Onchocerciasis


Onchocerciasis

Description, Causes and Risk Factors:

Alternative Name: River blindness, blinding disease, and volvulosis.

Onchocerciasis is the world's fourth leading cause of preventable blindness. 500,000 of those infected with onchocerciasis are severely visually impaired, and another 270,000 have been rendered permanently blind from the disease. Worldwide onchocerciasis is second only to trachoma as an infectious cause of blindness.

Onchocerciasis is caused by the parasitic worm Onchocerca volvulus. It is transmitted through repeated bites by blackflies of the genus Simulium. The disease is also called River blindness because the blackfly that transmits the infection lives and breeds near fast-flowing streams and rivers and the infection can result in blindness.

Infected flies carry a larvae form of the filarial parasitic worm O. volvulus (Onchocerca volvulus) from person-to-person. Larvae enter the skin at the black fly bite site and form nodules in the subcutaneous tissue where they mature. Adult females release millions of microscopic larvae - called microfilariae - into the surrounding tissue. The larvae move throughout the body and, when they die, contribute to a variety of health conditions including skin rashes and depigmentation (“leopard skin”), debilitating itching, and skin nodules. Visual impairment and blindness can also occur.

The people most at risk for acquiring onchocerciasis are those who live 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.

Symptoms:

    Debilitating skin rashes and lesions.People with onchocerciasis can have several hundred nodules on their skin. The nodules vary in size from one to five centimeters in diameter. They can cause discomfort, but are not usually painful.

  • Itching and depigmentation of the skin.

  • Serious visual impairment and 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 when it is put in physiologic solutions (e.g. normal saline). Typically 6 snips are taken from different areas of the body. Polymerase chain reaction (PCR) of the skin can allow for diagnosis if the larvae are not visualized.

In patients with nodules in the skin, the nodule can be surgical 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 US. 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 to onchocerciasis.

Treatment:

People who are found to be infected with O. volvulus should be treated in order to prevent the long-term skin damage and blindness. The treatment of choice for onchocerciasis is ivermectin (Mectizan®). Ivermectin paralyses and kills microfilariae, relieving intense skin itching and halting the progression towards blindness. It also prevents adult worms from producing more microfilariae for a few months following treatment, so reduces transmission. Before any treatment is begun, however, you need to make sure that you are not also infected with Loa loa (loiasis), another filarial parasite 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.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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