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Onchocerciasis is a condition caused by the infection of Onchocerca volvulus. This disease can be found primarily in Africa, in the equatorial zone. Clinical symptoms of onchocerciosis mainly affect the skin, lymph nodes and the eye.
Where does Onchocercosis occur?
Onchocercosis occurs primarily in Africa, in the equatorial zone. Limited outbreaks are also found in Mexico, Bolivia, Ecuador, Colombia and Brazil, as well as in Yemen. The disease occurs in small, clearly demarcated foci near rivers and streams. Dots of the genus Simulium are the carriers of onchocercosis. Flowing freshwater is essential for the development of the vector larvae. The midges have a range of up to 2 km, hence the transmission of onchocercosis is spatially limited. In the last dozen or so years, in some areas in Africa, the risk of contracting this phylariosis has increased as a result of the use of irrigation systems. People living in endemic areas for at least several months are susceptible to falling ill. Onchocercosis is not a significant risk for people staying there for a short time.
Onchocercosis – causes of formation
Onchocercosis is caused by an infection Onchocerca volvulus, a threadworm whose mature forms are 33-50 mikrom in length and 230-400 mikrom in diameter for females, and 19-42 mikrom in length and 130-210 mikrom in diameter in the case of males. Microfilariae are 220-360 mikro m in length and 5-9 mikrom in diameter, they are devoid of a halo. Mature forms live about 15 years, of which an average of 9 years produces microfilariae, the average life of microfilariae is about 2 years.
The mechanism of onchocercosis formation
During blood collection, the infected larva introduces the third larval stage, which from the bite site penetrates into the subcutaneous tissue, undergoes transformation into a mature form and forms a nodule in the subcutaneous tissue. The microfilariae produced by it penetrate the skin, and occasionally they can also be found in the blood and other body fluids, and show a special affinity for eye tissues. Mature parasites elicit a mild inflammatory reaction with the formation of a connective tissue capsule.
They are responsible for most of the clinical symptoms microfilariaespecially dead and dying, to a lesser extent alive. Live microfilariae produce the collagenase enzyme responsible for the slow degradation of collagen and elastin. This leads to the loss of elasticity of the skin and its early aging, which is characteristic of onchocercosis. Dead and decaying microfilariae cause an accumulation of inflammatory cells, mainly eosinophils and neutrophils. The chronic inflammatory process leads to hyperkeratosis, changes in pigment distribution, skin atrophy and dilation of the skin’s lymphatic vessels. The local lymph nodes can also be the site of increased inflammation.
In the eye, dead microfilariae cause clouding of the cornea, inflammatory changes in the choroid and the retina. Cell and tissue damage in inflammatory lesions is caused by the action of cytokines and other substances secreted by immunocompetent cells, the deposition of immune complexes and autoimmune phenomena.
Onchocercosis – symptoms
The clinical symptoms of onchocercosis are mainly related to skin, lymph nodes and the eye.
The earliest skin symptom is itching. It usually affects the lower torso, thighs and buttocks. The pruritus is often accompanied by papular and urticarial skin lesions, turning into vesicles and pustules in more severe cases. Sometimes the skin changes coalesce, sometimes erythematous or edema changes are observed. Scratching the lesions can lead to secondary bacterial infections and ulceration. In chronic maculopapular lesions, discoloration and post-inflammatory hyperpigmentation sometimes appear.
There is a separate form of onchocercosis sowa, i.e. lichenoid atrophic dermatitis, which occurs most often in young people – often located asymmetrically, on the limbs or torso. It is characterized by itchy, discolored lesions with hyperkeratosis and peeling of the epidermis. As the disease continues, these changes will blend together. It is often accompanied by local lymphadenopathy. The final image of these changes is often skin atrophy with loss of elasticity, thinning of the dermis and loss of subcutaneous tissue. In the course of sowdy, the loss of hair and sweat glands is also observed, which in combination with the enlargement of the inguinal lymph nodes sometimes gives the image of “hanging groin”.
Skin changes arising in the course of onchocercosis lead to the formation of foci of discoloration and discoloration – the so-called leopard skin. There are nodules in the subcutaneous tissue that contain mature forms of the parasite (onchocerkomata). They are of various sizes – single ones are 5-10 mm in diameter, there are conglomerates containing many nodules, up to 10 cm in diameter. They are painless, soft, and usually moveable in relation to the ground.
Ocular symptoms of onchocercosis include:
- conjunctivitis,
- transient and resolving point opacities in the cornea,
- irreversible hardening keratitis,
- inflammation of the iris and the front part of the choroid,
- inflammation of the retina and choroid,
- inflammation and atrophy of the optic nerve,
- post-inflammatory glaucoma.
Ocular symptoms most often occur in people who are exposed to infection for a long time, with massive infestation. Microfilariae can be detected in the eye using a slit lamp and can be found in all parts of the eye. Floaters in the vitreous or anterior chamber of the eye are most often the result of inflammation around the dead microfilariae, while the living floaters do not usually cause inflammation. Permanent eye damage and irreversible blindness are most often caused by sclerosing keratitis, but also due to chorioretinitis or damage to the optic nerve. Onchocercosis, especially its ocular involvement, can cause general cachexia, loss of fat and muscle tissue, and increased susceptibility to other infections.
How is onchocercosis diagnosed?
The diagnostic method of onchocercosis is parasitological examination of the skin section. A small patch of skin is immersed in a saline solution for 30-120 minutes. The microfilariae of the test piece get into the solution, which is initially examined under the microscope. The sensitivity of this method is not high, but in symptomatic cases it usually allows to confirm the infection.
Examination of skin sections with application PCR methods enables confirmation of infection even in cases of minor infestation. Histopathological and parasitological examination the content of the removed nodules, as well as the visualization of microfilariae in the eye, in a slit lamp examination, often allows for the diagnosis of onchocercosis. With a negative parasitological test result of skin sections and the inability to perform molecular tests, it can be used for diagnostic purposes. test Mazzotiego. It consists of the administration of 25 or 50 mg of diethylcarbamazine (DEC). People with onchocercosis develop itchy skin and develop a rash within a few hours. Administration of DEC to persons with massive invasions may lead to rapid deterioration, including irreversible changes to the eye. A positive result of this test may also occur in other filariasis, especially in the infection of Mansonia with streptocerca. A certain modification of the Mazzoti test, increasing the safety of its use, is the application of a 10% cream containing DEC to the skin, under an occlusive dressing – however, the sensitivity of this method is much lower.
Serological diagnostic methods they are only of limited importance as they do not distinguish the current infection from the past one. They are used in people suspected of being infected with O. volvuus, after a limited stay in endemic areas and in population studies to determine the degree of endemicity in a given area.
Differential diagnosis of onchocercosis
Skin changes occurring in onchocercosis should be differentiated from:
- atopic dermatitis,
- contact eczema,
- eczema in the course of food allergy,
- skin changes related to malnutrition,
- scabies
- skin mycoses.
Ocular varieties of onchocercosis require differentiation from:
- jaglic,
- tuberculosis,
- sarcoidosis,
- glaucoma,
- neurogenic keratopathy,
- herpes,
- syphilitic interstitial keratitis.
Onchocercosis – treatment
In the case of onchocercosis, the drug of choice is ivermectin. The drug is used at a dose of 150 mikrog / kg bw, every 6-12 months. It kills the microfilariae and causes a reduction in microfilariaemia lasting several months. Ivermectin appears to work to prevent the release of new microfilariae from mature females. The drug does not kill mature forms. The therapy should be continued until the end of life – this procedure prevents the occurrence of skin and eye changes.
Surgical removal of nodules containing mature parasites is recommended especially in the case of their location on the head due to the proximity of the eyes. As in lymphatic filariasis, treatments against Wolbachia appear to reduce the fertility of O. volvulus females and may become an alternative treatment for this infection.
Use DEC in the treatment of onchocercosis, it is contraindicated due to the rapid death of microfilariae, leading to an exacerbation of pathological changes. Suramin is a drug that acts on both microfilariae and mature parasites, but its high nephrotoxicity limits its use to cases where ivermectin is ineffective.
What is the prognosis in onchocerciasis?
Most cases of untreated onchocercosis progress, and in high-endemic foci, a significant number of inhabitants lose their eyesight. Treatment is considered effective if disease progression is inhibited, but numerous eye and skin changes are irreversible.
Source: J. Cianciara, J. Juszczyk, Infectious and parasitic diseases; Czelej Publishing House