Loa loa: A cutaneous filarial parasite of humans

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Biology and Epidemiology

Distribution

Human loiasis is confined to the rain forest and swamp forest areas of West Africa. It is especially common in Cameroon and on the Ogowe river.

(taken from Peters and Gilles1991)

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Life Cycle:

Loiasis is caused by the filarial nematode Loa loa which is transmitted to humans by day-biting Chrysops flies (see picture below taken from Peter and Gilles 1991).

Once inside the body the infective larvae develop slowly into a mature adult (the process takes about a year). During this period it lives and moves around the fascial layers of the skin. In periods of growth and development Loa loa often makes frequent excursions through the subdermal connective tissues where it is often noticed by the host. Once they reach maturity (measuring 3-3.4 cm x 0.35-0.43 mm for males and 5.7 x 0.5 mm for females) the adults mate and produce sheathed microfilariae 298 x 7.5 micrometers in size. The microfilariae closely resemble the microfilariae of W. bancrofti however in stained films they assume a stiff angular attitude. The cuticle sheath also does not stain with Giemsa (see picture below taken from Peters and Gilles 1991).

The microfilariae are diurnally periodic in synchrony with their vector and once they reinfect a fly they undergo two stages of growth into infective larvae (in about 10 days time) which can then be transmitted back to humans.

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Pathology

Most of the pathological problems observed in people infected with Loa loa are connected to periods when the migrating adult worms appear near the surface of the skin. The worms often appear around the eye where they can be easily seen and extracted before they damage the conjunctiva (See pictures below taken from Peters and Gilles 1991).

Immune reactions to the migrating worms can also cause calabar swellings in the arms and legs. Recurrent swelling can lead to the formation of cyst like enlargements of the connective tissues around the tendon sheaths. These swellings can be extremely painful when moved. Dying worms can also cause chronic abscesses followed by granulomatous reactions and fibrosis.