Onchocerciasis


Onchocerciasis is the world's second leading infectious cause of blindness.

The blackfly, Simulium damnosum, the vector of Onchocerca volvulus, the parasite which causes onchocerciasis (river blindness).

A patient infected suffering from river blindness. This elderly man shows nodules, skin changes and blindness, all manifestations of the disease.

Fast-flowing water, a typical breeding site for larvae of the blackfly

 


Distribution

35 countries in total.


Causative agent

A parasitic worm, Onchocerca volvulus, of the family filariidae, which lives in the human body for up to 14 years.

Onchocerca volvolus is a helminthic worm

  • male: 2-3 cm long
  • female: 60 cm long
  • Adults occur in the subcutaneous tissues and in nodules
  • microfilariae: 300 x 8 micrometer, 1000-3000 produced per day per adult female worm
  • Adult worms have a longevity of 10-15 years

Onchocerca volvulus (adult worms)


The vector and transmission of the disease

The vector is Simulium damnosum or blackfly.

Fast-flowing water, a typical breeding site for larvae of the blackfly, Simulium damnosum, the vector of Onchocerca volvulus, the parasite which causes onchocerciasis (river blindness).

Via the bite of infected blackflies (Simulium spp.) that carry immature larval forms of the parasitic worms from human to human.


The disease

Each adult female worm produces millions of microscopic larvae (microfilariae) (1000-3000 per day), that migrate throughout the body to cause a variety of symptoms.

Symptoms

Adult worms lodge in nodules under the skin, releasing large numbers of microfilariae into surrounding tissues. Immature worms move through the body and after dying, cause a variety of conditions including:

Nodules caused by the adult worms

A patient infected with onchocerca volvulus, the parasite which causes onchocerciasis (river blindness). This elderly man shows nodules, skin changes and blindness, all manifestations of the disease.

Onchocerca volvulus: nodules (removed from under the skin of infected people) contain the adult worms.

Eye infections by the microfilaria larvae

Ocular lesions in an onchocerciasis (river blindness) victim blinded from sclerosing Keratitis.

This woman has leopard skin and onchocercal skin lesions on both legs.


Diagnosis


Prevention and control

Catching adult Simulium blackflies, the vectors of the parasites which cause onchocerciasis, using a man as bait.

Medical doctor using an ultrasound machine to examine a male patient for signs of nodules and worms.

Drug treatment: the development of ivermectin in the 1980s provided a safe, effective drug for killing microfilariae in infected people. A single dose of the drug (being supplied free by the manufacturer, Merck & Co. Inc.) needs to be taken annually.

Drugs:

  • Diethylcarbamazine (DEC) only kills microfilaria
  • Ivermectine (150 microgram/kg single dose) only kills microfilaria
  • Suramin only kills macrofilaria

Masindi district: A Community-directed distributor (CDD) dispensing ivermectin tablets to a young girl during a central point distribution in her village


Onchocerciasis Control Programme in West Africa (OCP)

Vector control: insecticide spraying to control blackflies has proved successful in certain areas.Temephos as larvicide is effective but resistance develops

Helicopters and fixed wing aircraft are used to spray insecticides (chemical and biological) on rivers and fast-flowing water

OCP, a major control initiative

OCP's principal method for controlling onchocerciasis involved interrupting transmission by eliminating the blackfly vector. Simulium larvae are killed by applying insecticides via aerial spraying over breeding sites in fast-flowing rivers. Following interruption of transmission, the reservoir of adult worms dies out in humans after 14 years. To complement vector control activities, OCP also distributes ivermectin.


African Programme for Onchocerciasis Control (APOC)

Following the success of the OCP, the same co-sponsors and donors created APOC in 1995.


All pictures are reproduced with permission from WHO/TDR

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Last updated: 28 November 2002.

created by :Fred Opperdoes