Onchocerciasis: causes, symptoms, diagnosis, treatment
Last reviewed: 23.04.2024
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[Onchocerciasis developmental cycle
Infection with onchocerciasis occurs when a person bites Simush. The ultimate host is a man, the intermediate host (carrier) is the blood-sucking midge of the genus Simulium, living along the banks of swift, clean, fast-flowing rivers and streams. Coastal vegetation serves as a place for the day stay of Simuliidae. Moss attack a person in bright, coolest time of the day: from 6 to 10 am and from 16 to 18 hours. They bite mainly lower limbs. In the afternoon, when the air temperature is maximum, the activity of the midges decreases.
The life cycle of onchocerciasis is similar to the life cycles of other filarias. With a bite of a patient with onchocerciasis, microfilariae enter the digestive tract of midges, which after 6-12 days become invasive and migrate to her oral apparatus. At the time of human bite, larvae actively tear off the lining of the lower lip of the midges, disappear to the skin and penetrate into it, migrate to the lymphatic system, then into the subcutaneous adipose tissue, where they reach sexual maturity. Adult helminths are located in the nodes (onchocercomes) located under the skin, the size of a pea to a pigeon egg. Onchocercomes are nodules covered with a connective tissue capsule containing live and dead adult helminths. Most often, the nodes are located in the armpit, near the joints (knee, femoral), on the ribs, near the spine. Each node contains several females and males intertwined in a ball. The female generates up to 1 million larvae a year. The first microfilariae are born 10-15 months after infection. Life expectancy of larvae is from 6 to 30 months. Microfilariae are located along the periphery of the nodes. They can actively penetrate into the superficial layers of the skin, the lymph nodes, into the eyes. Adult helminths live 10-15 years.
Epidemiology of onchocerciasis
Endemic foci of onchocerciasis are found in African countries (Angola, Benin, Ivory Coast, Gabon, Gambia, Ghana, Guinea, Kenya, Liberia, Mali, Niger, Nigeria, Senegal, Sudan, Sierra Leone, Tanzania, Togo, Uganda, Chad, Ethiopia), Latin America (Venezuela, Ecuador, Guatemala, Colombia, Mexico). According to WHO, in 34 endemic countries about 18 million people suffer from the onchocera goat, 326 thousand have lost sight as a result of this disease.
Foci of onchocerciasis are usually formed in settlements located near rivers, so the disease is called river blindness. From a place of an ostrich midge can scatter on distance from 2 up to 15 km. Mice do not fly into living quarters.
The source of infection is infected people. In endemic onchocerciasis areas of West Africa, the disease is mainly affected by the rural population. As a rule, all villagers are affected from small children to old people. In Africa, there are two types of foci: forest and savanna types. Forest foci are common in the area diffusely. The infection index of the midges does not exceed 1.5%. Invasive population in these outbreaks is 20-50%, among them the share of the blind is 1-5%.
Foci of the savanna type are more intense. They occupy territories adjacent to fast-flowing streams along stony plateaus. The most intensive foci of onchocerciasis in the world are located in the West African savannahs, in the Volta basin. Infection of midges reaches 6%. The prevalence of onchocerciasis is 80-90%. The proportion of the blind in the adult population ranges from 30 to 50%. Foci of forest type can turn into savannas due to deforestation.
In America, foci of onchocerciasis 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. The workers of these plantations are most often ill with onchocerciasis. The incidence of eye damage is lower than in Africa.
In the world of onchocerciasis, about 50 million people are affected. Socio-economic importance of onchocerciasis is great: the population leaves endemic areas with fertile lands, fearing infection with onchocerciasis.
In Ukraine, there are single imported cases of onchocerciasis.
What causes onchocerciasis?
Onchocerciasis is caused by Onchocerca volvulus, a white filamentous nematode. Females have a length of 350-700 mm, a width of 0.27-35 mm, and males are 19-42 mm and 0.13-0.21 mm, respectively. Larvae (microfilariae) have a length of 0.2-0.3 mm, a width of 0.006-0.009 mm, do not have a sheath.
Pathogenesis of onchocerciasis
Pathogenic action is associated with the sensitization of the human body products of metabolism and decay of parasites. The body reacts with allergic reactions to substances released by parasites. The most vivid skin and eye manifestations appear in response to dead microfilariae, and not to live ones. A fibrous capsule is formed around adult parasites, surrounded by eosinophils, lymphocytes, neutrophils. Helminths gradually die, which reduces the intensity of infestation.
Microfilaria, born by mature females, migrate into connective tissue, skin, lymph glands, eyes. Manifestations of the disease are associated with the localization of parasites. Parasitizing helminths in the skin leads to the development of onchocerciasis dermatitis, leading to the development of hyper and depigmented spots, thinning and atrophy of the skin, formation of onchocerciasis. When larvae penetrate the eyes, the vascular membrane of the eye, the retina, the optic nerve, which can lead to loss of vision, are affected.
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 manifest after 1.5-2 months after infection.
Symptoms of onchocerciasis depend on the degree of infection of the patient. In people with low infection, the only manifestation of the disease can be itching. In 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 a few millimeters to several centimeters.
Itching is intense in the region of the thighs and lower legs, is intensified at night ("filariasis scabies"). It is caused by the ingestion of helminth larval antigens into the skin tissue and is so strong that people commit suicide. In addition to itching, onchocerciasis symptoms manifest papular rash. Papules can ulcerate, slowly heal and form scars. Often joins a secondary infection. The skin thickens, becomes wrinkled and becomes like a crust of orange. Some patients develop progressive skin hypertrophy with loss of its elasticity ("crocodile skin" or "elephant skin"). Often there is a xeroderma - dryness and peeling of the skin with a mosaic pattern ("lizard skin").
With long-term dermatitis, persistent spotty depigmentation of the skin appears ("leopard skin"). This sign is more often noted on the lower extremities, genitals, in the inguinal and axillary regions.
In the late stages of dermatitis, skin atrophy occurs. Some of its areas are similar to crumpled tissue paper ("flattened paper skin", senile dermatitis). Hair follicles and sweat glands completely atrophy. There are large folds of skin, similar to hanging bags. Patients of a young age with such skin changes are similar to decrepit old people. With the localization of lesions in the face, it acquires a characteristic appearance that resembles the face of a lion with leprosy ("lion face").
At a late stage of onodermatitis with atrophy of the skin pseudo adenocarcinomas develop. They are found in men and are large hanging bags containing subcutaneous tissue and lymph nodes. The local population calls them "gottentot apron" or "hanging groin", with localization in the armpit - "hanging armpit". Often inguinal and femoral hernias develop, which are very common in endemic onchocerciasis areas of Africa.
Lymphatic system disorders are manifested by lymphatic and lymphatic edema of the skin. Lymph nodes are enlarged, compacted and painless. Possible development of lymphangitis, lymphadenitis, orchitis, hydrocele.
In Central America and Mexico, in patients younger than 20 years, there is a severe form of onchocerciasis dermatitis, which proceeds according to the type of recurrent erysipelas. On the head, in the neck, on the chest and upper limbs, there are dark maroon, condensed and swollen skin areas. In the dermis, rough deforming processes develop, accompanied by itching, eyelid swelling, photophobia, conjunctivitis, iritis, common intoxication and fever.
Onchocerciasis is characterized by onchocercal development - 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, onchocercias are located more often in the pelvic region, especially over the crest of the ilium, around the hips, over the coccyx and the sacrum, around the knee joint, on the side wall of the chest.
In Central America, onchocercias are more often observed on the upper half of the body, near the elbow joints, in more than 50% of cases on the head. When localized by onchocerci in the joint region, the development of arthritis and tendovaginitis is possible.
Onchocercias are formed only in native inhabitants of endemic areas, in which the mechanism of the immune response to the antigens of the parasite has already been developed. In non-immune individuals with a prolonged course of the disease, adult onchocercles are found that lie freely in the subcutaneous tissue.
The most dangerous is getting microfilariae into the eyes. They can penetrate into all its shells and environments. Toxico-allergic and mechanical effects cause lacrimation, pain in the eyes, photophobia, hyperemia, edema and pigmentation of the conjunctiva. The most characteristic lesions are noted in the anterior chamber of the eyes. The severity of lesions 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 cloudiness, due to the similarity with snow flakes. Keratitis extends from the periphery to the center, and after a while 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. On the cornea ulcers and cysts are formed. Spikes, formed as a result of an inflammatory reaction around the perishing microfilaria, lead to a change in the shape of the pupil, which becomes pear-shaped. The crystalline lens becomes turbid. Pathological processes in the eye develop for many years and lead to a decrease in visual acuity, and sometimes complete blindness.
In connection with deep lesions of the eyes, the prognosis of the disease is serious.
Diagnosis of onchocerciasis
Differential diagnosis of onchocerciasis is carried out with leprosy, fungal diseases of the skin, hypovitaminosis A and B, as well as other filariasis. Imported cases of onchocerciasis to non-endemic territories are established with delay. The time from the return from the tropics to the diagnosis can be 2 years or more.
The diagnosis is established on the basis of a complex of clinical symptoms and an epidemiological anamnesis.
Laboratory diagnostics of onchocerciasis
A reliable method for diagnosing is the detection of microfilariae in bloodless pieces of skin, and sexually mature forms in remote onchocercomes. With the help of the Mazzoti reaction, onchocerciasis can be diagnosed in cases where other methods have proved ineffective.
Treatment of onchocerciasis
Treatment of onchocerciasis consists in the use of ivermectin, diethylarbamazine and antripole. Ivermectin (mektizan) is prescribed to adults only once at a rate of 0.2 mg / kg. In the case of renewal of reproductive function in the filarias, the treatment is repeated after 3-4 weeks. When taking medication, side effects are observed: headache, weakness, fever, abdominal pain, myalgia, arthralgia, itching, swelling.
Diethylcarbamazine (DEC) is prescribed on the first day at a dose of 0.5-1 mg / kg once. In the next 7 days - 2-3 mg / kg three times a day. The maximum daily dose is 400 mg. The drug has an effect only on larvae (microfilariae).
To destroy adult helminths after the course of DEC should appoint an antipol. A freshly prepared 10% solution of this drug is administered intravenously slowly. The subsequent 5-6 injections are carried out at a weekly interval of 1 g of the drug (10 ml of a 10% solution) per administration. After 3-4 weeks, the second course of DEC treatment is carried out according to the same scheme as the first one.
When allergic reactions are prescribed antihistamines, in severe reactions - corticosteroids. Onchocerciasis nodes are removed surgically.
How to prevent onchocerciasis?
To reduce the intensity of foci of onchocerciasis, larvicides are used to kill larvae of Simuliidae in their breeding areas. Treatment of water with insecticides for 20-30 min leads to the death of larvae over 200 km downstream from the site of their application. Treatments are repeated every 7 days. Individual protection is provided by clothing that should be treated with repellents.
If you need to stay in endemic centers, you should avoid staying outside the settlement or outside the living quarters in the early morning and evening hours. Onchocerciasis can be prevented by chemoprophylaxis with ivermectin at 0.2 mg / kg orally every 6 months.