Encyclopedia of Parasitology

2016 Edition
| Editors: Heinz Mehlhorn

Onchocerciasis, Man

Reference work entry
DOI: https://doi.org/10.1007/978-3-662-43978-4_2207




Onchocerciasis is a tissue infection with   Onchocerca volvulus transmitted by  blackflies (genus   Simulium ), with microfilariae migrating in the connective tissue. The third or fourth-stage larvae enter with the bite of the fly. Blackfly bites are often identifiable by a small petechial spot, sometimes with blood seepage, surrounded by marked inflammatory reaction and itching, or with a small scab on the surface. The larvae develop subcutaneously and when they become adults they frequently congregate to form subcutaneous  nodules. Microscopy shows intertwined worms sometimes surrounded by an eosinophilic coagulate of plasma, suggesting the Splendore-Hoeppli phenomenon and separated by connective tissue containing lymphocytes, plasma cells, and eosinophils ( Pathology, Fig. 18e). The  microfilariae are born, live and migrate through superficial tissues, especially the skin and the cornea. When they are intact they appear innocuous, but when they die they elicit edema, chronic granulomatous inflammation, and fibrosis. With heavy infections there is marked dermatitis with desquamation. Microabscesses ( Abscess) or granulomas with eosinophils are sometimes found around individual dead microfilariae in the skin. There is a relationship between the number of microfilaria dying and the intensity of the proinflammatory response, and an inverse relationship to specific mechanisms suppressing inflammation. The pathogenesis can be duplicated in an accentuated fashion after an injection of diethylcarbamazine, which kills many of the microfilariae and produces intensified inflammation and itching, followed by desquamation. In Africans and Amerindians the lesions are more benign, but contain more filariae; in Europeans and Yemenites lesions are more inflammatory, although with fewer microfilariae. Onchocercal lymphadenitis may give rise to lymphedema with skin swelling and scarring and occasional elephantiasis.

Ocular onchocerciasis results from migrating mirofilariae, some of which die in the eye ( Eye Parasites). This can give rise to punctate keratitis around each dying larva in the cornea, progressing to diffuse sclerosing keratitis with numerous microfilariae, and with the inflammatory reaction followed by vascularization and opacification of the cornea ( Eye Parasites, Fig. 4). Microfilariae in the anterior chamber give rise to iridocyclitis with a possibility of fibrosis, formation of adhesions, or synechiae, and the development of glaucoma. Microfilariae dying in the retina lead to retinochoroiditis with destruction of retinal cells, and depigmentation alternating with proliferation of the pigment epithelium. Inflammation of the optic nerve eventually leads to optic atrophy. All of these lesions, especially those in the cornea and retina, impair vision, and sometimes lead to blindness (e.g., “ river blindness”, Roble’s disease) with heavy or prolonged infections.
  • Main clinical symptoms: Skin nodules, chronic dermatitis, xerodermic conjunctivitis, keratitis, chorioretinitits, atrophia of the nervus opticus.

  • Incubation period: 3–4 months.

  • Prepatent period: 9–30 months.

  • Patent period: 10–16 years.

  • Diagnosis: Microscopic determination of microfilarie from  skin-snips, serodiagnostic methods.

  • Prophylaxis: Avoid the bite of the vector.

  • Therapy: Treatment see  Nematocidal Drugs, Man.

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© Springer-Verlag Berlin Heidelberg 2016