Causal Agent:

Loiasis is caused by the nematodes (roundworm) Loa loa that can inhabit the lymphatics and subcutaneous tissues of humans.

Life Cycle:

Loa loa lifecycle

The vector for Loa loa filariasis are flies from two 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 number 1. The larvae develop into adults that commonly reside in subcutaneous tissue The number 2. 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 number 3. The fly ingests microfilariae during a blood meal The number 4. After ingestion, the microfilariae lose their sheaths and migrate from the fly’s midgut through the hemocoel to the thoracic muscles of the arthropod The number 5. There the microfilariae develop into first-stage larvae The number 6 and subsequently into third-stage infective larvae The number 7. The third-stage infective larvae migrate to the fly’s proboscis The number 8 and can infect another human when the fly takes a blood meal The number 1.

Life cycle image and information courtesy of DPDx.

Page last reviewed: January 20, 2015