The Morphology and Biology of Onchocerca volvulus

Onchocerca volvulus is a large filaroid worm that causes the disease onchocerciasis in millions of Africans (at least 30 million), Arabians, Guatemalans, Mexicans, Venezuelans, Colombians and other people of tropical nations. The disease is commonly called River Blindness, because people infected with this worm may become blind. The worm attacks the eye and causes destructive effects. The worm also causes skin diseases, such as serious skin lesions.

Morphology of Onchocerca volvulus

There are slight morphological differences of O. volvulus to Wuchereria bancrofti worms. O. volvulus worms are entangled together in pairs or groups in subcutaneous tissues. They are slender and rounded at both ends. Buccal capsule and lips are not present and 2 circles of 4 papillae each encircle the mouth. The division of the esophagus cannot be clearly discerned.

Males are 19 cm to 42 cm long by 130 micrometers to 210 micrometers wide, while females are 33.5cm to 50.0 cm long by 270 micrometers to 400 micrometers wide. The vulva of the female is located just behind the posterior end of the esophagus. The male’s posterior end is coiled ventrally and does not have alae; it bears 4 pairs of adanal and 6 or 8 pairs of postanal papillae. The microfilariae are not covered with membrane sheaths. (Mehlorn and Armstrong 2001)

Biology of Onchocerca volvulus

Adult O. volvulus worms position themselves underneath the skin, where they become encapsulated by the host’s immune reactions. Prominent nodules appear if the worms are situated over a bone, such as at a joint or the skull. The locations of these nodules differ according to geographical region. In Central America, they are usually found above the waist, while in Africa they are usually below the waist. These distributions conform to the biting preferences of insect vectors in the two regions. As a result, microfilariae are concentrated at body parts where the insect vectors prefer to bite. (Mehlorn and Armstrong 2001)

Unsheated microfilariae stay in the skin, where they can be eaten by the black fly Simulium damnosum, which serves as intermediate host for the parasites. Black flies are widely distributed and become infected when they feed on human blood. Their mouthparts are not capable of deep piercing, so much of their meal consists of tissue juices, which hold numerous microfilariae in infected individuals. Upon reaching the stomach of the fly, microfilariae travel towards the thoracic muscles (Lehmann et al. 1995), where they develop (molt) into sausage shape juveniles. Another molting follows to become infective filariform juveniles. These infective juveniles travel to the labium (a mouthpart of the black fly) and can infect a new host (human) when the insect next feeds. In less than a year, mature worms are already noticeable in human skin.

African slaves probably introduced the parasite into the Americas during the boom of slave trade centuries ago. O. volvulus in the Americas has undergone numerous mutations within four centuries, which explains why clinical symptoms in American cases differ to that of the Africans. The parasites also adapted to have a high reproductive capacity.

Different species of black flies (Simulium spp) have different susceptibility to microfilariae infection; some are infected, while others are resistant because they have effective defense mechanisms (Ham et al. 1995). Furthermore, it is still a mystery why humans appear to be the only natural definitive hosts for O. volvulus.

References

Ham PF, Hagen HE, Baxter AJ, and J Grunewald. 1995. Mechanisms of resistance to Onchocerca infection in blackflies. Parasitology Today 11:63-67

Lehman T, Cupp SM, and WE Cupp. 1995. Chemical guidance of Onchocerca lienalis microfilariae to the thorax of Simuliam vittatum. Parasitology. 110:329-337.

Mehlorn Heinz and PM Armstrong. 2001. Encyclopedic Reference of Parasitology. Published by Springer.