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Filarial Worms Causing Subcutaneous Filariasis 107

Onchocerca volvulus

Distribution
Mainly in tropical Africa, Central and South America. A small focus of infection
exists in Yemen and south Arabia.
Habitat
The adult worms are in nodules in subcutaneous connective tissue of infected
persons.
Morphology
The adult male worm measures not more than 5 cm in length and the female mea-
sures 50 cm.
The microfilariae are unsheathed and non-periodic. They measure about 300 μm in
length. The microfilariae are found in the skin and subcutaneous lymphatics in the vicin-
ity of parent worms. They may also be found in the eye and not in peripheral blood.

Life Cycle (Fig. 10.18)
(1) The vector, black fly, takes a blood meal and L3 larvae enter the bite wound.
(2–3) The larvae travel to the subcutaneous tissue and develop into adults which are

Onchocerca volvulus
Blackfly Stages Human Stages
1 Blackfly (genus Simulium)
takes a blood meal
(L3 larvae enter bite wound)
2
9 Migrate to head Subcutaneous tissues
and blackfly’s proboscis

i
8 L3 larvae

3 Adults in subcutaneous
nodule
7
L1 larvae Adults produce
5 Blackfly takes 4 unsheathed microfilariae
a blood meal
(ingests microfilariae) that typically are found in
skin and in lymphatics of
connective tissues, but
Microfilariae penetrate
6 blackfly’s midgut also occasionally in
and migrate to peripheral blood, urine,
and sputum.
thoracic muscles
d
i = Infective Stage
d = Diagnostic Stage
Fig. 10.18 Life cycle of Onchocerca volvulus (Reproduced from https://www.cdc.gov/dpdx/
onchocerciasis/index.html)