Onchocerca volvulus

Onchocerca volvulus can be found mainly in West Africa, and also in Central and South America. Most likely, this species was originally only in Africa, and was introduced to the Americas with African slaves. There are two strains of this species, distinguishable at the DNA level by the O-150 PCR test. One strain is typically found in the savanna regions of West Africa and the Americas, while the other strain is commonly found in rain forest areas. (Ogunrinade, et al., 1999; Roberts and Janovy, Jr, 2000)

The habitat of Onchocerca volvulus is the body of its host. In the blackfly, microfilariae reside in the thoracic flight muscles. In humans, microfilariae can be found in the skin, lymph vessels, lymph nodes, blood, urine, cerebrospinal fluid, and internal organs (especially the eyes). Adults can be found knotted together in pairs or groups in the subcutaneous tissues. Due to host immunological reactions, these groups of worms often become encapsulated in fibrous tissue mainly composed of collagen. When these sites are located over bones such as a joint or skull, a conspicuous nodule called an onchocercoma appears, which is generally about 3 cm in diameter. Nodule location depends on geographical area, with most infections in Africa exhibiting nodules below the waist (especially on the knees and pelvic area), and those in Central America exhibiting nodules above the waist (especially on the neck and head). This relationship is based on the biting preferences of the blackfly vector, with African flies preferring to bite below the waist, and Central American flies preferring to bite above the waist.

Onchocerca volvulus will generally be found causing disease in savanna and rain forest areas, although occasionally it can be found in arid savannas and desert areas as well. (Burnham, 1998; Roberts and Janovy, Jr, 2000)

Adults of Onchocerca volvulus are long and slender, have a smooth cuticle, and have blunt anterior and posterior ends. There are no lips or buccal capsule, and the mouth is surrounded by 2 circles made up of four papillae each. The esophagus doesn't appear to be divided. Males range in length from 19-42 cm and range in width from 130-210 micrometers. Lacking alae, their tails are curled ventrally and bear 6 or 8 pairs of postanal and 4 pairs of adanal papillae. Females are larger than males, ranging in length from 33.5-50 cm and in width from 270-400 micrometers. Their vulva is directly behind the posterior end of the esophagus. The microfilariae released by adult females are 250-300 micrometers long, are unsheathed, and have sharply pointed and curved tails. (Burnham, 1998; Roberts and Janovy, Jr, 2000)

Onchocerca volvulus has a complex life cycle involving a definitive and intermediate host. The only natural definitive host is humans. The intermediate host is a blackfly in the genus Simulium. Common blackfly hosts include Similium damnosum and S. neavei in Africa and S. ochraceum, S. metallicum, S. callidum, and S. exiguum in the Americas. Adult worms typically live from 8-15 years in human hosts, and microfilariae can live up to 2 years. Adults can be found grouped together in various regions of the human body, with groups containing about half as many males as females. The males move from group to group, inseminating the females. Fertilized eggs develop into live microfilariae in 3-12 weeks, and the females release the microfilariae into the host body. A single female worm may release 1300-1900 microfilariae per day for 9-11 years. The microfilariae migrate away from the adult worms, through blood vessels, lymphatics, skin and connective tissue, and generally settle in the skin.

Since the mandibles of the blackfly are not useful for deep piercing, a blackfly feeding on an infected human host will ingest blood and tissue fluid containing microfilariae. Once ingested, the microfilariae are attracted to the fly's thoracic flight muscles, and travel there. In 6-12 days, the microfilariae then develop into juvenile stage 1 (J1), molt into juvenile stage 2 (J2), and molt into juvenile stage 3 (J3), which is the infective, filariform stage. J3 worms then migrate to the labium of the blackfly host, and can infect new human hosts when the female fly next feeds on blood. Upon gaining entry into a human host, the J3 worms develop into adults in 1-3 months. After initial infection, microfilariae from the adults will appear in 10-20 months. (Burnham, 1998; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)

Adult worms typically live from 8-15 years in human hosts, and microfilariae can live up to 2 years. Adults can be found grouped together in various regions of the human body, with groups containing about half as many males as females. The males move from group to group, inseminating the females.

Females may produce a phermomone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the female during copulation. Nematode sperm are amoeboid-like and lack flagella.

Fertilized eggs develop into live microfilariae in 3-12 weeks, and the females release the microfilariae into the host body. A single female worm may release 1300-1900 microfilariae per day for 9-11 years. (Barnes, 1987; Burnham, 1998; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)

There is no parental investment beyond the time the female releases the microfilariae.

Microfilariae concentrate themselves in the areas of the body where their insect vector prefers to bite. They migrate all over the human body, often entering the eye. (Roberts and Janovy, Jr, 2000)

Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). When mating, females may produce a pheromone to attract males. (Barnes, 1987)

Adults receive nutrients either from ingesting blood, or from absorption via a highly convoluted cuticle. It is common for rich vascular networks to develop surrounding groups of worms (the worms probably stimulate vessel formation), and these vessels bring the worms in close proximity with blood and nutrients. (Burnham, 1998)

These animals are probably not preyed on directly but are ingested. Egg and larval mortality are high since the parasites often do not reach appropriate hosts.

Onchocerca volvulus has a complex life cycle involving a definitive and intermediate host. The only natural definitive host is humans. The intermediate host is a blackfly in the genus Simulium. Common blackfly hosts include Similium damnosum and S. neavei in Africa and S. ochraceum, S. metallicum, S. callidum, and S. exiguum in the Americas. (Burnham, 1998; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)

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Onchocerca volvulus is the pathogen responsible for the disease onchocerciasis, or river blindness. Infecting at least 17.7 million people worldwide, this non-fatal but severely debilitating disease is endemic in 37 countries (30 in Africa, 6 in the Americas, and Yemen). The pathology of onchocerciasis is based on host immune responses to the adult worm and the microfilariae, with the microfilariae being much more immunogenic. Little inflammation is caused by live microfilariae, but dead and degenerating microfilariae in the skin cause severe dermatitis, intense itching, skin depigmentation, skin thickening, skin discoloration, cracking of skin, and loss of skin elasticity. Quite often, microfilariae migrate to the eye, and their death causes intense inflammation. Over years this sclerosing (scarring) keratitis hardens the cornea and causes blindness. Worldwide, onchocerciasis impairs the vision of 500,000 people, and causes complete blindness in another 270,000. In some communities, the frequency of visual impairment is as high as 30%, and the frequency of blindness can be more than 10%. Additionally, elephantiasis may result (from microfilarial infestation of the lymphatic system), along with associated hernias. Furthermore, this disease results in decreased worker productivity, and the skin disfiguration can reduce marital prospects, reduce dowry size, and disrupt social relationships. (Burnham, 1998; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)

Onchocerciasis is called "river blindness" because the intermediate host of Onchocerca volvulus, the blackfly, lives its egg, larval, and pupal stages in fast-running streams and rivers. This makes the flies most abundant near rivers, and makes areas near rivers the most likely place to get infected.

There are a number of methods for controlling onchocerciasis. First, there is the practice of excision of onchocercomas (especially those around the head) in order to reduce the rate of visual impairment and the number of new infections. Secondly, chemotherapy with the drug ivermectin has proved very successful. Donated in 1987 by Merck & Co., Inc. for mass treatment in developing nations, ivermectin is tolerated by humans, and a single oral dose given annually eliminates skin microfilariae, interrupts the transmission cycle by suppressing adult female microfilariae release, improves skin disease, and eventually kills adult worms. Thirdly, larvicides have been used to reduce the blackfly vector population. Programs such as the African Program for Onchocerciasis Control (APOC) and the West African Onchocerciasis Control Program (OCP) have used these control methods to combat the disease. (Hougard, et al., 1997; Molyneux and Davies, 1997; Richards and Hopkins, 2000; Roberts and Janovy, Jr, 2000)