Unsheathed microfilaria is seen in:
a. All have sheathed microfilaria except Onchocerca volvulus.
b. Onchocerciasis is second in the world only to trachoma as an infectious cause of blindness.
c. A Simulium female black fly (intermediate host) takes a blood meal on an infected human host(definite host), and ingests microfilaria.
d. The microfilaria enter the gut and thoracic flight muscles of the black fly, progressing into the first larval stage.
e. The larvae mature into the second larval stage , and move to the proboscis and into the saliva in its third larval stage.
f. The black fly takes another blood meal, passing the larvae into the next human host’s blood.
g. The larvae migrate to the subcutaneous tissue form nodules as they mature into adult worms over six to 12 months.
h. After maturing, adult male worms mate with female worms in the subcutaneous tissue to produce between 700 and 1,500 microfilaria (unsheathed) per day.
i. The microfilaria migrate to the skin during the day, and the black flies only feed in the day, so the parasite is in a prime position for the female fly to ingest it.
j. The microfilariae migrate to the surface of the cornea
k. Ocular involvement provides the common name associated with onchocerciasis, river blindness, and may involve any part of the eye from conjunctiva and cornea to uvea and posterior segment, including the retina and optic nerve.