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Onchocerciasis: causes, symptoms, diagnosis, treatment
Medical expert of the article
Last reviewed: 04.07.2025
Onchocerciasis development cycle
Onchocerciasis is transmitted when a person is bitten by the midge Simuhum. The final host is a person, the intermediate host (vector) is the blood-sucking midges of the genus Simulium, which live along the banks of rapids, clean, fast-flowing rivers and streams. Coastal vegetation serves as a daytime habitat for midges. Midges attack people during daylight, the coolest time of day: from 6 to 10 am and from 4 to 6 pm. They bite mainly the lower limbs. During the day, when the air temperature is at its highest, midges are less active.
The life cycle of onchocerciasis is similar to the life cycles of other filariae. When a person with onchocerciasis is bitten, microfilariae enter the midge's digestive tract, which become invasive after 6-12 days and migrate to its mouth apparatus. When a person is bitten, the larvae actively tear the membrane of the midge's lower lip, disappear on the skin and penetrate it, migrate to the lymphatic system, then to the subcutaneous fat, where they reach sexual maturity. Adult helminths are located in nodes (onchocercomas) located under the skin, ranging in size from a pea to a pigeon's egg. Onchocercomas are nodules covered with a connective tissue capsule, containing live and dead sexually mature helminths. Most often, the nodes are located in the armpit, near joints (knee, hip), on the ribs, near the spine. Each node contains several females and males intertwined into a ball. The female produces up to 1 million larvae per year. The first microfilariae are produced 10-15 months after infection. The lifespan of larvae is from 6 to 30 months. Microfilariae are located along the periphery of the nodes. They can actively penetrate the superficial layers of the skin, lymph nodes, and eyes. Adult helminths live 10-15 years.
Epidemiology of onchocerciasis
Endemic foci of onchocerciasis are located in African countries (Angola, Benin, Ivory Coast, Gabon, Gambia, Ghana, Guinea, Zaire, Yemen, Cameroon, Congo, Kenya, Liberia, Mali, Niger, Nigeria, Senegal, Sudan, Sierra Leone, Tanzania, Togo, Uganda, Chad, Ethiopia), Latin America (Venezuela, Guatemala, Colombia, Mexico, Ecuador). According to WHO, in 34 endemic countries, about 18 million people suffer from onchocerciasis, 326 thousand have lost their sight as a result of this disease.
Onchocerciasis foci usually form in settlements located near rivers, so the disease is called river blindness. From the breeding site, midges can fly away over a distance of 2 to 15 km. Midges do not fly into residential premises.
The source of the spread of the invasion are infected people. In the endemic onchocerciasis areas of West Africa, the disease mainly affects the rural population. As a rule, all villagers are affected, from small children to the elderly. There are two types of foci in Africa: forest and savanna types. Forest foci are diffusely distributed in the area. The infection index of midges does not exceed 1.5%. The infected population in these foci is 20-50%, among them the proportion of blind people is 1-5%.
Savannah-type foci are more intense. They occupy territories adjacent to fast-flowing rivers on rocky plateaus. The most intense onchocerciasis foci in the world are located in the West African savannas, in the Volta River basin. The infection rate of midges reaches 6%. The incidence of onchocerciasis in the population is 80-90%. The proportion of blind people among the adult population fluctuates between 30 and 50%. Forest-type foci can turn into savannas due to deforestation.
In America, onchocerciasis outbreaks are few and not as intense as in Africa. They occur in hilly areas at an altitude of 600-1200 m above sea level, where the areas are occupied by coffee plantations. Workers on these plantations are most often affected by onchocerciasis. The incidence of eye lesions is lower than in Africa.
Onchocerciasis affects about 50 million people worldwide. The socioeconomic significance of onchocerciasis is great: people leave endemic areas with fertile lands, fearing infection with onchocerciasis.
In Ukraine, there are isolated imported cases of onchocerciasis.
What causes onchocerciasis?
Onchocerciasis is caused by Onchocerca volvulus, a white thread-shaped nematode. Females are 350-700 mm long and 0.27-35 mm wide, while males are 19-42 mm and 0.13-0.21 mm wide. Larvae (microfilariae) are 0.2-0.3 mm long and 0.006-0.009 mm wide and have no sheath.
Pathogenesis of onchocerciasis
Pathogenic action is associated with sensitization of the human body by the products of metabolism and decay of parasites. The body reacts with allergic reactions to substances secreted by parasites. The most striking skin and eye manifestations occur in response to dead microfilariae, not to living ones. A fibrous capsule is formed around adult parasites, surrounded by eosinophils, lymphocytes, and neutrophils. Helminths gradually die, which reduces the intensity of invasion.
Microfilariae born by mature females migrate to connective tissue, skin, lymph glands, and eyes. Manifestations of the disease are associated with the localization of parasites. Parasitism of helminths in the skin leads to the development of onchocercal dermatitis, which leads to the development of hyper- and depigmented spots, thinning and atrophy of the skin, and the formation of onchocercomas. When larvae penetrate the eyes, the vascular membrane of the eye, retina, and optic nerve are affected, which can lead to loss of vision.
Symptoms of onchocerciasis
The incubation period of onchocerciasis lasts about 12 months, in some cases up to 20-27 months. Sometimes the first signs of the disease can appear 1.5-2 months after infection.
Symptoms of onchocerciasis depend on the degree of infection of the patient. In individuals with low infection, the only manifestation of the disease may be skin itching. During this period, subfebrile temperature and eosinophilia in the blood may appear. An early symptom of onchocerciasis is hyperpigmentation of the skin. The spots have a diameter from several millimeters to several centimeters.
The itching is intense in the thighs and shins, and intensifies at night ("filarial scabies"). It is caused by the entry of antigens of helminth larvae into the skin tissue during their molting and can be so severe that people commit suicide. In addition to itching, symptoms of onchocerciasis include papular rash. Papules can ulcerate, heal slowly and form scars. A secondary infection often occurs. The skin thickens, becomes covered with wrinkles and resembles an orange peel. Some patients develop progressive hypertrophy of the skin with a loss of its elasticity ("crocodile skin" or "elephant skin"). Xeroderma often occurs - dryness and peeling of the skin with a mosaic pattern ("lizard skin").
With long-term dermatitis, persistent spotty depigmentation of the skin ("leopard skin") appears. This symptom is most often noted on the lower extremities, genitals, in the groin and armpit areas.
In the later stages of dermatitis, skin atrophy occurs. Some areas resemble crumpled tissue paper ("flattened paper skin", senile dermatitis). Hair follicles and sweat glands atrophy completely. Large folds of skin appear, resembling hanging bags. Young patients with such skin changes look like decrepit old men. When lesions are localized in the facial area, it acquires a characteristic appearance resembling the muzzle of a lion with leprosy ("lion face").
In the late stage of onchodermatitis with skin atrophy, pseudoadenocysts develop. They occur in men and are large hanging sacs containing subcutaneous tissue and lymph nodes. The local population calls them "Hotentot apron" or "hanging groin", when localized in the axillary region - "hanging armpit". Inguinal and femoral hernias often develop, which are very common in areas of Africa where onchocerciasis is endemic.
Lesions of the lymphatic system are manifested by lymphostasis and lymphatic edema of the skin. The lymph nodes are enlarged, compacted and painless. Lymphangitis, lymphadenitis, orchitis, and hydrocele may develop.
In Central America and Mexico, patients under 20 years of age develop a severe form of onchocerciasis dermatitis, which occurs as a recurrent erysipelas. Dark-burgundy, compacted and edematous areas of skin appear on the head, neck, chest and upper limbs. Gross deforming processes develop in the dermis, accompanied by itching, eyelid edema, photophobia, conjunctivitis, iritis, general intoxication and fever.
Onchocerciasis is characterized by the development of onchocercomas - dense, painless, round or oval formations visible to the eye or determined only by palpation. Their sizes vary from 0.5 to 10 cm.
In Africans, onchocercomas are most often located in the pelvic area, especially above the iliac crest, around the hips, above the coccyx and sacrum, around the knee joint, and on the lateral wall of the chest.
In Central America, onchocercomas are most often observed on the upper half of the body, near the elbow joints, and in more than 50% of cases on the head. When onchocercomas are localized in the joint area, arthritis and tendovaginitis may develop.
Onchocercomas are formed only in indigenous inhabitants of endemic areas, who have already developed a mechanism of immune response to parasite antigens. In non-immune individuals, with a long course of the disease, adult onchocerci are found, lying freely in the subcutaneous tissue.
The most dangerous place for microfilariae to enter is the eye. They can penetrate all of its membranes and environments. Toxic-allergic and mechanical effects cause lacrimation, eye irritation, photophobia, hyperemia, edema and pigmentation of the conjunctiva. The most typical damage is observed in the anterior chamber of the eye. The severity of the damage is directly proportional to the number of microfilariae in the cornea. Early damage to the cornea is manifested by punctate keratitis, the so-called snow clouding, due to its resemblance to snowflakes. Keratitis spreads from the periphery to the center, and after some time the entire lower half of the cornea is completely covered with a network of blood vessels - "sclerotic conjunctivitis". With onchocerciasis, the upper segment of the cornea remains clear until the last stage of the disease. Ulcers and cysts form on the cornea. The adhesions that form as a result of the inflammatory reaction around the dying microfilariae lead to a change in the shape of the pupil, which becomes pear-shaped. The lens becomes cloudy. Pathological processes in the eye develop over many years and lead to a decrease in visual acuity, and sometimes to complete blindness.
Due to the deep damage to the eyes, the prognosis for the disease is serious.
Diagnosis of onchocerciasis
Differential diagnostics of onchocerciasis is carried out with leprosy, fungal skin diseases, hypovitaminosis A and B, and other filariases. Imported cases of onchocerciasis to non-endemic areas are established with a delay. The time from returning from the tropics to establishing a diagnosis can be 2 years or more.
The diagnosis is established on the basis of a complex of clinical symptoms and epidemiological history.
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Laboratory diagnostics of onchocerciasis
A reliable method for diagnosis is the detection of microfilariae in bloodless skin scraps, and adult forms in removed onchocercomas. The Mazzotti reaction can diagnose onchocerciasis in cases where other methods have proven ineffective.
Treatment of onchocerciasis
Treatment of onchocerciasis involves the use of ivermectin, diethylarbamazine and antripol. Ivermectin (mectizan) is prescribed to adults once at a rate of 0.2 mg/kg. In case of resumption of reproductive function in filariae, the treatment is repeated after 3-4 weeks. When taking the drug, side effects are observed: headache, weakness, fever, abdominal pain, myalgia, arthralgia, itching, edema.
Diethylcarbamazine (DEC) is prescribed on the first day at a dose of 0.5-1 mg/kg once. In the following 7 days - 2-3 mg/kg three times a day. The maximum daily dose is 400 mg. The drug acts only on larvae (microfilariae).
To destroy adult helminths, antripol should be prescribed after a course of DEC. A freshly prepared 10% solution of this drug is administered intravenously slowly. The following 5-6 injections are administered at weekly intervals, 1 g of the drug (10 ml of 10% solution) per administration. After 3-4 weeks, a second course of DEC treatment is administered according to the same scheme as the first.
In case of allergic reactions, antihistamines are prescribed, in case of severe reactions - corticosteroids. Onchocerciasis nodes are removed surgically.
How to prevent onchocerciasis?
To reduce the intensity of onchocerciasis outbreaks, larvicides are used to destroy midge larvae in their breeding sites. Treatment of water with insecticides for 20-30 minutes results in the death of larvae for more than 200 km downstream from the place of their introduction. Treatments are repeated every 7 days. Personal protection is provided by clothing, which should be treated with repellents.
If it is necessary to live in endemic foci, one should avoid being outside the populated area or outside living quarters in the early morning and evening hours. Onchocerciasis can be prevented by chemoprophylaxis with ivermectin 0.2 mg/kg orally once every 6 months.