Onchocerciasis

Other Names: River blindness; Robles' disease Onchocerciasis, or river blindness, is a neglected tropical disease (NTD) caused by the parasitic worm Onchocerca volvulus. It is transmitted through repeated bites by blackflies of the genus Simulium. The disease is called river blindness because the blackfly that transmits the infection lives and breeds near fast-flowing streams and rivers, mostly near remote rural villages. The infection can result in visual impairment and sometimes blindness. Additionally, onchocerciasis can cause skin disease, including intense itching, rashes, or nodules under the skin. Worldwide onchocerciasis is second only to trachoma as an infectious cause of blindness.

Epidemiology

The Global Burden of Disease Study estimated that in 2017 there were at least 20.9 million people are infected worldwide, of which 14.6 million had skin disease and 1.15 million had vision loss.

Risk Factors

• The people most at risk for acquiring onchocerciasis are those who live or work near streams or rivers where there are Simulium blackflies.
• Most of the areas where the blackflies are found are rural agricultural areas in sub-Saharan Africa.
• Onchocerca infections are found in tropical climates. The main burden is in 31 countries in sub-Saharan Africa.
• The parasite is also found in limited areas in South America and in Yemen in the Middle East.
• In the Americas, four countries have been verified by the World Health Organization (WHO) as free from onchocerciasis: Colombia, Ecuador, Mexico, and Guatemala. Only a single small transmission zone remains in South America, crossing the border between the Bolivarian Republic of Venezuela and Brazil.

Cause

Onchocerciasis is the disease caused by the nematode (roundworm) Onchocerca volvulus when it inhabits subcutaneous tissues.

symptoms

Some people do not experience symptoms while infected with O. volvulus, as the larvae can migrate through the human body without provoking a response from the immune system. But many people do have symptoms, which include itchy skin rashes, nodules under the skin, and vision changes. There can be non-painful swelling of lymph glands, but this is not common. Most symptoms of onchocerciasis are caused by the body’s response to dead or dying larvae (also called microfilariae). The inflammation caused in the skin, in addition to causing itching, can result in long-term damage to the skin. This can cause changes in the color of the skin that result in a “leopard skin” appearance, and can cause thinning of the skin with loss of elasticity that gives the skin a “cigarette-paper” appearance and can contribute to conditions such as “hanging groin.” The inflammation caused by larvae that die in the eye results initially in reversible lesions on the cornea that without treatment progress to permanent clouding of the cornea, resulting in blindness. There can also be inflammation of the optic nerve resulting in vision loss, particularly peripheral vision, and eventually blindness.

Diagnosis

The diagnosis of onchocerciasis can be difficult in light infections, which are more common in persons who have travelled to but are not residents of affected areas. There are multiple ways that the diagnosis can be made:

The most common method of diagnosis is the skin snip. A 1 to 2 mg shaving or biopsy of the skin is done to identify larvae, which emerge from the skin snip and can be seen under a microscope when the shaving or biopsy (“skin snip”) is put in physiologic solutions (e.g. normal saline). Typically six snips are taken from different areas of the body.

Polymerase chain reaction (PCR) of the skin snip can allow for diagnosis if the larvae are not visualized. In patients with nodules in the skin, the nodule can be surgically removed and examined for adult worms. Infections in the eye can be diagnosed with a slit-lamp examination of the anterior part of the eye where the larvae, or the lesions they cause, are visible. Antibody tests have been developed to test for infection, though they are not widely available in the United States. These tests cannot distinguish between past and current infections, so they are not as useful in people who lived in areas where the parasite exists, but they are useful in visitors to these areas. Some of the tests are general tests for infection with any filarial parasite and some are more specific for onchocerciasis.

Treatment

People who are found to be infected with O. volvulus should be treated in order to prevent long-term skin damage and blindness. The recommended treatment is ivermectin, which will need to be given every 6 months for the life span of the adult worms (i.e., 10–15 years) or for as long as the infected person has evidence of skin or eye infection. Ivermectin kills the larvae and prevents them from causing damage but it does not kill the adults.

There is a promising treatment using doxycycline that kills the adult worms by killing the Wolbachia bacteria on which the adult worms depend in order to survive. If you are infected, it is possible that your doctor will want to treat you both with ivermectin and with doxycycline.

Before any treatment is begun, however, you need to make sure that you are not also infected with Loa loa, another filarial parasite found in central Africa that is sometimes found in the same areas where O. volvulus is found, because Loa loa can be responsible for severe side effects to the medications used to treat onchocerciasis. The medication(s) listed below have been approved by the Food and Drug Administration (FDA) as orphan products for treatment of this condition.

• Moxidectin (Brand name: Moxidectin)approved for the treatment of onchocerciasis due to Onchocerca volvulus in patients aged 12 years and older.

Prevention & Control

There are no vaccines or medications available to prevent becoming infected with O. volvulus. The best prevention efforts include personal protection measures against biting insects. This includes wearing insect repellant such as N,N-Diethyl-meta-toluamide (DEET) on exposed skin, wearing long sleeves and long pants during the day when blackflies bite, and wearing permethrin- treated clothing.

NIH genetic and rare disease info

• NIH info on Onchocerciasis

Onchocerciasis is a rare disease.

Onchocerciasis Resources
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