River blindness (onchocerciasis) is a filarial disease and the second most common cause of infective blindness in the world. It is caused by Onchocerca volvulus, microfilaria which is predominantly found in the high flow rivers in Sub-saharan Africa, Yemen, and Central America.
Pathogenesis
The filarial worm onchocerca is transmitted by the bite of an infected black fly (Simulium). The worm takes up to 4 months to mature and they can survive for more than 15 years in a human host. They usually live just underneath the skin. A lot of worms can accumulate under the skins and they can also invade the eyes. The dead worms releases a variety of allergens under the skin which may cause severe allergic reaction.
Clinical features
Symptoms start about a year after the initial infection. Some common skin symptoms are:
- Itching
- Allergic reactions
- Skin swelling
- Subcutaneous nodules
Some common eye symptoms are
- Conjunctival irritation
- Keratitis
- Corneal opacities
- Iridocyclitis
- Choroiditis
- Glaucoma
- Optic atrophy
- Blindness (river blindness)
Diagnosis
The clinical features are very obvious for endemic area doctors. Therefore, they often diagnose it by history and clinical features. Some of the useful tests are:
- Full blood count can show a high eosinophil count.
- Visualization of microfilaria in the eye by slit-lamp examination.
- Visualization of adult worms in a biopsy of skin nodules.
- Skin snips from the iliac crest or scapular region are taken and then allowed to stand in saline for 4 hours. Then the microscopy shows onchocerca wriggling free on the slide.
- Diethylcarbamazine is applied to the skin under an occlusive dressing. This will provoke an allergic rash.
- Serological and PCR tests are rarely used.
Treatment
- Ivermectin single dose is very effective against microfilariae. It also prevents further infection for up to 6 months. But adult worms are not easily killed. Therefore, annual Ivermectin treatment is required. Sometimes adult worms can be surgically removed.
- Ivermectin should be used cautiously in a patient with loa loa coinfection due to the risk of fatal encephalitic reaction.
- Doxycycline is an Ivermectin alternative in treating onchocerciasis.
