Onchocerca volvulus - Life Cycle, Pathogenesis, Pathology
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