Human loiasis geographical distribution is restricted to the rain forest and swamp forest areas of West Africa, being especially common in Cameroon and on the Ogowe River. Humans are the only known natural reservoir. It is estimated that 12-13 million humans are infected with the Loa loa larvae.
The vector for Loa loa filariasis are flies from two hematophagous species of the genus Chrysops, C. silacea and C. dimidiata. During a blood meal, an infected fly (genus Chrysops, day-biting flies) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound. The larvae develop into adults that commonly reside in subcutaneous tissue. The female worms measure 40 to 70 mm in length and 0.5 mm in diameter, while the males measure 30 to 34 mm in length and 0.35 to 0.43 mm in diameter. Adults produce microfilariae measuring 250 to 300 μm by 6 to 8 μm, which are sheathed and have diurnal periodicity. Microfilariae have been recovered from spinal fluids, urine, and sputum. During the day they are found in peripheral blood, but during the noncirculation phase, they are found in the lungs. The fly ingests microfilariae during a blood meal. After ingestion, the microfilariae lose their sheaths and migrate from the fly's midgut through the hemocoel to the thoracic muscles of the arthropod. There the microfilariae develop into first-stage larvae and subsequently into third-stage infective larvae. The third-stage infective larvae migrate to the fly's proboscis and can infect another human when the fly takes a blood meal.
Filariasis such as loiasis most often consists of asymptomatic microfilaremia. Some patients develop lymphatic dysfunction causing lymphedema. Episodic angioedema (Calabar swellings) in the arms and legs, caused by immune reactions are common. When chronic, they can form cyst-like enlargements of the connective tissue around the sheaths of muscle tendons, becoming very painful when moved. The swellings may last for 1-3 days, and may be accompanied by localized urticaria (skin eruptions) and pruritus (itching). Subconjunctival migration of an adult worm to the eyes can also occur frequently, and this is the reason Loa loa is also called the "African eye worm." The passage over the eyeball can be sensed, but it usually takes less than 15 min. Gender incidence of eyeworms have approximately the same frequency, but it tends to increase with age. Eosinophilia is often prominent in filarial infections. Dead worms may cause chronic abscesses, which may lead to the formation of granulomatous reactions and fibrosis.
Identification of microfilariae by microscopic examination is the most practical diagnostic procedure. Examination of blood samples will allow identification of microfilariae of Loa loa. It is important to time the blood collection with the known periodicity of the microfilariae. The blood sample can be a thick smear, stained with Giemsa or hematoxylin and eosin (see staining (biology)). For increased sensitivity, concentration techniques can be used. These include centrifugation of the blood sample lyzed in 2% formalin (Knott's technique), or filtration through a Nucleopore membrane.
Antigen detection using an immunoassay for circulating filarial antigens constitutes a useful diagnostic approach, because microfilaremia can be low and variable. Identification of adult worms is possible from tissue samples collected during subcutaneous biopsies or worm removal from the eye. Antibody detection is of limited value. Substantial antigenic cross reactivity exists between filaria and other helminths, and a positive serologic test does not distinguish between past and current infection.