Onchocerciasis: causes, symptoms, diagnosis, and treatment

Onchocerciasis, also known as river blindness, is a parasitic disease caused by onchocerca volvulus parasitizing the eye or subcutaneous tissue.


Causes

Onchocerca volvulus adults are filiform, white, with attenuate and blunt ends. Females are 33.5 - 50mm x 0.27 -0.40mm, and males are 19 - 42mm x 0.13 - 0.21mm in size. Microfilariae are highly mobile and without sheath. Microfilariae have two sizes, the large ones are 285 - 368μm x 6 - 9μm, and the small ones are 150 - 287μm x 5 - 7μm. When the intermediate host Simulium bite the infected humans, the microfilariae enter the body of Simulium, where they develop into infective larvae. When Simulium containing infective larvae bite humans, the larvae enter the human body through the wound, and develop into adults in about one year in the human body. Adults can live generally for 8 - 10 years, up to 15 years, and microfilariae can survive for 3 - 5 years.


Signs and Symptoms

Microfilariae enter the eye, causing mechanical damage. The toxic secretions and dead microfilariae can cause inflammation, resulting in corneal lesions, particularly corneal opacity. Microfilariae can invade deep eyeballs, causing inflammation of the iris, ciliary body, retina, and choroid. The invasion of optic nerve may lead to blurred vision, and total blindness may occur in severe patients.

Adult worms parasitize the lymph nodes in the subcutaneous tissue, causing local inflammation. Fibrous nodules occur on the skin. The nodules are 2 - 25mm in diameter, indurated, round or long, painless, sharply demarcated, and can be distributed in various parts of the body. Each nodule contains two or more adult worms and many microfilariae. The toxic effect of microfilariae can cause severe dermatitis, mostly manifested by rash on the face, neck, and shoulders. There are severe pruritus and pigmentation in the early stage, and there are thickening, lichenification, discoloration, cracking, inelasticity of the skin in the late stage.

The lymph nodes can be swollen, solid, and painless, and contain microfilariae. Fibrotic lymph nodes can cause inguinal hernia, scrotal hydrocele, and external genital elephantiasis.

Microfilariae can damage directly or indirectly the pituitary gland, leading to nanism.

Microfilariae can also infect the urogenital tract, lung, liver, spleen, and other organs.

Microfilariae can cause eosinophilia in the peripheral blood.


Diagnosis

If there are asymmetric pruritus, acute skin rash, and swelling of the extremities in the epidemic area, the disease should be suspected.

If there are microfilariae found in the cornea, aqueous humor, or skin, the disease can be definitively diagnosed.


Treatment

The treatment regimen is ivermectin 150mcg/kg orally once every 6 - 12 months. Ivermectin is contraindicated in pregnant women, patients with severe central nervous system diseases or acute diseases, children under 5 years of age, individuals with body weight less than 15kg, and lactating women within 1 week after delivery.

Diethylcarbamazine can be administered in patients contraindicated to ivermectin, and the dose is 0.5mg/kg once a day on the first day, 0.5mg/kg twice a day on the second day, followed by 2mg/kg 3 times a day for 10 days if there are not severe adverse reactions.

Suramin can also be considered, and the dose is 0.5g initially, followed by 1g/week, not exceeding 4.5g totally.