Onchocerciasis (River Blindness)

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.

              Onchocerciasis (River Blindness)

What is onchocerciasis?

Onchocerciasis, also known as river blindness, is a disease that affects the skin and eyes. It’s caused by the worm Onchocerca volvulus.

Onchocerca volvulus is a parasite. It is transmitted to humans by the bite (blood meal) of the female blackfly (Simulium). This type of blackfly is found near rivers and streams. That’s where the name “river blindness” comes from. Although the majority of people infected reside in sub-Saharan Africa (for example, Nigeria), it also occurs in Central and South American countries and Yemen but much less frequently. After trachoma, it is the second-leading cause of infection-caused blindness worldwide.

                       Black fly

Causes

The cause of onchocerciasis is the transfer of larvae of the parasite Onchocerca volvulus by the female blackfly when the fly gets a blood meal (bites) a human. The larvae enter the subcutaneous tissues and develop into adult male and female worms (filarial nematodes). These reproduce in the human tissue and form microfilariae that migrate to other areas of connective tissue and occasionally to the blood, urine, and sputum. In addition, both the adult worms and the microfilaria are colonized with bacteria termed Wolbachia that help these parasites survive. When the worms die, a host immune response ensues that can destroy optical tissue in the eye. The life cycle of the parasites is continued when a blackfly bites a human and, during its blood meal, ingests microfilariae. The complex life cycle is shown below.

Risk Factors 

Living in and visiting sub-Sahara Africa and the few other areas where the blackfly is endemic is a major risk factor for developing onchocerciasis. The disease usually occurs after repeated exposures to blackfly bites so short-term travelers through these areas pose little risk; however, missionaries, volunteer health workers, and others who may spend a few months in the areas have an increased risk of infection.

Is onchocerciasis contagious?

Onchocerciasis is not contagious person to person. Transmission of the disease is through the bite of female blackflies (usually occurring during the daytime near rapidly flowing rivers and streams). Multiple bites are usually needed before being infected.

Symptoms

The symptoms and signs of onchocerciasis are as follows:

•  Skin inflammation that is very itchy and forms papules on the skin

•  Nodules in the skin (subcutaneous nodules or bumps)

•  Scarred, saggy, or drooping areas of skin

•  Patchy skin depigmentation (leopard skin)

•  Lymph node inflammation (lymphadenitis)

•  Eye (ocular) lesions (eye itching, redness, or swelling)

•  Visual problems (visual impairment and/or inability to distinguish certain colors, partial or complete blindness)

•  Eosinophilia (unusually high levels of eosinophils in the blood)

•  "Sowda" is a term used to describe the severe itching and skin discoloration (darkening), that is often confined to one limb, that can be found with onchocerciasis.

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.

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:

1.  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.

2.  Removing and Examining Nodules: If nodules are under the skin, one or more can be surgically removed to see if adult worms are inside.

3.  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.

4.  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.

Treatment

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

1.  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.

2.  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.

1.  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.

2.  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.

3.  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.

Conclusion

•  Onchocerciasis is a parasitic disease that may cause blindness. It occurs mainly in sub-Saharan Africa and a few isolated areas in the Americas (Brazil, Colombia, Guatemala, Mexico, and Venezuela) and is transmitted by blackfly bites.

•  It is called river blindness as the blackfly that transmits the disease lives and breeds near fast-flowing rivers.

•  Symptoms include skin changes, itching, nodules, and alterations in vision.

•  Symptoms may not appear for months to years.

•  Onchocerciasis is diagnosed by finding the parasites in skin nodules, skin biopsies, or skin scrapings.

•  The treatment is the antiparasite drug ivermectin, given once or twice a year for about 10-15 years; the antibiotic doxycycline may also be utilized.

•  The major risk factor for onchocerciasis is residing or having extended visit time in areas where blackflies are endemic.

•  There are no vaccines or drugs that prevent onchocerciasis. Avoiding areas where blackflies are endemic and the use of personal protection items (clothing, insect repellents) reduce the chance of infection.

•  The earlier that onchocerciasis is diagnosed and treated, usually the better is the prognosis.