River blindness, or onchocerciasis 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 worldwide are currently infected with the parasite, nearly all of whom are in sub-Saharan Africa. Small foci exist in Yemen, southern Mexico, Guatemala, Ecuador, Colombia, Venezuela, and the Brazilian Amazon. Blindness due to river blindness or onchocerciasis is fairly rare in North America. 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.
Although river blindness remains a significant neglected tropical disease, huge strides have been made to reduce the number of new diagnosed infections worldwide.
Causes and Risk Factors
The distribution of river blindness or onchocerciasis is linked to the location of blackflies which are naturally found close to the fast-running streams and rivers in the inter-tropical zones.
River blindness, or onchocerciasis, is caused by the tiny parasitic worm Onchocerca 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.
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.
Symptoms and signs
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 is done to the eyes and skin becomes permanent or leads to blindness and disfigurement.
The symptoms and signs of river blindness or 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.
Diagnosis of River Blindness
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.
Can It Be Treated?
Yes, river blindness can be treated. Medications can kill the larvae and potential worms inside the body, which can help limit symptoms and further damage.
Medications like Ivermectin reduces microfilariae in the skin and eyes and decreases production of microfilariae for many months. It does not kill adult female worms, but cumulative doses decrease their fertility. Ivermectin is given as a single oral dose of 150 mcg/kg, repeated at 6- to 12-month intervals. The optimal duration of therapy is uncertain. Although treatment could theoretically be continued for the life span of female worms (10 to 14 years), it is often stopped after several years if pruritis has resolved and no evidence of microfilariae is detected by skin biopsy or eye examination.
Doxycycline can kill the endosymbiont bacteria Wolbachia, which O. volvulus requires for survival and embryogenesis. Doxycycline kills > 60% of adult female worms and sterilizes or decreases the fertility of those that survive. An alternative regimen includes one dose of ivermectin 150 mcg/kg, followed in 1 week by doxycycline 100 mg orally once or twice a day for 6 weeks; ivermectin is then continued at 6- to 12-month intervals as above. It is unclear whether the likely adherence difficulties of this regimen would result in improvement on the results of ivermectin-only treatment.
Surgical removal of accessible river blindness onchocercomas can reduce skin microfilaria counts, but it has been replaced by ivermectin therapy.
How To Prevent River Blindness
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 the 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.
- Msdmanual : River blindness
- Medicinenet : what causes onchocerciasis
- Verywellhealth : River blindness
- WHO : ochocerciasis disease information
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