acanthocheilonema perstans and Mansonella ozzardi and are not in most cases associated with specific symptoms. The prevention of filariasis relies heavily on insecticides and insect repellents.