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Onchocerca volvulus is a filarial nematode that causes a disease commonly called River Blindness or Onchocerciasis; the vector of Onchocerca volvulus is the Similium species of black fly. This roundworm is a common cause of blindness worldwide.
Onchocerca volvulus Epidemiology
Onchocerca volvulus is the second leading cause of infectious blindness worldwide after trachoma; it affects about 40 million people worldwide with majority living in Africa and has caused blindness in about 400,000 people worldwide.
Onchocerca volvulus Characteristics and Morphology
- The Adult female Onchocerca volvulus has a life span of 8-10 years with a maximum of 15yrs.
- The adult female worms usually produce the first stage larvae that are called microfilariae which can live for up to 12 to 24 months. In heavy worm infection there may be 50 to 200 million microfilariae in the skin at any point in time. This heavy worm in infestations allows the microfilariae to be seen in other body tissues such as blood, glomerular capillaries, pulmonary capillaries and the choroid plexuses of the ventricles. Microfilariae can also be seen urine, sputum and C.S.F
- The parasites are white or cream colored and transparent
- The adult female worm measures about 30 to 80cm long while the male measures about 3 to 5cm long
- The female worm may produce 1600 microfilariae per day and generate a body load of 150 million
- The microfilariae may migrate in the dermis and its lymphatics and die after a couple of years if they do not pass into a black fly
- The microfilariae may enter the eye directly from the conjunctiva or through the bloodstream entering the eyes causes ocular disease.
Onchocerca volvulus Mode of Transmission
Onchocerca volvulus is spread by a bite from an infected black fly called Simulium spp which serves as the vector for transmission of the microfilariae. The black fly needs well oxygenated water for the development of their larvae. In west Africa, there are two common species of the black fly which are Simulium damnosum and Simulium bovi while in East Africa the vector is Simulium naevei and in America the vector is Simulium metallicum.
Onchocerca volvulus Life cycle
- Microfilariae under the skin are ingested whenever the female black fly bites while feeding on blood
- The microfilariae develop into infective larvae stage in the fly and pass to its mouth where it can be transmitted during the next bite
- After moulting, the larvae develop into adult worms in the body and cause the symptoms often experienced such as skin rash that are itchy. The adult worms can live in fibrous nodules (and are known as Onchocercomata) in the skin for 15 years
- Adult worms then produce microfilariae that can be found mostly in the skin and in the lymphatic drainage system of connective tissues; sometimes the microfilariae can be found in peripheral blood, urine and sputum. Whenever another fly takes a blood meal, it ingests the microfilariae in the skin, lymphatics of connective tissue or in the blood and the life cycle of Onchocerca volvulus is completed
Onchocerca volvulus infection symptoms and signs
- Common symptom of Onchocerca volvulus infection is appearance of skin rash that itches
- Skin nodules especially over bony prominences and sites of trauma
- Patients may have enlarged inguinal lymph nodes (hanging groin) that may cause inguinal hernia
- Heavy worms infection may cause severe wasting (weight loss)
- Skin lesions may cause wrinkling of skin and epidermal atrophy leading to hypopigmentation than hyperpigmentation
- Superficial lymph nodes may become enlarged
- Eczematous dermatitis may occur especially in the lower extremities.
- Visual impairment occurs when the microfilariae migrate to the eyes and this is the most serious complication of onchocerciasis as it may lead to secondary glaucoma or blindness (as a result of intense inflammation that surrounds the dying microfilaria)
- Early eye symptoms of Onchocerca volvulus infection include conjunctivitis (red painful eyes) with fear of light (photophobia), inflammation of the inner chamber of the eyes causing iridocyclitis and sclerosing keratitis may also occur and it is the cause of the blindness in river blindness disease
Onchocerca volvulus Laboratory Tests for Diagnosis
- Skin snip biopsy to identify microfilaria
- Use of slit lamp examination of the eyes to check for microfilaria
- Excision and examination of skin nodules for adult worms
- Mazotti Test or Mazzotti reaction using DEC (Diethylcarbamazine) provocation test this is life threatening and should not carried out in patients with heavy worm infestation. It is usually performed by topically applying DEC under an occlusive dressing: this will provoke an allergic rash in the majority of people infected with Onchocerca volvulus (this reaction is named as the modified Mazzotti reaction)
- Antibodies against the worms can be found in blood
Onchocerca volvulus Treatment
- Onchocerca volvulus infection is treated using Ivermectin (100 to 200microgram/kg single dose with retreatment after 6 to 12 months until adult worms die) a drug that destroys microfilariae and prevents the microfilariae from leaving the uteri of the adult worms . Ivermectin was developed by Merck and was donated to the World Health Organization to help in the eradication programme of Onchocerca volvulus infection. Ivermectin is contraindicated (should not be used) in co-infection with loa loa, in pregnant women or lactating mothers and also in children under the age of 5 years. Side effects of Ivermectin includes fever, headache, arthralgia, pruritus, painful gIandular swelling, tachycardia and postural hypotension.
- Excision of the adult worms under the skin (Nodulectomy)
Onchocerca volvulus Prevention
- Use of protective clothing in endemic regions
- Use of insecticides to control the insects population
- Treatment of those infected to prevent transmission
- Large scale Ivermectin chemotherapy strategy such as the Onchocerciasis Elimination Programme in the Americas (OEPA) and African Programme for Onchocerciasis control (APOC)