Onchocerciasis (River Blindness)
- Onchocerciasis (also called river blindness or filariasis) is a disease caused by the nematode (worm) Onchocerca volvulus.
- Onchocerciasis is a significant cause of blindness globally, and especially in sub-Saharan Africa.
- It often presents with pruritus (itching), with or without a rash, but can progress to permanent skin damage.
- Because the incubation period can take up to 3 years, onchocerciasis may present in children and youth several years after their arrival in Canada.
- Look for this disease in newcomer children and youth, especially those coming from sub-Saharan Africa.
- Diagnosis is by a skin snip and slit lamp examination for eye involvement, or by serology.
- Treatment is with ivermectin, which is given every 6 to 12 months for an extended period of time. Consultation with an infectious disease specialist is recommended.
Onchocerciasis (also known as river blindness) is a filiarial disease caused by the nematode (worm) Onchocerca volvulu. Infection can result in a systemic illness that commonly affects the skin and eyes. Disease is transmitted by infected blackflies of the Simulium species.1 Adult female O. volvulus can live under the skin in a human host for up to 15 years, producing many prelarval microfilariae2 which can survive for 2 to 3 years. Most consequences of this infection relate to the immune response to dead or dying microfilaria in the skin and eyes. A heavy microfilarial load can result in intense itchiness of the skin that can become severely excoriated and disfiguring. Microfilaria in the eyes can cause visual impairment and, if left untreated, lead to blindness.
More than 17 million people worldwide are estimated to be infected with O. Volvulus,2 making it the second-most common infectious cause of blindness, after trachoma. The prevalence in young newcomers to Canada is not known. The U.S. Centers for Disease Control and Prevention have detailed information on the life cycle of Onchocerca.
The vast majority of infections occur in sub-Saharan Africa, with a small number of cases in the Middle East (notably Yemen) and in Central and South America.3 Figure 1 shows high risk regions.
People who live in rural areas near fast-flowing rivers and streams in an endemic region (especially Sub-Saharan Africa) are at highest risk of infection.3
|Figure 1. Distribution of onchocerciasis|
|Source: Reproduced, with the permission of the publisher, from “Onchocerciasis (river blindness) - disease information”, Geneva, World Health Organization: www.who.int/blindness/partnerships/onchocerciasis_disease_information/en/, accessed May 5, 2014.|
While onchocerciasis is not commonly seen in newcomers or travellers from endemic areas, as a preventable cause of blindness and other serious morbidities it should be suspected in patients with a compatible travel history and the following symptoms:2,4
- Pruritus (itchiness) with or without rash. The rash can lead to severe excoriation and hypertrophy of the skin. Secondary bacterial skin infections are common.
- Nodules under the skin.
- Non-painful swelling of lymph glands (uncommon).
- Ocular disease ranges from mild symptoms (e.g., itching, redness, pain, photophobia, diffuse keratitis and blurred vision) to severe morbidities (such as decreased visual fields or blindness).
Symptoms usually develop 1 to 2 years after infection; however, the incubation period can be highly variable.2 Some infected individuals are asymptomatic.4 Without treatment, reversible lesions on the cornea can lead to permanent clouding of the cornea and blindness. Inflammation of the optic nerve can also lead to vision loss and blindness.4 Long-term damage to skin can result in scattered rashes, hyperpigmentation (which may be associated with edema), skin atrophy (thinning of the skin with loss of elasticity) and depigmentation (which can look like “leopard skin”, especially on the lower legs).4 Persistent and severe itchiness can contribute to chronic insomnia or depression.
Visit Vision Screening for more information on assessing young immigrants and refugees new to Canada.
Diagnosis is usually made by serology at the National Parasitology Laboratory in Montreal, Quebec. However, a definitive diagnosis can also be made by observing live microfilaria in a stained skin snip biopsy, or by more sensitive methods, such as polymerase chain reaction (PCR) or enzyme-linked immunosorbent assay (ELISA) testing or immunolabelling.2 An adult worm can be detected by ultrasound or by excision of a nodule. If ocular onchocerciasis is suspected, an ophthalmology consultation should be requested. Slit lamp examination to detect microfilariae in the anterior chamber of the eye is another diagnostic.
Treating onchocerciasis is crucial to prevent long-term skin damage and blindness,4 and consulting with an infectious diseases specialist in all cases is recommended.
Patients with onchocerciasis infection are at risk of coinfection with another, less common, filarial parasite, loa loa, which is endemic to many regions of West and Central Africa. It is important to rule out the presence of loa loa, because treatment with ivermectin with loa loa coinfection can result in encephalopathy.5
Infection can be treated using ivermectin (200µg/kg),1,2 which kills larvae and prevents adult worms from producing more microfilariae for a few months following treatment. However, ivermectin does not kill the macrofilariae (adult worms) and does not cure infection. It does control the disease when administered at intervals of every 6 to12 months for the expected lifespan of the parasite, which is often more than 10 years.2 In Canada, ivermectin is obtained through the Special Access Programme of Health Canada. The safety of treating children who weigh less than 15 kg with ivermectin has not been demonstrated.1,4 In most cases, doxycycline is added to treat another organism, Wolbachia, that lives within onchocercia cells, promoting survival of the parasite.
At present, there is no vaccine against onchocerciasis. Personal protective measures to prevent insect bites should be used when visiting endemic regions.2 More information about such precautions can be found in the Travel-related Illness section.
- WHO. Onchocerciasis
- Public Health Agency of Canada. Onchocerca volvulus. Pathogen safety data sheet — infectious substances. Ottawa, ON: PHAC; 2011.
- USAID. Onchocerciasis
- Centers for Disease Control and Prevention. Onchocerciasis
- Centers for Disease Control and Prevention. Parasites –Onchocerciasis (also known as River Blindness): Resources for health professionals.
- Anna Banerji, MD
Last updated: March, 2018