Biology of Parasite

Parasite:
Onchocerca volvulus (filarial parasitic worm)
onchos (Greek): hook, barb
kerkos (Greek): tail
volvo, volutus (Latin): turn, roll (involve, revolve, evolve)

Synonyms:
River blindness. Whitewater disease. Blinding filarial disease. Sowda (Arabia). Galfilarienne and craw craw (Africa).
Spanish: Erisipela de la costa (Guatemala). Enfermedad de Robles. Mal morado (Mexico). Ceguera de los rios (Africa)

Agent Taxonomy [1]:
Phylum: Aschelminthes (roundworms)
Class: Tissue-dwelling nematode, Secernentea
Subclass: Spriruria
Order: Sprirurida
Superfamily: Filarioidea
Family: Onchocercidae
Genus: Onchocerca
Species: volvulus


Morphology:

Microfilariae (larval form of O. volvulus):
Worm-like unsheathed eggs, with sharply pointed, curved tails. Are 150 to over 350 µm long, and 5-9 µm in diameter [2].
The lifespan of microfilariae is 6-30 months [3].
Microfilariae exit the nodules created around the adult worms and migrate actively through the dermis and connective tissues, not only in the vicinity of the nodules, but also at some distance from them. Microfilariae are found in the fluid within the nodules and in the dermal layers of the skin. Rarely, microfilariae may be found in urine, blood, sputum, and the eye (during heavy infections) [4].


Microfilaria
Courtesy of Liz Webb (Leeds University)

Microfilariae in subcutaneous tissue
Courtesy of Tropical Medicine Central Resource


Adults (macrofilariae):
Wire-like, whitish color.
Worms lie coiled up in pairs or groups within subcutaneous fibrous tissue capsules.
The worms are slender and blunt at both ends. Lips and a buccal capsule are absent, and two circles of four papillae each surround the mouth [1].
Females are 33-50 cm long, and 0.4 mm in diameter [4]. The vulva lies just behind the posterior end of the esophagus [1].
Males are considerably shorter, and are 1.9 - 4.2 cm long, and 0.2 mm in diameter [4]. The tail of the male is a curled ventrad and lacks alae; it bears four pairs of adanal and six or eight pairs of postanal papillae [1].
Adult worms develop within 6-12 months, and can live up to 14 years. Females can produce microfilariae for 9 years [4].


Nodules containing adult worms
Courtesy of Liz Webb (Leeds University)

Intact nodule, and coiled adult male and female worm in worm nest
Courtesy of Tropical Medicine Central Resource