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Onchocerca volvulus - Life Cycle, Pathogenesis, Pathology

Last Modified: June 9, 2023

Life Cycle of Onchocerca volvulus

The life cycle of Onchocerca volvulus is completed in two hosts – the intermediate host and the definitive host.

Definitive host: Man

Intermediate host: Female

  • the human host acquires infection when the infective larvae (J3 juveniles) of Onchocerca volvulus are injected into the skin by the bite of an infected female black fly (Simulium)

  • after reaching the skin, the parasitic larvae migrate to the subcutaneous tissue

  • inside the host tissue

  • over a period of 6 to 12 months, they molt, develop, and mature into either male or female filarial nematode

  • with the progression of infection, Onchocerca volvulus worms induce the formation of fibrous nodules where they remain encapsulated

  • in the lymphatic spaces or in subcutaneous tissues, the adult parasites live singly or in a mass of two to three females and one to two males

  • adult males migrate from nodules to subcutaneous tissue to mate with adult females

  • following the pre-patent period of 7 to 34 months, viviparous females produce 500 and 1,500 unsheathed stage one microfilaria per day

  • these microfilariae migrate out of the nodule to enter the dermis of the skin, connective tissue

  • the Onchocerca volvulus microfilariae may also migrate to the eye and other host organs but do not enter the blood circulation

  • when a female black fly (Simulium) bites an infected person during a blood meal, these microfilariae are ingested from the skin

  • once in the vector’s gut, the microfilaria penetrate the stomach wall and reach the thoracic flight muscles

  • in the fly’s thoracic flight muscles, they develop into the first juvenile phase, J1

  • after maturation of J1 into the second juvenile phase, J2, these larva migrates to the proboscis and are found in the vector’s saliva

  • in the saliva, the J2 stage juveniles matures into stage three juveniles, J3, the infectious larvae

  • when the black fly vector harboring J3 larvae in its saliva bites another human host during a blood meal, the Onchocerca volvulus is transmitted and the life cycle is continued

Onchocerca volvulus life-cycle (Source: CDC)

Pathogenesis, Pathology of Onchocerca volvulus

Adult Onchocerca volvulus parasites present in the subcutaneous tissues' inflammation and nodules to a varying degree. These subcutaneous nodule appears 3 to 4 months after infection and may be surrounded by a granulomatous inflammatory reaction and formation of granuloma, fibrosis, or calcification.

The microfilariae of Onchocerca volvulus are more pathogenic. Pathological lesions in the eye and inflammatory dermatitis in the skin are due to trauma caused by microfilariae and hypersensitivity reactions of the host against dead microfilariae.

Onchocercoma

  • also known as subcutaneous nodules

  • the typical pathological lesion in Onchocerca volvulus infection

  • are firm, smooth, rubbery, and contain adult worms

  • causes severe itching

  • the shape varies from round to elongated

  • size ranges from a few millimeters to several centimeters in diameter

  • can be freely moving or attached to the underlying tissue

  • site of distribution of nodules depends upon geography

    * Africa – nodules found in the lower part of the host body such as the coccyx, sacrum

    * Central America, African Savannah – nodules found in upper body parts such as the head, neck, shoulders

    * Yemen – nodules are found in lower limbs

    * in children below the age of 9, sub-cutaneous nodules containing Onchocerca volvulus are found in the head

Lymphadenopathy

  • in lymphadenopathy caused by Onchocerca volvulus, lymph nodes of inguinal and femoral areas who mild to moderate infection

  • due to enlarged lymph nodes and accumulated fluid, a condition known as ‘hanging groin’ occurs

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