What is onchocerciasis?

Definition

Onchocerciasis is a parasitic infestation by filarial worms (
Onchocerca volvulus) that affects persons in Africa, Latin America, and the Arabian Peninsula. Worm larvae enter the body from black-fly bites, causing nodular skin swelling that may progress to a harmful eye disease known as river blindness. Almost one-half of the adult population in the West African savanna has some visual impairment caused by onchocerciasis, the second-leading infectious cause of blindness worldwide.

Causes

Black flies (genus
Simulium) feed on the blood of infected people and ingest microfilariae, the embryos of worms, which then mature into larvae in the gut of the fly within seven days. The larvae are then deposited into other persons through fly saliva. The larvae develop into adult worms then live and reproduce in firm nodules in the subcutaneous and deeper layers of skin. Adult worms produce numerous microfilariae, which travel from the parent nodule and move throughout the skin. The presence of the microfilariae, dead and alive, causes the body to have a powerful immune response, leading to a severe inflammatory reaction that damages surrounding skin and eye tissue.

Risk Factors

Persons who live in Africa, Latin America, and the Arabian Peninsula near
streams and rivers, the breeding habitat for the black fly, are at the greatest
risk for developing onchocerciasis.

Symptoms

Symptoms of onchocerciasis may not appear until three to fifteen months after
infection. Early indicators include skin nodules that contain two or more adult
worms. The migration of microfilariae causes a severe rash and painful, hot, or
swollen skin. Lymph nodes in the neck and groin can become enlarged. Chronic
infection may lead to thickened, pigmented, or depigmented skin, often in a lizard
or leopard pattern. If the microfilariae migrate to the eye, the body’s
immune
system responds by destroying the eye tissue, leading to
deteriorating vision. People who have been infected with only a few larvae may not
experience any noticeable symptoms at all.

Screening and Diagnosis

Diagnosis of onchocerciasis is commonly made by analysis of skin snips that
contain microfilariae or by excision of the nodule containing adult worms. A
dipstick test for the presence of an antigen in urine or tears has been
developed; however, results do not distinguish between current and past
infection.

Treatment and Therapy

In 1986, the World Health Organization (WHO) worked collaboratively with
pharmaceutical company Merck to develop ivermectin, the most effective treatment
for onchocerciasis. Ivermectin is administered in two doses, six months apart,
every three years. Community directed treatment programs have been established to
provide treatment at no cost. Damage done to skin and eyes cannot be reversed, but
treatment prevents further deterioration by killing the microfilariae. Although
treatment does not kill the adult worms, it prevents them from reproducing.

Prevention and Outcomes

WHO has initiated the several prevention programs, including the Onchocerciasis
Control Programme, the African Programme for Onchocerciasis Control, and the
Onchocerciasis Elimination Program for the Americas. Preventive measures by these
programs include spraying of insecticides to prevent black-fly
breeding in affected areas and distributing ivermectin treatment to persons and
communities in need.

Bibliography

Stingl, Peter. “Onchocerciasis: Developments in Diagnosis, Treatment, and Control.” International Journal of Dermatology 48 (2009): 393-396.

Taylor, Mark, et al. “Lymphatic Filariasis and Onchocerciasis.” The Lancet 376 (2010): 1175-1185.

World Health Organization. Special Programme for Research and Training in Tropical Diseases. “Eliminating River Blindness.” Available at http://www.who.int/tdrold/publications/publications/ elimin_riverblind.htm.