The Pathogenesis of Onchocerciasis

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Microfilariae and adult worms of O. volvulus contribute to the pathogenesis of onchocerciasis, both through consequences of host immune response. Adult worms are the least pathogenic, usually causing no symptoms at all and at worst, stimulate the development of noticeable subcutaneous nodules called onchocercomas. These nodules are usually situated over bony prominences. The African O. volvulus strain differs to Central American strains with respect to the region of the body where the nodules are found in high frequency. For the African strain, the nodules are most frequent in the pelvic area, with a few along the chest, spine, and knees. But for central American strains, the nodules are mostly found above the waist, especially on the head and neck. The presence of nodules doesn’t cause pain or ill health to patients but does cause some disfigurement to the body. The number of nodules per individual may vary from 1 to over 100. Collagen fibers are the main structural components of a nodule; these fibers surround one to several adult O. volvulus worms (George et al. 1985). The worms rarely calcified or degenerate to form an abscess.

The formation of nodules is sometimes followed by elephantiasis (enlargement of body parts) and another condition called “hanging groin”. A loss of the skin’s elasticity causes the drooping of the groin into pendulous sacs, usually filled with lymph nodes. Scrotum and testes are unaffected, and hydrocoele does not come with the condition. Females are also affected. In Africa, onchocerciasis commonly causes hernia, “a condition in which part of an internal organ projects through the wall of the cavity that contains it (Microsoft Encarta 2008).”

According to Kazura et al. (1993), live microfilariae elicit little inflammatory response, although degenerating juveniles in the skin result in a severe dermatitis. The dermatitis is the result of inflammation due to the release of Wolbachia bacteria from dead juveniles. Eradicating the bacteria by using the antibiotic doxycycline effectively treats the skin lesions (v. Saint et al. 2002). The first symptoms of dermatitis are itching, bacterial infection, and abnormal pigmentation but later on followed by the thickening, loss of pigmentation, and creation of skin cracks (lichenification). The last stage of dermatitis is exhibited by the loss of the skin’s elasticity, which gives the patient the appearance of premature aging.

The impairment of vision is the most terrible complications of onchocerciasis. In some villages of Africa, the rate of impaired vision may get to 30%. Ten percent of the village’s population may be totally blind. In Guatemala, it is typical to see a child with normal vision leading a string of blind adults to the local market. Eye complications are uncommon in African rain forest areas but are so common in the savanna regions, a difference possibly related to the differences among O. volvulus strains (Pearlman 1996). Eye impairment takes years to develop, which is why affected individuals are those over 40 years old.

Live microfilariae attack several parts of the eye, but here again may cause small reaction; their deaths lead to eye lesions. Eosinophils and neutrophils would cover the dead worms, which then followed by fibroblast propagation and chronic inflammatory infiltrates (Kazura 1993). According to Pearlman (1996), the most significant cause of blindness is sclerosing (scarring) keratitis characterized by the hardening inflammation of the cornea. It shows that degranulating eosinophils, as reaction to worm antigens, interrupt fibril arrangement in cornea.

References

v. Saint Andre, A, Blackwell NM, Hall LR, Hoerauf A, Brattig NW, Volkmann L, Taylor ML, Ford L, Hise AG, Lass JH, Diaconu E, and E Pearlman. 2002. The role of endosymbiotic Wolbachia bacteria in the pathogenesis of river blindness. Science 295:1892-1895.

Pearlman E. 1996. Experimental onchocercal keratitis. Parasitology Today. 12:261-267.

Kazura JW, Nutman TB, and BM Greene. 1993. Flariasis in K.S. Warren (Ed), Immunology and molecular biology of parasitic infections. 3d ed. Boston: Blackwell Scientific Publications.

George GH, Palmieri JR and DH Connor. 1985. The onchocercal nodule: Interrelationship of adult worms and blood vessels. Am. J. Trop. Med. Hyg. 34:1144-1148.

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