River blindness is a disease that is caused by parasites. The scientific name for this disease is called Onchocerciasis. It derived this huge name because it is caused by the parasite Onchocerca volvulus. The parasite does not get into human beings on its own. Just like malaria, which has mosquitoes to carry the parasite to humans when it feeds on your blood. Onchocerca volvulus is carried by a biting insect, a blackfly from the genus (Simulium) which bites during the day.
The name River blindness was given to this disease because the blackfly that transmits this parasite lives or breed along fast-flowing rivers and streams in rural areas and infection from Onchocerciasis can lead to blindness.
Onchocerca volvulus is the name of the parasite that causes this infection. They are a type of minute worms that cannot be seen with naked eyes unless through a microscope. These worms are dioecious meaning, they have both male and female types which mate to release microfilariae. The males move in groups inseminating (inserting their sperms into) the females. The female is usually bigger than the male and can release about 500-1000 microfilariae(minute larvae) within a day. The adult worm can live in the human being for up to 15 years. Incredible, right?
It is these microfilariae that are taken by the blackfly, then introduced into humans when the blackfly bites humans. Thus, the parasite requires an intermediate host(blackfly) and a definitive host(humans) to cause an infection.
Did you know that in 2017, River blindness affected about 20.9 million people globally? Out of this number, 1.15 million people lost their sight? Furthermore, 99% of all infections were from African countries with Ghana being inclusive?
Blackflies just like, all living organisms need to eat and reproduce. They lay microfilariae(minute larvae) which later hatch through different stages to become an adult blackflies. However, they need protein from human blood to develop their eggs and continue with their reproductive cycle. Not all blackflies carry the parasite. They become infected when they suck blood from a person who already has the disease-Onchocerciasis.
The parasite is usually at its basic stage when the blackfly ingests them. The microfilariae(minute larvae) move from the gut of the blackfly into the thoracic muscles where it develops into larvae (micro larvae). It then migrates to the head and into the proboscis (biting mouthpart of an insect). At this stage, a blackfly is infected and ready to spread the larvae into humans when it feeds on the human’s blood. It should be noted that the parasite has 3 main cycles:
Microfilariae- Microlarvae- Adult worm
which is completed in 2 hosts: blackfly and humans.
Stages of infection:
1. An infected blackfly bites a person, takes a blood meal and in the process deposit the larvae of the parasites in the skin. The larvae find their way through the bite wound.
2. The larvae enter the subcutaneous tissue of the skin.
3. The larvae develop into an adult and reside in nodules. The adult is made up of both the male and female parts which mate to form microfilariae. It takes about 12-18 months for the larvae to develop into adults.
4. Adults can live in the nodules for approximately 15 years. Some nodules may contain numerous male and female worms. Females measure 33 to 50 cm in length and 270 to 400 µm in diameter, while males measure 19 to 42 mm by 130 to 210 µm. In the subcutaneous nodules, the female worms are capable of producing microfilariae(eggs) for approximately 9 years. The microfilariae typically live in the skin but occasionally be present in urine, bloodstream and sputum.
5. The infection spreads when a blackfly bites an infected person on the skin and become infected.
6. After ingestion, the microfilariae migrate from the blackfly’s midgut through the hemocoel to the thoracic muscles
7-8. The microfilariae develop into larvae.
9. The third-stage infective larvae migrate to the blackfly’s proboscis and can infect another human when the fly takes a blood meal.
Symptoms of River blindness
River blindness affects both the eyes and the skin. This is because the microfilariae(minute larvae) will travel to other organs in the body. As it does, the immune system tries to fight it when they die. It is the response of the immune system that causes the symptoms. Sometimes, the immune system will not respond to them thus no symptoms will be observed.
· The immune system tries to clear them usually when they die and by so doing, it causes itching and rashes on the skin. The adult worm can also form nodules.
· The inflammation in the skin as a result of the immune system getting rid of the microfilariae can cause long-term damage to the skin. Examples are colour changes in the skin “leopard” skin appearance.
. Also, loss of elasticity that gives a “cigarette-paper” appearance.
· Furthermore, the inflammation caused by microfilariae that spread to the eye and die leads to lesions on the cornea. This situation can be reversed if medical treatment is sought early. If that this does happen, it will cause permanent damage leading to blindness.
· Lastly, inflammation of the optic nerve can lead to vision loss and eventually blindness.
People who are at risk
It has to be noted that, a person does not get the infection by just a single bite. One must be repeatedly bitten by an infected blackfly of the genus Simulium to become infected.
People found in Sub-Saharan Africa are at an increased risk. Regardless, it is only individuals from these areas who live near fast-flowing rivers and streams where there are Simuliumblackflies. The parasite can also be found in limited areas in South America and Yemen.
Visitors who travel into such regions such as researchers, missionaries and volunteers in such regions may become infected.
The best way is to abstain from such regions but that may not be realistic and practical. The insects bite only during the day. When visiting such areas, one can put on insect repellant creams and wear protective coverings. There are no vaccines or medications to prevent a person from becoming infected. But some drugs can control the disease once a person becomes infected. Quickly, visit a healthcare facility when you suspect this infection.
· In a nutshell, Onchocerciasis popularly known as river blindness is a disease of the eye and the skin. Onchocerciasis is transmitted to humans through exposure to repeated bites of infected blackflies of the genus Simulium. It can cause disfiguring symptoms on the skin and blindness. Always visit the nearest healthcare when you suspect this infection.
1. World Health Organization. Onchocerciasis. Retrieved from https://www.who.int/news-room/fact-sheets/detail/onchocerciasis.
2. Centers for Disease Control and Prevention. Parasites - Onchocerciasis. Retrieved from https://www.cdc.gov/parasites/onchocerciasis/index.html .
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