Lymphatic filariasis is caused by threadlike worms (nematodes) that live in the lymphatic system; it is transmitted by the bite of a mosquito carrying infective larvae. The larvae develop into adult worms in the lymphatic vessels, causing severe damage and swelling (lymphedema). The most common filarial species is Wuchereria bancrofti, but Brugia malayi and Brugia timri cause the infection in Asia. It takes several mosquito bites over many months to cause the infection. People living in tropical and subtropical settings are at the greatest risk for infection. Cutaneous filariasis caused by Loa loa is transmitted by the mango fly or deer fly; Onchocerca volvulus transmits microfilariae via the black fly; and Mansonella streptocerca transmits infection via a midge. Dirofilaria immitis may cause lesions in the lung periphery. Depending on the species, incubation may be 3 to 12 months.

Filarial infection can cause personal and socioeconomic hardships due to its debilitating consequences. Meningoencephalopathy and localized necrotizing granulomas are severe complications of L. loa infection. D. immitis may cause pulmonary infarction if the larvae lodge in the branches of the pulmonary arteries. Lymphatic filariasis may result in hydrocele or elephantiasis of the extremities or genitals. A secondary bacterial infection may cause elephantiasis, blindness, and infection of the skin and lymphatic system.

Assessment findings are based on the type of infection present and the body site affected. Signs and symptoms may be acute or chronic. Patients with lymphatic filariasis may be asymptomatic or may complain of fever and edema of the lymph system, commonly affecting the legs but also seen in the arms, breasts, and genitals. Thickening and hardening of the skin (elephantiasis) as well as skin exfoliation may occur. Filariasis affecting the pulmonary system causes a dry cough, dyspnea, wheezing, and anorexia. Cutaneous filariasis causes itching, subcutaneous lumps, localized pain, and lymphadenitis and may lead to ocular lesions and eye disorders such as glaucoma, optic atrophy, and corneal fibrosis.