Treating River Blindness

Onchocerciasis Can Be Treated, Perhaps Even Eradicated, But Not Yet

Jul 9, 2007 Rosemary Drisdelle

A few drugs have been used for treating river blindness, but none so far has proved to be the answer to curing onchocerciasis and eradicating this parasitic disease.

The parasitic worm that causes river blindness, Onchocerca volvulus, was first described in 1893 and the relationship between the worm and blindness was recognized by 1915. A treatment for the parasitic disease, however, was elusive. Today, the worm still infects millions, and confounds health organizations.

The biggest obstacle in treating and controlling river blindness is that most infected people live in remote villages and other small communities in underdeveloped countries. The victims of O. volvulus are poor and have limited access to health care. Because infected people are a continuing source of infection in others, a control program requires widespread treatment. Because everyone can’t realistically be treated in a health care facility, the drug used has to be safe, inexpensive, and easy to transport, distribute, and administer. Those attempting a treatment program must contend with poor roads, lack of infrastructure, tropical climates, cultural differences, language barriers, and sometimes war.

No drug to date has been the perfect answer to treating river blindness, but a handful of drugs and other treatments have been widely used and have brought some relief.

Surgical removal of worms

Surgical procedures to remove worms are relatively simple and help to reduce the risk of blindness, especially if the worms are encapsulated on the head. However, some adult O. volvulus worms remain migratory in the skin and so do not become encapsulated in a nodule. As well, some nodules are located in areas where they are not obvious and may be missed. Thus, surgical removal of worms does not cure the disease and may not even have much effect on the numbers of young (microfilariae) in the skin.

Diethylcarbamazine (DEC)

Diethylcarbamazine, usually referred to as DEC, was discovered in 1948. Diethylcarbamazine kills microfilariae but does not kill adult worms, making continued treatment necessary. It has been a useful treatment for other filarial diseases (diseases caused by species of worms that produce microfilariae); however, in onchocerciasis, the drug causes serious side effects, making its widespread use impossible. Side effects tend to be worst in people who are heavily infected, and include headache, fever, a dangerous drop in blood pressure, damage to vision, and an itchy skin rash. Treatment of onchocerciasis with DEC must be carefully monitored.

Suramin

Suramin is another drug that has been used to treat human parasites—in onchocerciasis it kills both adult worms and microfilariae. Like DEC, however, suramin has serious side effects, notably kidney damage, when used in onchocerciasis. Suramin must be administered by intravenous injection making it a poor candidate for widespread treatment. It is sometimes used in combination with DEC to achieve a complete cure—a risky treatment course.

Ivermectin

Ivermectin is currently the drug of choice for treating Onchocerca volvulus. It is easy to distribute, has few side effects, and although it does not kill adult worms, it kills microfilariae and prevents adult females from producing microfilariae. To halt the disease and prevent transmission, all that is needed is a yearly dose of Ivermectin—if the treatment is continued until adult worms die, the patient is cured. Donated in 1987 by Merck and Co. Inc. to treat millions infected worldwide, ivermectin has been our greatest hope of a drug to eradicate onchocerciasis; however, as often happens when a single drug is used to treat a parasitic disease, O. volvulus is apparently becoming resistant to ivermectin. Some adult worms continue to produce microfilariae after exposure to ivermectin, making a 2020 goal for eradication unlikely—unless we find a new drug.

Related content:

What is River Blindness?

Eliminating River Blindness.

Sources:

Clinical Parasitology 9th ed. Beaver, Paul Chester, Rodney Clifton Jung, and Eddie Wayne Cupp. Philadelphia: Lea and Febiger, 1984.

Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.

Prevalence and Intensity of Onchocerca volvulus infection and efficacy of ivermectin in endemic communities in Ghana: a two-phase epidemiological study. Osei-Atweneboana M Y, J K Eng, D A Boakye et al. Lancet (2007); 369(9578): 2021-9.

The copyright of the article Treating River Blindness in Microbiology is owned by Rosemary Drisdelle. Permission to republish Treating River Blindness in print or online must be granted by the author in writing.