Life Cycle of Onchocerca volvulus

Onchocerca volvulus is a filarial (arthropod-borne) nematode (roundworm) that causes onchocerciasis (river blindness) and is the world’s second-leading cause of infection-related blindness after trachoma. It is one of the 20 tropical diseases neglected by the World Health Organization.

Scientific Classification 

Kingdom: Animalia

Phylum: Nematoda

Class: Chromadorea

Order: Rhabditida

Family: Onchocercidae

Genus: Onchocerca

Species: Onchocerca volvulus

In 1874, Irish surgeon John O’Neill discovered Onchocerca volvulus as the causative agent of craw-craw, a skin disease found in West Africa. Rodolfo Robles, a Guatemalan doctor, was the first to link it to visual impairment in 1917.

Humans are the only known definitive host of Onchocerca volvulus, which is primarily found in Sub-Saharan Africa. It is transmitted from person to person by female Simulium biting blackflies.

The parasite Onchocerca volvulus obtains nutrients from the human host by ingesting blood or diffusing through their cuticle. Because dense vascular networks frequently surround the worms, they may be able to stimulate blood vessel formation. The presence of deep transverse striations distinguishes them from other human-infecting filarial nematodes.

It is a dioecious species with distinct males and females that form nodules under human skin. Mature female worms live in these fibrous nodules indefinitely, whereas male worms are free to move around the subcutaneous tissue. Males are shorter than females, measuring 23 mm in length compared to 230-700 mm in females.

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The presence of a male worm does not affect the release of oocytes (eggs) in female worms, though they may attract male worms using unidentified pheromones. The first larval stage, microfilariae, is 300 m long and unsheathed, which means that when they mature into microfilariae, they exit the egg envelope.

Life Cycle of Onchocerca volvulus 

  1. A person is bitten by an infected blackfly, which deposits Onchocerca larvae in the skin. The larvae then crawl into the bite wound.
  1. The larvae enter the tissues beneath the skin (subcutaneous tissues) and form lumps (nodules).
  1. In the nodules, the larvae mature into adult worms. Adult females live in these nodules for up to 15 years.
  1. Mature female worms produce eggs after mating, which develop into immature forms of the worm known as microfilariae. Every day, a worm can produce 1,000 microfilariae. Microfilariae are most commonly found in the skin and lymph vessels, but they can also be found in the bloodstream, urine, and sputum.
  1. When a blackfly bites an infected person and becomes infected with microfilariae, the infection spreads.
  1. After being ingested, the microfilariae travel to the fly’s midgut and then to the muscles in its midsection (thoracic muscles)   The microfilariae grow into larvae there.
  1. When the fly bites another person, the larvae travel to the fly’s mouth parts (proboscis) and can be transmitted to them.
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Adult worms have a 15-year lifespan on average, and mature females can produce 500 to 1,500 microfilariae per day. The average microfilarial lifespan is 1.0 to 1.5 years; however, their presence in the bloodstream causes little to no immune response until the microfilariae or adult worms die or degrade.

Diseases Caused by Onchocerca 

Onchocerciasis is caused by Onchocerca volvulus, which causes severe itching. Long-term infection can lead to keratitis, an inflammation of the cornea in the eye, and eventually blindness.

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Symptoms of Onchocerca volvulus Infection 

Symptoms may not appear for up to a year after infection. Here are the following ailments that take place when the infection occurs.

  1. Lymphadenitis
  2. Lesions of the eyes (eye itching, redness, or swelling)
  3. Visual impairment and/or inability to distinguish certain colors, as well as partial or complete blindness)
  4. High levels of eosinophilia in the blood 
  5. “Sowda” is a term used to describe the severe itching and skin discoloration (darkening) associated with onchocerciasis, which is often confined to one limb.
  6. It is possible that skin nodules and itching will develop.
  7. Bumps beneath the skin
  8. Rashes

Treatment of Onchocerca volvulus Infections 

The most effective treatment is to use ivermectin, but resistance to this drug has been reported. For several months, ivermectin prevents female worms from releasing microfilariae, relieving symptoms and temporarily preventing transmission. However, because this does not kill adult worms, it must be repeated once a year as long as adult worms are present.