Onchocerciasis is an infectious disease that can cause blindness and permanent skin damage.
The source of onchocerciasis is a parasitic worm called Onchocerca volvulus.
A person can contract this parasite after sustaining a bite from an infected blackfly. These insects exist mainly in Africa and some parts of Central and South America.
This article outlines the symptoms of onchocerciasis and describes how a person may acquire it. It also provides information on treating and preventing onchocerciasis.
Onchocerciasis is a skin and eye infection that can cause blindness. It is caused by the parasitic worm O. volvulus. People can contract O. volvulus as a result of sustaining a bite from a blackfly that is infected with the parasite.
Infected blackflies live and breed in rural areas, close to fast-flowing rivers or streams. As a result, onchocerciasis is commonly known as river blindness.
Onchocerciasis tends to affect countries with the following characteristics:
- high levels of poverty
- limited access to healthcare
- a lack of clean water
- poor sanitation
Blackflies are bloodsucking insects that feed on humans and other mammals. A blackfly that is infected with O. volvulus may bite a person and deposit the parasitic worm larvae onto their skin. These larvae can then enter the skin through the bite wound.
The O. volvulus larvae live in nests, or “nodules,” beneath the skin, where they develop into adult worms. Each worm may live for up to 15 years.
Adult female worms produce up to 1,000 larvae per day. These larvae move through the body to the skin and eyes. Most of the larvae die inside their human host, triggering an inflammatory reaction that can cause skin lesions or blindness.
A person may not always develop symptoms of onchocerciasis.
However, those who do develop symptoms may experience issues with their skin or eyes. The following sections will discuss the possible symptoms in more detail.
The rash associated with onchocerciasis can vary from person to person. There are different patterns of skin damage associated with the disease, and a person may have more than one.
These patterns of damage are as follows:
- An itchy rash that resembles eczema: A person may have small, itchy lumps that can develop into pus-filled blisters. The rash may affect the following areas:
- the face
- the trunk
- the hands
- the feet
- An itchy rash with raised, dark patches: This type of skin damage may develop across the following areas:
- the hands
- the feet
- the buttocks
- the shoulders
- Thick, scaly, and dark patches: These skin patches may develop across the legs and feet.
- Areas of thin, dry, and wrinkled skin: This type of skin damage may affect the lower back and buttocks.
- Lumps beneath the skin: Adult worm nests may resemble lumps beneath the skin. These nests, or nodules, vary in size, with some measuring up to several centimeters across.
- Patchy loss of skin pigment: The skin may lose pigment in places, resulting in a patchy “leopard skin” appearance. Pigment loss usually occurs around the hair follicles and often affects the shins.
Worms that die inside the eye can trigger inflammation in that area. This inflammation can lead to increased pressure inside the eye, which may damage the nerves that enable vision.
In some cases, a person may develop a filmy tissue across the cornea at the front of the eye. This affects vision and can lead to blindness.
O. volvulus is the parasitic worm that causes onchocerciasis. Its life cycle depends on its human host and on the blackflies that transmit it from one host to the next.
The life cycle of O. volvulus begins inside a human host. It consists of the following four stages:
- Stage 1: Young parasitic worms called microfilariae move toward the skin of their human host. A blackfly bites the human host, contracting the microfilariae.
- Stage 2: While inside the blackfly, the microfilariae develop into larvae. This process takes around a week.
- Stage 3: The infected blackfly bites a person, transferring the worm larvae into their body. Here, the larvae develop into adult worms in a process that takes 6–12 months. Swollen nodules form around the adult worms.
- Stage 4: Adult worms live inside the body for up to 15 years. Adult females produce microfilariae, which may survive inside their human host for 12–15 months. Most die inside the body, triggering an inflammatory immune reaction. Depending on the location of the worms within the body, this inflammatory reaction may cause skin or eye damage.
Receiving treatment for onchocerciasis can prevent long-term issues, such as blindness and permanent skin damage.
A doctor may prescribe the antiparasitic medication ivermectin to kill the worm larvae and prevent them from causing further damage.
However, the medication will not kill adult worms or prevent them from reproducing. Because of this, a person will need to take the medication every 6 months for 10–15 years, which is the potential lifespan of the parasite.
To kill the adult worms, a doctor may also prescribe doxycycline off-label. Prescribing a drug off-label means using it in a way that differs from its approved use. This antibiotic essentially starves the worms to death by killing the bacteria they feed on.
Before prescribing a treatment for onchocerciasis, a doctor will run tests to check that the person does not have a separate parasite called Loa loa. This parasite can cause severe side effects to the medications that doctors use for treating onchocerciasis.
Without treatment, onchocerciasis can lead to permanent blindness. Worldwide, onchocerciasis is the second most common cause of blindness due to infection.
Untreated onchocerciasis may also result in permanent skin damage and scarring.
The people most at risk of developing onchocerciasis are those who live in countries that the disease primarily affects. People who live or work near rivers with infected blackflies are particularly at risk.
A person usually only develops this disease after sustaining several blackfly bites. Therefore, residents and travelers in affected countries for long periods of time have a higher risk of developing onchocerciasis than those who visit for a short period.
For this reason, travelers with the highest risk include:
- field researchers
Receiving early treatment for onchocerciasis can limit or prevent damage to the skin and eyes.
A person should see a doctor if they develop any symptoms of onchocerciasis and they live in or have traveled to an area where the disease is prevalent.
There are a few tests for onchocerciasis. A person may receive more than one of the following tests:
- Skin snip test: A doctor will remove several small shavings of skin from different parts of the body. They will then inspect the shavings under a microscope to check for the presence of worm larvae.
- Removal of nodules: If skin nodules are present, a doctor may remove and examine them to check for the presence of worms.
- Slit lamp examination: A slit lamp is a device that shines a thin sheet of light into the eye. This allows a doctor to view the eye in great detail. The slit lamp examination will help them detect any larvae or lesions within the eye.
- Antibody tests: A doctor may request a blood test to check for the presence of antibodies. These are proteins that the body makes to inhibit or destroy infectious pathogens. The presence of certain antibodies can show whether or not a person currently has onchocerciasis or has previously had the disease.
To best prevent onchocerciasis, a person should try to protect themselves from blackfly bites.
The Centers for Disease Control and Prevention (CDC) recommend that people take the following precautions when traveling to countries where blackflies are common:
- wearing protective clothing that fully covers the arms and legs
- wearing clothing treated with the insecticide permethrin
- applying an insect repellent, such as diethyltoluamide, to exposed areas of skin
Programs that aim to eliminate onchocerciasis focus on making ivermectin available to people in communities where the disease is prevalent.
Ivermectin kills the young parasitic worms before they can reproduce. If enough people in a community take this medication, it can end the disease in that community.
Onchocerciasis is an infectious disease caused by the parasitic worm O. volvulus. This worm is transmissible to humans through bites from infected blackflies.
People who develop onchocerciasis require long-term treatment, usually with a combination of ivermectin and doxycycline. Without prompt treatment, the disease can lead to permanent skin damage and blindness.
Most cases of onchocerciasis occur in sub-Saharan Africa, with a smaller number of cases now occurring in South America and the Arabian Peninsula.
When traveling to one of these areas, a person should try to wear protective clothing and insect repellent to reduce the risk of blackfly bites.