Onchocerciasis – how flies can cause ‘river blindness’

By William Craig

Onchocerciasis, also known as ‘river blindness’, is a neglected tropical disease (NTD) mainly prevalent in areas of Africa, but also found in Latin America, Yemen and Asia. It is caused by the parasitic worm Onchocerca volvulus, and is transmitted to humans through the bites of infected blackflies from the Simulium genus usually found near rivers or wetlands, hence the name ‘river blindness’. WHO (2019) estimated that in 2017 there were 20.9 million cases of onchocerciasis worldwide, with over 1.5 million of those cases resulting in visual impairment or blindness. Symptoms include severe itching, skin conditions and a range of visual impairment problems, including permanent blindness (WHO, 2019).

Onchocerca volvulus infect blackflies by entering their midgut and migrating to their thoracic muscles, where they reside as microfilariae (a precursor to the worm’s larvae) and eventually develop into larvae. The larvae will then occupy the blackfly’s head and proboscis (nose) until it enters the human body. A female blackfly needs human blood for ovulation, so when they feed on humans, the Onchocerca volvulus enter the human body through the bite wound. The larvae inhabit the subcutaneous layer of tissue under the skin, and develop into adult filariae. The adult filariae then migrate to and occupy the nodules, or abnormal tissue growths, of the subcutaneous tissue layer, where then can live for up to 15 years. Female adult worms can grow up to 50 cm in length and measure from 270 to 400 μm in diameter, while males are much smaller at 19 to 42 mm in length and 130 to 210 μm in diameter. The mix of female and male worms in nodules leads to the reproduction of Onchocerca volvulus, and females are capable of producing microfilariae for about 10-15 years. This abundance of microfilariae, which have a lifespan of about 12-15 months, means that they can easily be found blood, urine and even sputum, a mix of saliva and mucus from the respiratory tract (CDC, 2019).

Symptoms of onchocerciasis are mainly caused by the body’s response to the abundance of dead or dying Onchocerca volvulus larvae. It is very rare for people to not observe symptoms once infected, and in most cases, symptoms include itchy skin rashes, lumps under the skin caused by the nodules, and changes in vision. The inflammation caused by the dead larvae under the skin can change the colour of the skin, leading to a ‘leopard-like’ appearance. It can also cause the skin to thin out, which contributes to a loss in elasticity, causing the skin to have a ‘cigarette paper-like’ quality. The inflammation caused by larvae that die in the eye cause lesions, or damage to the tissue on the cornea. Without treatment, this can progress into permanent clouding of the cornea, leading to eventual blindness. There is often damage to the tissue in the optic nerve, which transfers visual information from the retina in the eye to the brain. Therefore, this causes vision loss as electrical impulses are weakened and signals are lost between the eyes and the brain, resulting in vision loss and permanent blindness (CDC, 2019).

Other symptoms of onchocerciasis can be quite severe, and include hernias and a condition known as ‘hanging groin’. A hernia is the predisposition of a ‘hanging groin’, and is formed when an internal organ is pushes through its surrounding tissue, causing a swelling or lump in the skin. The damage to the tissue nodes by the dead larvae can cause hernias to form around the bowel and intestinal regions. These enlarged and inflamed nodes can then cause the skin around it to become inelastic and fold, and it protrudes to turn into a sac of atrophied skin that hang from the groin area (Stanford, 2018).

The standard treatment for onchocerciasis is a medication called ivermectin, which is commonly used to treat parasite infections. Ivermectin kills Onchocerca volvulus microfilariae by blocking postsynaptic glutamate-gated chloride ion channels, inhibiting electrical transmission and paralyzing the parasitic worm. It also enhances the effect of immune responses from the host against Onchocerca volvulus, and also has a significant oncogenic effect on female adult worms. Onchocerciasis requires constant ivermectin treatment every 3 months over the span of a year before the effects of the medication can be seen, such as clearing of skin lesions. However, an adverse effect of mass ivermectin treatment is that it is transforming the genetics of Onchocerca volvulus to make the worm clinically resistant to the medication, and it may be only a matter of time before Onchocerca volvulus becomes almost fully resistant to ivermectin. Aside from abnormal skin reactions, the main side effect of ivermectin treatment on the human body is a decrease in the number of eosinophils in the peripheral blood. According to Udall (2007), ivermectin treatment correlated with an increase in sequestration and degranulation of eosinophils, meaning that there was increase in eosinophil-derived neurotoxin in the blood plasma (Udall, 2007).

References:

WHO (2019), Onchocerciasis, Available from: https://www.who.int/news-room/fact-sheets/detail/onchocerciasis [Accessed 2nd October 2020]

CDC (2019), Onchocerciasis – Biology, Available from: https://www.cdc.gov/parasites/onchocerciasis/biology.html, [Accessed 2nd October 2020]

CDC (2019), Onchocerciasis – Disease, Available from: https://www.cdc.gov/parasites/onchocerciasis/disease.html [Accessed 2nd October 2020]

Stanford (2018), Onchocerciasis Clinical Presentation, Available from: https://web.stanford.edu/group/parasites/ParaSites2006/Onchocerciasis/Clinical%20Presentation.html [Accessed 2nd October 2020]

Udall, D N. (2007), Recent Updates on Onchocerciasis: Diagnosis and Treatment, Available from:

https://academic.oup.com/cid/article/44/1/53/433399 [Accessed 2nd October 2020]

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