Life Cycle and Pathophysiology

Life Cycle:

Human stages:
(1) During a blood meal, an infected blackfly introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound.
(2) In subcutaneous tissues, the larvae develop into adult filariae within 6-12 months.
(3) These adult worms commonly reside in nodules in subcutaneous connective tissues. Adults can live in the nodules for approximately 14 years. Usually, 2-3 female worms are found coiled up in the nodules, while male worms migrate in subcutaneous tissues, traveling through the body to locate female worms to mate with in other nodules. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 9 years. The female can produce between 700-1900 eggs a day [3].
(4) Microfilariae are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues [4].

Blackfly stages:
(5) A blackfly ingests the microfilariae during a blood meal. Fly saliva acts as a chemoattractant for the microfilariae in the surrounding subcutaneous tissues [3].
(6) After ingestion, the microfilariae migrate from the blackfly's midgut through the hemocoel to the thoracic (flight) muscles of the fly.
(7) There, the microfilariae develop into first-stage (L1) larvae after 28 hours. After 96 hours, the L1 larvae develop into second-stage (L2) larvae.
(8) By day 6-10, the second moulting process occurs, and the L2 larvae develop into third-stage (L3) infective larvae [5].
(9) The L3 infective larvae migrate to the blackfly's proboscis and can infect another human when the fly takes its next blood meal, thus bringing us back to step one (1) of the life cycle.


Courtesy of CDC image library


Pathophysiology:

Nodules:
The classic lesion of onchocerciasis is the onchocercoma, a firm, painless nodule in the subcutaenous tissue. Nodules usually form within a year after infection. They are most often subcutaneous, but may occur in connective tissues deeper within the body. The nodules’ size ranges from a few mm to several cm in diameter. The nodule usually contains 2-3 females and daughter microfilariae encapsulated in a fibrous coat. Dead worms may calcify in the nodules. [5]

Blindness:
Microfilariae migrate through the body through the lymphatic system, bloodstream, or along nerves, collecting in the eyes and skin. At any one time, a human can house millions of these larvae in the skin and eyes [5]. Visual impairment and eventually blindness occur when the microfilariae migrate through and collect in ocular tissues, causing lesions of the cornea, anterior chamber and iris [2].

Skin lesions:
Skin lesions, on the other hand, are caused by an inflammatory response to dead microfilariae. This reaction can be suppressed by the administration of corticosteroids Most microfilariae die as immature worms in the human. Their death causes an intense inflammatory reaction, which is responsible for most of the disease morbidity. Antigens of the microfilariae induce cell-mediated and humoral responses, as circulating immune complexes and high levels of Immunoglobulin E (IgE) have been identified in the inflammatory response. It is possible that the deposition of immune complexes in tissues, as a result of the constant antigenic stimulation by the microfilariae, causes the symptoms of inflammation, fibrosis and eventual obstructive lymphadenitis seen in some patients [1].

Note: for pictures of each of these conditions, please refer to the section on Clinical Presentation