An Overview of Onchocerciasis (River Blindness)

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Onchocerciasis, or river blindness, is a neglected tropical disease that can cause disfigurement and blindness. The parasitic worm responsible for the condition is spread from person to person through bites from infected blackflies, predominantly in remote agricultural areas with fast-moving rivers. While symptoms of the disease can be severe, the most serious cases are typically due to repeated infections over the course of several years.

At least 25 million people are currently infected with the parasite, nearly all of whom are in sub-Saharan Africa. Hundreds of thousands of people have been blinded by the disease, and many more are left with permanent damage to their skin or eyesight because of it. While river blindness remains a significant neglected tropical disease, huge strides have been made to reduce the number of new infections worldwide, especially in South America.

Symptoms

Not everyone with onchocerciasis will have symptoms, especially not right away. Most new infections won't start to show any symptoms for a year or two, and those with mild cases might never even know they've been infected with the parasite. While the most well-known symptom of onchocerciasis is blindness (thus the nickname), the disease affects both the skin and the eyes and can cause a range of symptoms, from irksome to disfiguring.

Symptoms Include:

  • Nodules under the skin where the adult worms are
  • Intense itchiness
  • Swelling
  • Changes in the skin, sometimes called “leopard” or “lizard” skin
  • Lesions on the eye
  • Altered vision
  • Blindness

It's important to note that it often takes more than one blackfly bite to get infected with onchocerciasis. The most serious and debilitating outcomes associated with the condition typically happen after years of repeated exposure to the parasite. The more infections a person experiences throughout their lifetime, the more likely the damage done to the eyes and skin becomes permanent or leads to blindness and disfigurement.

According to the World Health Organization, there are communities in West Africa where roughly half of all men over age 40 are blind due to onchocerciasis.

Causes

Onchocerciasis, or river blindness, is caused by the tiny parasitic worm O. volvulus, which is passed on to humans after being bitten by an infected blackfly. Most of the physical discomforts people experience when they have river blindness happen when the worms die, which can happen years after they first become infected.

Life Cycle of Onchocerca volvulus

People are infected with the worm’s larvae after being bitten by an infected blackfly. Once inside the body, the larvae mature into adult worms—a process that can take up to a year—and live in nodules just under the skin where they can spend the better part of a decade producing their offspring, or microfilariae. These microscopic and immature larvae live in the blood and are picked up by blackflies when the bugs bite someone who is infected.

Inside the flies, the larvae grow and morph several times over the course of two weeks, before they become infectious and finally make their way into the insect’s proboscis—that is, the long tube they use to eat. When the fly bites someone during a blood meal, the now infectious larvae hop a ride into the person’s body, starting the whole cycle over again.

Role of Blackfies

Blackflies are essential to the life cycle of the worm. Without the flies, the larvae can’t mature and become infectious. These flies live near fast-running rivers and streams near remote agricultural communities but can only spread the parasite in areas where humans are already infected.

Diagnosis

There are a few ways health care providers can test for and diagnose river blindness, most of which include looking for the larvae or adult worms in some capacity. These tests include:

  • Skin Snip Biopsy: The most common test used to diagnose onchocerciasis is through a skin snip. This procedure takes small shavings of the skin from different parts of the body and places them in saline for 24 hours to draw out the larvae. This makes it much easier to spot the parasite through a microscope. The biggest downside to this method is that it doesn't always catch the disease in people who have a light infection, like travelers who only briefly visited an area with onchocerciasis.
  • Removing and Examining Nodules: If nodules are under the skin, one or more can be surgically removed to see if adult worms are inside.
  • Slit-lamp Examination: This method uses a special microscope and lights—like the kinds used during a routine eye exam—to see to the back of the eye where larvae (and the damage they cause) might be lurking.
  • Antibody Tests: Some tests can detect if the body has responded to an onchocerciasis infection, but they can't tell the difference between past or current infections. For that reason, they aren't all that helpful in places where the parasite is common, though they can help diagnose the disease in people who have visited such areas. This test is also fairly rare outside of research settings.

    Because river blindness is most common in remote villages, health care providers might not be available to diagnose infections, and if they are, they might not have access to the tools needed to perform these diagnostic tests. For this reason, people are frequently treated for the parasite without first receiving an official diagnosis by a medical professional.

    Treatment

    Medications can kill the larvae and potentially worms inside the body, which can help limit symptoms and further damage. Treatment options include:

    Ivermectin

    The most common way to treat onchocerciasis is by using ivermectin, a common drug used to treat other parasitic infections like scabies and elephantiasis. It's dosed by weight and typically given by mouth every three to six months until there are no longer signs of an infection.

    The upside to this treatment is that it kills the larvae and sterilizes the adult females inside the body, stalling the life cycle of the parasite. The drawback is that the drug doesn't kill the adult worms themselves.

    Doxycycline

    A relatively new treatment possibility for river blindness is doxycycline. Doxycycline is an antibiotic—not an anti-parasitic medication—but research shows it kills bacteria that the adult worms need to survive.

    After six weeks of treatment, studies show the drug kills more than 60 percent of the adult females but not the larvae, suggesting it would need to be used in combination with ivermectin. The safety of this combination, however, is unclear. As a result, doxycycline isn't yet considered to be a standard treatment for the condition—but the research on it is promising.

    Preventive Methods

    The most serious symptoms of river blindness are the result of repeated exposure to the parasites. This is why preventing future infections is an important part of treatment. There's no vaccine or drug that can prevent an onchocerciasis infection, but there are things you and whole communities can do to lower the odds of getting one.

    • Personal Protection: The best way to prevent river blindness is to avoid blackfly bites. This means wearing bug spray with DEET, as well as long sleeves and long pants treated with permethrin during the day when the flies are most likely to bite. Protecting against blackflies has the added benefit of protecting against other disease-carrying insects, which can help prevent other serious tropical diseases like dengue fever.
    • Vector Control: One step countries are taking to reduce the number of cases of river blindness is by eliminating its vector: the black fly. The parasite can't spread in a community without the help of blackflies. Spraying the flies' breeding sites with insecticide can disrupt the life cycle of the parasite and stop new infections from happening in the area.
    • Mass Treatment Programs: Another way areas are working to prevent the parasite from spreading is by proactively treating everyone in a particular community with ivermectin, regardless of whether they've been diagnosed with the parasite. Not only does this help treat light infections that might not have been caught with diagnostic tests, but it also disrupts the life cycle of the parasite. Blackflies spread the larvae from person to person (not the adult worms), so by giving everyone in an area treatment to kill the larvae, the blackflies have nothing to pass along, and communities are able to stop new infections from happening for a while.

      A Word From Verywell

      Nearly all serious cases of river blindness are the result of repeated infections over long periods of time. Travelers only briefly visiting areas where the parasite is common are unlikely to get infected, and if they do get the disease, it probably won't lead to serious issues like blindness.

      That said, if you plan on being in an area with onchocerciasis for several months or years (ex. while serving in the Peace Corps or on a mission), be sure to take steps to prevent blackfly bites like wearing bug spray and long sleeves and pants to reduce your chances of infection. Likewise, if you've recently traveled to remote areas of sub-Saharan Africa, talk to your doctor right away if you start to experience any of the symptoms mentioned above.

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