What is Onchocerciasis?
Onchocerciasis (Onchocercosis) is a chronically flowing filariasis characterized by a predominant lesion of the skin, subcutaneous tissue and eyes.
Onchocerciasis is common in forest areas along rivers and streams. Onchocerciasis is widespread in several African countries, and foci of onchocerciasis exist in Brazil, Mexico, Costa Rica, Venezuela, and Guatemala. Worldwide, about 13 million people suffer from it.
Onchocerciasis ranks second among the infectious causes of blindness.
Causes of Onchocerciasis
It was assumed that there are two types of onchocercias that can cause human diseases: O. volvulus (Africa) and O. caecutiens Brumpt (South America). Now it is proved that morphologically and biologically, these parasites are identical. Onchocerki have a filamentous body, thinning towards the ends. The length of the parasite is 19-50 mm, width – 0.13-0.4 mm. Females are larger than males. Microfilariae can be large, with a size of 0.29-0.37 x 0.09 mm, and small, with a size of 0.15-0.29 x 0.05-0.07 mm.
The final owner is a man, intermediate hosts (carriers of the disease) – female midge of the genus Simulium. Adult parasites are localized in the fibrous nodes located under the skin, the aponeurosis of the muscles, the periosteum. Females of the parasite produce microfilariae that live mainly in the surface layers of the skin, often in the eyes, less often in the lymph nodes and internal organs, and very rarely in the blood.
Pathogenesis during Onchocerciasis
The source of the invasion is man. The carriers of onchocerciasis in Guatemala and Mexico are Simulium ochraceum, S. callidum, S. metallicum, and possibly other species, in Africa in most parts of the continent – S. damnosum, and in Kenya, Uganda and Congo, in addition – S. NeaveL. Blackflies of the genus Simulium breed in rivers and reservoirs. Females attack humans more often in the morning and evening. In residential areas, they usually do not fly. The midges are invaded by the bite of an infected person. In their body microfilariae reach the invasive stage in 6-7 days. The maximum lifespan of midges is 20-35 days. The high number of carriers is associated in the tropics with the season of the year and depends mainly on the amount of moisture.
Of great importance in the pathogenesis of onchocerciasis are common and local allergic reactions.
When onchocerciasis affects the skin, eyes and lymph nodes. The main manifestations of the disease are due to microfilariae, not adult parasites, as in lymphatic filariasis. The skin has a mild chronic inflammation, leading to the destruction of elastic fibers, atrophy and fibrosis. Onchocercoms consist mainly of connective tissue surrounding adult parasites. On the periphery of foci inflammatory infiltration is often noted. If the eyes are damaged, vascular growth, scarring and clouding of the cornea leading to blindness, iritis, chorioretinitis and optic nerve atrophy are observed. It is known that the point corneal opacities are caused by an inflammatory response to dead or dying microfilariae, but the pathogenesis of the remaining manifestations of onchocerciasis has not yet been clarified.
Symptoms of Onchocerciasis
Nodes become visible by eye 3-4 months after infection. Females are able to produce microfilariae after approximately one year of parasitism in the human body.
The duration of the incubation period is about 1 year. The disease begins with general malaise and fever. Note dry and flaky skin, itchy papular rash. Sometimes papules transform into pustules with ulceration. With significant microfilariah, the skin takes on the appearance of lemon peel, shagreen or ivory. Over time, skin depigmentation sites appear.
Onchocercoms. The most characteristic sign of onchocerciasis is the presence under the skin of dense, mobile, often painful fibrous nodes ranging in size from 1-2 to 5-7 cm. Nodes can be in different parts of the body, but are more common on the head, in the pelvic area and around the joints. In Africa, in patients with onchocerciasis, the nodes are located in the pelvic region, less frequently in the area of the scapula, and even less frequently on the head. Most often they are found above the protrusions of the bones, where the subcutaneous fatty tissue is thinner. Carriers S. damnosum, S. neavei mainly attack the lower parts of the body, while in Central America the carrier S. ochraceum attacks the head and neck. In South America, nodes in patients are more often located in the occipital and temporal regions.
Skin changes are one of the characteristic symptoms of onchocerciasis. The skin becomes firm, wrinkles, flakes, periodically there is a small, intensely itchy rash. The body temperature increases, symptoms of general intoxication appear (general weakness, headaches). On the papules then vesicles or pustules appear, which subsequently ulcerate. Ulcers heal slowly with scar formation. Often occurring dermatitis is very reminiscent of erysipelas of the skin. In these cases, the skin on the affected areas becomes edematous, dark red in color, the body temperature reaches 39-40 °, swelling of the lips and auricles appears. Exacerbations of dermatitis lasting from several days to several weeks gradually lead to the fact that the skin on the affected areas thickens, becomes edematous, there is an increase in the auricles, they are bent anteriorly. Depigmented skin appears on the neck and back.
In some cases (mainly Europeans), fibrous nodes are not formed, although a huge number of parasites can be found in the skin. In this disease, elephantiness of the scrotum, lower extremities, face, hydrocele, orchitis, localized abscesses, arthritis, and perforations of the cranial bones causing epileptiform seizures are described. In advanced cases, patients with onchocerciasis men sometimes have skin bags that contain enlarged sclerotic femoral or inguinal lymph nodes.
Eye damage. When microfilariae get into the eye, symptoms of chronic conjunctivitis appear, the mucous membrane thickens, especially at the site of cornea passage into the sclera, where a roller of a hyperemic conjunctiva 2-3 mm thick is formed. An early objective sign of damage to the cornea is the appearance of small gray-white spots in the surface layers. In this period of the disease photophobia, lacrimation, blepharospasm develop. The lesions gradually spread from the periphery to the center of the cornea, causing persistent opacification with severe visual impairment. In onchocerciasis, Pannus often has a triangular shape with a base around the periphery and apex near the center of the pupil. The iris is often depigmented and atrophied. In the anterior chamber of the eye exudate is detected brown. Severe ocular complications of onchocerciasis are cataract, glaucoma, chorioretinitis and optic nerve atrophy.
The defeat of the lymph nodes. Often there is a slight or moderate increase in lymph nodes – especially superficial and deep inguinal. Under the action of gravity, hanging skin pockets form in the groin, into which the enlarged lymph nodes are shifted. This increases the risk of inguinal and femoral hernias.
Other manifestations. With severe invasion, cachexia sometimes develops with the loss of adipose tissue and muscle atrophy.
Among adults who have lost sight, the mortality rate is 3-4 times higher than usual.
Diagnosis of Onchocerciasis
The presence of fibrous nodes under the skin, damage to the organ of vision, taking into account the epidemiological data help the clinical diagnosis of onchocerciasis. The diagnosis is confirmed by the detection of microfilariae in the eye using a corneal microscope or an ophthalmoscope, as well as in a thin layer of skin, cut with a razor or dermatome. Intradermal allergy test, complement fixation test, agglutination test are not strictly specific.
Treatment of Onchocerciasis
WHO recommends several treatment regimens for patients with onchocerciasis with the indicated means in various combinations. The choice of treatment method and self-treatment of patients is carried out only by specially trained doctors, which is associated with the development of severe allergic reactions with the death of parasites by the type of Mazzoti reaction, with the possibility of exacerbation of eye lesions during treatment, as well as with the toxicity of a number of drugs.
The main tasks are to prevent irreversible changes and alleviate the condition of the patient. When onchocercian is located on the head, surgical treatment is recommended, since sexually mature parasites are in close proximity to the eyes. In other cases, the main method of treatment is medication.
Ivermectin, a semisynthetic macrocyclic lactone, is effective against microfilariae and serves as the drug of choice for onchocerciasis. The drug is administered orally at a dose of 150 mg / kg 1-2 times a year. Side effects are rare – itching, urticaria and maculopapular rash occur in 1-10% of cases. Ivermectin is contraindicated in children under 5 years of age, pregnant and lactating. It can not be used in diseases of the central nervous system, which can contribute to the penetration of ivermectin through the blood-brain barrier (in particular, meningitis). A single dose of ivermectin significantly reduces the number of microfilariae, but its effect sometimes lasts only 6 months.
Suramin, a potent but toxic antiparasitic drug, is active against sexually mature worms. It is prescribed only if a complete cure is necessary. In connection with the nephrotoxic effect of suramin during treatment carefully monitor renal function.
Prevention of Onchocerciasis
While there is no effective vaccine or medication for onchocerciasis, it is therefore imperative to observe protective measures against insect bites in their habitats. Fortunately, travelers who stay in high-risk areas for a short period (less than three months) rarely develop onchocerciasis.