Onchocerciasis

• Onchocerciasis also known as river blindness, is a disease caused by infection with the parasitic worm –Onchocerca volvulus.
• It is the second most common cause of blindness due to infection, after trachoma.

Transmission

The parasite worm is spread by the bites of a black fly (Simulium fly).

These flies live near rivers therefore the name of the disease.

Signs and symptoms

Skin

• Intense itching
• Swelling
• Acute papular onchodermatitis – scattered pruritic papules
• Depigmentation – ‘leopard skin’ appearance, usually on anterior lower leg
• Scaly, wrinkled skin (Lizard’s skin)
• Hanging groin
• Skin nodules
• Lymphadenopathy

Ocular involvement

• Provides the common name associated with onchocerciasis, river blindness
• Sclerosing Keratitis
• Iridocyclitis
• Corneal opacities
• Blindness in extreme cases

Others

• Onchocerciasis has been associated with an unusual form of epilepsy and Nodding Disease Syndrome.
• Growth retardation
• Elephantiasis of scrotum 

Life cycle

1. A Simulium female black fly takes a blood meal on an infected human host, and ingests microfilaria.
2. The microfilaria enter the gut and thoracic flight muscles of the black fly, progressing into the first larval stage.
3. The larvae mature into the second larval stage, and move to the proboscis and into the saliva in its third larval stage. Maturation takes about seven days.
4. The black fly takes another blood meal, passing the larvae into the next human host’s blood.
5. The larvae migrate to the subcutaneous tissue and undergo two more molts. They form nodules as they mature into adult worms over six to 12 months.
6. After maturing, adult male worms mate with female worms in the subcutaneous tissue to produce between 700 and 1,500 microfilaria per day.
7. The microfilaria migrate to the skin during the day, and the black flies only feed in the day, so the parasite is in a position for the female fly to ingest it. Black flies take blood meals to ingest these microfilaria to restart the cycle.

Diagnosis

• From history and examination
• Skin snip (biopsy)
• Blood for microfilaria, microscopy
• Immune diagnosis- it looks at hypersensitivities
• Excision of nodules for adult worms

Treatment

• Ivermectin (Mectizan); infected people can be treated with two doses of ivermectin 150mcg/kg, six months apart, repeated every three years.
• Antibiotics: doxycycline is used to kill the Wolbachia bacteria that live in adult worms.

Prevention and control

• Ivermectin treatment of affected people
• Prophylaxis treatment with Evermectin in endemic areas (mass treatment)
• Destruction of simulium flies with insecticides or river dosing
• Health education about settlement along rivers