Parasitology

ONCHOCERCIASIS (caused by Onchocercia volvulus)

Written by MicroDok

Onchocerciasis is a parasitic disease that is characterized by the formation of nodules on the skin (skin dermatitis) and other inflammatory reactions which can lead to blindness in affected human host. The disease which can also be referred to as “river blindness disease” is a major public health problem in most parts of the world especially in tropical developing nations including African countries. Onchocerciasis also occurs in some parts of Latin America and Asia. Onchocerciasis is caused by a filarial worm known as Onchocercia volvulus. O. volvulus can also be called blinding worm because the parasite causes blindness in its human host when it migrates to the eye from the skin.   

Type and morphology Onchocercia volvulus

Onchocercia volvulus is a filarial parasite and tissue nematode that exist as microfilaria. It is a parasitic worm that is notorious for hiding in the subcutaneous tissue of the skin of its human host. The worm can also penetrate the muscle tissues, eye, urine and blood of its human host. Mature parasites of O. volvulus are sexually distinct i.e. the worm can exist as male and female Onchocerca. The larva of O. volvulus is known as microfilariae while the adult worm is called macrofilariae.

Vector, reservoir and habitat of Onchocercia volvulus

The vector and intermediate host of O. volvulus is a blackfly in the genus Simulium. This particular species of black flies that transmit O. volvulus to man are usually found around banks of rivers and streams in tropical countries where the insect vector lays its egg. Humans are the only definitive hosts and reservoir of O. volvulus.

Clinical signs and symptoms of Onchocercia volvulus infection

The clinical manifestation of onchocerciasis may include skin dermatitis (i.e. nodule formation on the skin) which can be located anywhere on the body, loss of skin elasticity, complications in the lymphatic systems, and inflammatory reactions in the eye. The nodules formed on the skin of onchocerciasis patients are generally known as onchocercoma. Onchocercoma are usually elongated or round, firm and smooth, and they measure about 5 mm transversely and 50 mm when in clusters. In children, onchocercomas are mainly found on the head region while in adults, the nodules are usually found on the upper or lower parts of the body. Pain and tenderness can occur in infected individuals when onchocercoma are located over a joint in the body.   

Pathogenesis of Onchocercia volvulus infection

Onchocercia volvulus is transmitted to humans through the bite of blackfly in the genus Simulium as the insect vector takes a blood meal (Figure 1). This introduces the infective larva of the parasite into the subcutaneous tissues of infected human hosts. Infective larva of O. volvulus transmitted to humans by Simulium blackfly usually takes weeks to months before the worm fully develop into a sexually mature parasite. The larva develops into adult filarial worms which can live for up to 10 years and even more in the subcutaneous and connective tissues of the human host. Female worms produce many microfilariae (larvae) which can penetrate body fluids including blood, urine and CSF. The presence of microfilariae and adult worms (macrofilariae) in host’s tissue sparks up inflammatory reactions and other pathological reactions which culminate to nodule formation, skin diseases and blindness when the worm migrates to the eye. The worm can penetrate the cornea and iris, thus causing irritation and redness of the eye. Blindness is the most serious complication of onchocerciasis in endemic regions where the Simulium blackfly is widespread. However, the risk of complications associated with O. volvulus infection depends on the duration and intensity of the disease, and on the strain of the infecting parasite.

Figure 1: Life cycle of Onchocerca volvulus. 1. During a blood meal, an infected blackfly (in the genus Simulium) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound created by the insect vector. 2. In subcutaneous tissues the larvae develop into adult filariae. 3. Adult filariae commonly reside in nodules in subcutaneous and connective tissues of the human host. 4. Adults can live in the nodules for approximately 15 years. Some nodules may contain numerous male and female worms (female’s measure 33 to 50 cm in length and 270 to 400 μm in diameter, while males measure 19 to 42 mm by 130 to 210 μm). In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 9 years (the microfilariae, which measures 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span that may reach 2 years). Microfilariae are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues in the human host. 5. A blackfly ingests the microfilariae during a blood meal. 6. After ingestion, the microfilariae migrate from the blackfly’s midgut through the hemocoel (empty space) to the thoracic muscles. 7. Microfilariae develop into first-stage larvae in the thoracic muscles of the insect vector. 8. The first-stage larvae subsequently develop into third-stage infective larvae. 9. The third-stage infective larvae migrate to the proboscis of the Simulium blackfly’s until the next blood meal from a human host. CDC

Laboratory diagnosis Onchocercia volvulus infection

Onchocerciasis is diagnosed in the laboratory by detecting and identifying the microfilariae of the parasite in skin snip specimen which can be obtained anytime of the day from infected individuals. Thin skin snips are usually obtained carefully from the skin area of infected human hosts where nodules appear. The skin snip are either examined directly in a wet preparation or centrifuged and examined in wet preparation. Skin snips should be examined immediately in normal saline or distilled water under the microscope. Nodules or biopsies can be surgically obtained from infected patients and examined for adult parasites or microfilariae. In severe forms of the disease (i.e. river blindness), microfilariae of the parasite can be detected in the anterior cavity of the eye by an ophthalmologist.

Treatment of Onchocercia volvulus infection

The antiprotozoal agent Ivermectin is the preferred choice of drug for the treatment of onchocerciasis. Ivermectin is very potent against the microfilariae of O. volvulus. But repeated therapy with Ivermectin is necessary to kill the adult worm (macrofilariae) of O. volvulus. No vaccine currently exists for O. volvulus infection. Treatment with Ivermectin can cause some hypersensitivity reactions such as itching in infected hosts.

Control and prevention of Onchocercia volvulus infection

Though there has been a significant decline in the occurrence of onchocerciasis in endemic regions, it is still important to sustain the measures geared towards eradicating the disease from across the globe. Repeated use of insecticides in onchocerciasis endemic regions through the aerial spraying of chemicals that targets the insect vector of O. volvulus and its larval stage are critical to breaking the life cycle of the disease. Selective spraying of insecticides along river banks where the Simulium blackfly thrives will also help to disrupt the developmental stages of the insect vector. Identification of O. volvulus infected individuals and their subsequent treatment with Ivermectin is important in slowing and preventing the progression of onchocerciasis from skin dermatitis to river blindness. Protective clothing’s should always be worn people in endemic regions and those who live along river banks in order to avoid bite by the Simulium blackflies that transmits O. volvulus to humans. Transmission of O. volvulus in endemic regions has been strongly interrupted by the onchocerciasis control programme (OCP) of the World Health Organization in parts of the world where onchocerciasis used to be widespread.

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