NAME: Onchocerca volvulus.
CHARACTERISTICS: O. volvulus is a filarial nematode with a five-stage life cycle: 4 larval stages (microfilariae), and an adult stage (macrofilariae) Footnote 9, Footnote 11. Females are 30 cm to 80 cm in length, whereas males are only 3 cm to 5 cm long appear to migrate from nodule to nodule to inseminate the females Footnote 2, Footnote 11. Female macrofilariae can remain sessile and live coiled-up in subcutaneous tissue (forming a “nodule” or “onchocercoma”) for up to 15 years, producing more than 700-1,500 microfilariae per day Footnote 13. Microfilariae measure 220 to 360 μm in length and can survive in humans for 2 to 3 years Footnote 5.
PATHOGENICITY/TOXICITY: Onchocerciasis is a chronic systemic illness associated with extensive and disfiguring skin changes, musculoskeletal complaints, weight loss, and changes in the immune system Footnote 3. The principal organ affected in onchocerciasis is the skin; however, infection with O. volvulus can lead to severe visual impairment and blindness Footnote 5, Footnote 7, Footnote 11. In Sub-Saharan Africa, onchocerciasis is the second most common cause of preventable blindness, and has caused visual impairments in 500,000, and blindness in 270,000 persons Footnote 3, Footnote 4. The pathogenesis of onchocerciasis is believed to be largely due to an immune reaction to dying or dead microfilariae that have localised in the skin and eyes Footnote 2, Footnote 3. In heavily infected persons, 100,000 or more microfilariae can die every day Footnote 3. There are 5 main categories of onchocercal skin disease: acute papular onchodermatitis, chronic papular onchodermatitis, lichenified onchodermatitis, atrophy, and depigmentation Footnote 7.
Acute papular onchodermatitis: Primarily affects the face, small pruritic papules may be scattered on limbs, trunk, shoulders and extremities. Lesions may progress to vesicles and pustules Footnote 7.
Chronic papular onchodermatitis: Often affects the shoulders, buttocks and extremities and consists of a severely itchy maculopapular rash containing scattered flat-topped papules and hyperpigmented macules Footnote 7.
Lichenified onchodermatitis: Consists of hyperkeratotic and hyperpigmented confluent plaques most often affecting the lower extremities and associated with lymphadenopathy Footnote 7.
Atrophy: Typically affecting the buttocks and lower back, and consisting of large atrophic plaques with finely wrinkled inelastic skin resembling cigarette paper Footnote 7.
Depigmentation: Often referred to as leopard skin and consists of vitiligo-like lesions with hypopigmented patches containing perifollicular spots of normally pigmented skin. Onchocercal depigmentation often affects the shins and is rarely associated with itch and excoriations Footnote 7.
Other clinical pictures include; “lizard skin” with dry ichthyoses-like lesions with a mosaic pattern resembling the scales of a lizard, and “hanging groin” with folds of atrophic inelastic skin in the inguinal region associated with lymphadenopathy Footnote 7. The different clinical patterns are not mutually exclusive and may be present simultaneously or one pattern may evolve into another.
Onchocercal ocular disease: The disease ranges from mild symptoms such as itching, redness, pain, photophobia, diffuse keratitis, and blurred vision, to more severe symptoms such as corneal scarring, night blindness, intraocular inflammation, glaucoma, visual field loss, and, eventually, blindness Footnote 7. Ocular lesions are usually bilateral and can affect various structures of the anterior and posterior segments of the eye resulting in uveitis, iridocyclitis, conjunctivitis, chorioretinitis, cataracts and glaucoma Footnote 5, Footnote 11. Increasing corneal opacity is not caused by O. volvulus but is believed to be from a host inflammatory response to its endosymbiotic Wolbachia bacteria, which are released by dying microfilariae Footnote 14.
EPIDEMIOLOGY: O. volvulus infection is estimated to affect more than 17 million people worldwide in 34 countries in Africa, the Middle East, South America and Central America Footnote 2, Footnote 3, Footnote 5, Footnote 8, Footnote 9. Countries with the highest prevalence of onchocerciasis include 11 sub-Saharan West African nations, including Ghana, Nigeria, Liberia, and parts of Mali Footnote 1, Footnote 9. The endemicity also extends latitudinally across the entire continent of Africa and into Southwest Asia, with foci in Yemen and Oman in the Arabian Peninsula Footnote 5, Footnote 9. Small foci also exist in Ecuador, Venezuela, Columbia, southern Mexico, and Guatemala. In West Africa, there are 3 major strains of O. volvulus: a forest strain with low ocular pathogenicity associated with high nodule numbers and severe skin disease, a dry savannah strain with high ocular pathogenesis and an associated high rate of blindness, and a humid savannah strain with an intermediate pattern Footnote 2, Footnote 3.
INFECTIOUS DOSE: Unknown.
MODE OF TRANSMISSION: O. volvulus is spread by blackflies (genus Simulium), which breed in fast-flowing rivers Footnote 2, Footnote 8, Footnote 9, Footnote 15. Infection occurs during the bloodmeal of a blackfly, which results in the dissemination of an O. volvulus larva (at stage 3 of life cycle) into the host Footnote 9. Female worms stay permanently in a fibrous capsule, while males are able to move freely in skin tissues and subcutaneous regions.
COMMUNICABILITY: No evidence for direct human-to-human transmission Footnote 1; however, infection can be spread indirectly between humans via blackfly bites Footnote 2, Footnote 3, Footnote 5, Footnote 9. People can infect flies as long as living microfilariae occur in the skin.
ZOONOSIS: No, the Onchocerca species found in animals cannot infect humans Footnote 1.
VECTORS: Blackfly (Simulium spp.) Footnote 1-Footnote 6, Footnote 6, Footnote 8, Footnote 11, Footnote 15. In Africa, the Simulium damnosus sensu lato (s.l.) species complex is the main vector and is responsible for more than 95% of onchocerciasis cases worldwide Footnote 3, Footnote 6. In Uganda, Tanzania, Ethiopia, and the Congo, the vectors are the S. neavei s.l. complex, and S. albivirgulatum s.l. is the vector that is only found in the Congo Basin Footnote 3. In Latin America, S. ochraceum s.l., S. exiguum s.l., S. metallicum s.l., and S. guianense s.l. are the main vectors Footnote 3, Footnote 6.
DRUG SUSCEPTIBILITY: Microfilariae are susceptible to ivermectin, diethylcarbamazine, and albendazole, although diethylcarbamazine is no longer used due to adverse side effects Footnote 1-Footnote 4, Footnote 6, Footnote 8, Footnote 9. Adult worms are susceptible to suramin, which is no longer used due to extreme toxicity, in favour of a new drug, moxidectin, which is safe for use in humans and has already undergone phase II trials Footnote 2, Footnote 9. Symbiotic Wolbachia spp. present within O. volvulus are sensitive to doxycycline treatment Footnote 15. Although the use of ivermectin against infection has proven to be effective, constant and regular treatment is necessary as studies have found live and possibly fertile worms present in patients even after 6 years of treatment Footnote 16.
SUSCEPTIBILITY TO DISINFECTANTS: Unknown. In a study testing the efficacy of Rifampin and Azithromycin on O.volvulus, the instruments used to obtain skin biopsies from patients were disinfected with a combination of disinfectants, which included the sequential use of full-strength bleach (30 seconds), 95% ethanol (30 seconds), distilled water (30 seconds), 95% ethanol (30 seconds), and then the instruments were air dried Footnote 17.
PHYSICAL INACTIVATION: Unknown; however, air drying, in combination with several common disinfectants, has been used clinically to disinfect instruments that have come into contact with O.volvulus Footnote 17.
SURVIVAL OUTSIDE HOST: Unknown.
SURVEILLANCE: Onchocerciasis should be considered in persons from endemic areas or expatriate visitors who present with itching with or without a rash Footnote 3. Onchocerciasis can be diagnosed based on the observation of living microfilaria in skin biopsies Footnote 2, Footnote 3, Footnote 5, Footnote 9, Footnote 10. Bloodless “skin snips” can be obtained from the shins, buttocks, and the iliac crests, and then placed in saline and examined microscopically for the presence of microfilariae Footnote 2, Footnote 3, Footnote 5, Footnote 9, Footnote 10. Microscopic demonstration of microfilaria may not be sensitive, and, therefore, methods such as PCR, ELISA, and immunolabelling are used to detect microfilaria in skin snips with greater sensitivity Footnote 1-Footnote 3, Footnote 9, Footnote 10. These methods still utilize skin snips which can be painful and increase the risk of blood borne contaminations, furthermore non-invasive tests, such as the dipstick assay, may prove to be more useful for the diagnosis of onchocerciasis Footnote 2, Footnote 10. Microfilariae can also be detected in the anterior chamber of the eye with a slit lamp Footnote 3, Footnote 5. Adult worms may be found in excised nodules.
FIRST AID/TREATMENT: The drug of choice for the treatment of onchocerciasis is ivermectin (Stromectol, Mectizan) Footnote 1-Footnote 3, Footnote 5, Footnote 6, Footnote 8, Footnote 9, Footnote 12, Footnote 15. The drug is administered once per year and for the expected lifespan of the parasite (often more than 10 years) Footnote 2, Footnote 3, Footnote 5, Footnote 6. Co-treatment with ivermectin and doxycycline (for 6 weeks) has been shown to be more effective, since doxycycline will interrupt the embryogenesis of O. volvulus for a few months by depleting symbiotic Wolbachia endobacteria Footnote 15. Another treatment, based on removal of nodules (nodulectomy, particularly from the head) is available in Mexico and Guatemala and is thought to reduce the number of microfilariae that can enter the eye and hence limit the number of cases of blindness Footnote 2, Footnote 3.
PROPHYLAXIS: Although no chemoprophylaxis exists, measures such as insect nets and protective clothing, insecticide, and larvicides are used in endemic regions Footnote 1-Footnote 3, Footnote 6, Footnote 8, Footnote 10. Onchocerciasis Elimination Program for the Americas – ivermectin q6m to prevent disease and interrupt transmission.
LABORATORY-ACQUIRED INFECTIONS: None reported to date.
PRIMARY HAZARDS: Accidental autoinoculation Footnote 18.
RISK GROUP CLASSIFICATION: Risk Group 2.
CONTAINMENT REQUIREMENTS: Containment Level 2 facilities, practices and containment equipment for activities involving the infective stages of the parasite and potentially infectious tissues or body fluids Footnote 18.
PROTECTIVE CLOTHING: Lab coat. Gloves when direct skin contact with infected materials or animals is unavoidable Footnote 18. Eye protection must be used where there is a known or potential risk of exposure to splashes.
OTHER PRECAUTIONS: All procedures that may produce aerosols, or involve high concentrations or large volumes should be conducted in a biological safety cabinet (BSC). The use of needles, syringes, and other sharp objects should be strictly limited. Additional precautions should be considered with work involving animals or large scale activities Footnote 18.
SPILLS: Allow aerosols to settle and, wearing protective clothing, gently cover spill with paper towels and apply an appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up Footnote 18.
DISPOSAL: Decontaminate all wastes that contain or have come in contact with the infectious organism by autoclave, chemical disinfection, gamma irradiation, or incineration before disposing Footnote 18.
STORAGE: The infectious agent should be stored in leak-proof containers that are appropriately labelled Footnote 18 .
REGULATORY INFORMATION: The import, transport, and use of pathogens in Canada is regulated under many regulatory bodies, including the Public Health Agency of Canada, Health Canada, Canadian Food Inspection Agency, Environment Canada, and Transport Canada. Users are responsible for ensuring they are compliant with all relevant acts, regulations, guidelines, and standards.
UPDATED: September 2011
PREPARED BY: Pathogen Regulation Directorate, Public Health Agency of Canada.
Although the information, opinions and recommendations contained in this Pathogen Safety Data Sheet are compiled from sources believed to be reliable, we accept no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information. Newly discovered hazards are frequent and this information may not be completely up to date.
Public Health Agency of Canada, 2011