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During a blood meal,
an infected blackfly (genus Simulium) introduces third-stage filarial
larvae onto the skin of the human host, where they penetrate into the bite
wound .
In subcutaneous tissues the larvae
develop
into adult filariae, which commonly reside in nodules in subcutaneous
connective tissues
. Adults
can live in the nodules for approximately 15 years. Some nodules may
contain numerous male and female worms. Females measure 33 to 50 cm in
length and 270 to 400 μm in diameter, while males measure 19 to 42 mm by 130
to 210 μm. In the subcutaneous nodules, the female worms are capable of
producing microfilariae for approximately 9 years. The microfilariae,
measuring 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span that
may reach 2 years. They are occasionally found in peripheral blood, urine,
and sputum but are typically found in the skin and in the lymphatics of
connective tissues
. A
blackfly ingests the microfilariae during a blood meal
. After
ingestion, the microfilariae migrate from the blackfly's midgut through the
hemocoel to the thoracic muscles
. There
the microfilariae develop into first-stage larvae
and
subsequently into third-stage infective larvae
. The
third-stage infective larvae migrate to the blackfly's proboscis
and can
infect another human when the fly takes a blood meal
.
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