Epidemiology and Pathology

The transmission of onchocerciasis between people requires an insect vector, which in this disease is the blackfly. Many species of blackfly (or Simulium spp.) act as vectors in different geographic regions. A requirement by all simuliids is the need to breed in fast-flowing, well-oxygenated rivers. Blackflies are daytime feeders and have a limited flight range of a few kilometres from their aquatic breeding sites. Onchocerciasis is therefore mainly a rural disease in tropical regions where there is forest or savanna containing fast-flowing rivers with rapids.

The microfilariae of O. volvulus (Leuckart 1893) have been known in Africa since the late 1800s and the adult worms were described in Africa and Central America by the early 1900s. Knowledge of the life cycle involving the blackfly vector was soon to follow. The interested reader is referred to an excellent historical account of onchocerciasis by Grove and another by Studeman, Fishback and Connor.

The life cycle is similar to that in lymphatic filariasis in that there is a human definitive host and an insect vector that facilitates transmission between people. The infection begins when the blackfly vector, Simulium spp., bites skin. Infective third-stage larvae are released onto the wound at the time of the meal and migrate into deeper tissues, molt twice, and develop to adults. Much of the migration and development is unknown, but adults finally come to reside in subcutaneous nodules (Figs. 26.22-26.24). The ratio of adult female to male worms is 3:1. Female nematodes are 40-70 cm in length by 0.4 mm in width. Males are 2.5 cm by 0.2 mm. Females deposit large numbers of unsheathed microfilariae into the nodules (larviposition). These microfilariae then wander throughout the human host, mainly within the dermis. The prepatent period, from blackfly bite to larviposition, ranges from 6 to 30 months. The larval development within the simuliid is quicker - only 6-8 days. Following ingestion of microfilariae during a blood meal by the blackfly, the microfilariae exit the stomach, penetrate the blackfly's thoracic muscles, and then migrate to the proboscis. During this time molting and development take place, resulting in infective third-stage larvae.

The pathology in onchoceriasis can be conveniently divided into two categories: that caused by microfilariae and that caused by adult worms. The most significant lesions are caused by wandering microfilariae which can occur in loads as high as 2,000 larvae/mg of skin. Microfilariae are found in all layers of skin, but are most concentrated in the dermal papillae. This is perhaps an adaptation to the feeding habits of Simulium spp. because the dermal papillae are closest to the epidermis, where the cutting mouthparts and saliva may free and attract microfilariae, respectively. The skin and eyes are the sites where the clinical manifestations are most severe (with lymph nodes also involved). Onchocercal dermatitis is inflammatory in origin and is progressive over time, ultimately leading to fibrosis and replacement of normal skin elements. Changes in skin pigmentation, edema, thickening of the dermis and scarring of the epidermis are hallmarks of this disease. Dermal elastic fibres are lost gradually, resulting in wrinkled, droopy skin.

Autopsy data have revealed that larval and adult worms migrate throughout the body. Worm nests composed of adult male and female nematodes form nodules in the skin called onchocercomata. These onchocercomata are common over skeletal promontories, are painless, and are variable in size from a few millimetres to several centimetres. In Africa, the skin nodules are more often in the lower trunk and limbs, whereas in Central America they occur more often in the thorax, head, and neck (Figs. 26.22, 26.23). This has been explained as an evolutionary adaptation of the parasite to the different feeding site preferences by the blackfly - this is species-specific.

Histopathologically, onchocercomata are firm and have three discrete layers: (1) an outer fibrous layer of granulation and scar tissue, (2) a middle layer of inflammatory cells, especially eosinophils, plasma cells, and foamy macrophages, and (3) a central core which is soft and contains adult female and male nematodes surrounded by Splendore-Hoeppli material (Figs. 26.23 B, 26.25). Huge giant cells are also seen.

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Fig. 26.22A,B. Three South American boys with scalp nodules due to onchocerciasis.

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Fig. 26.23. A Excision of an onchocercoma under local anesthesia. B A nodule opened to show coiled adult worms in the central soft core. There is a fibrotic outer layer.

Fig. 26.24. Coiled adult male and female. O. volvulus in a worm nest digested from a pelvic nodule in an African patient. An intact nodule is shown on the left. (From Marty and Anderson 1995).

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Fig. 26.25. A Microfilariae - Onchocerca volvulus - in the subcutaneous tissue. B An onchocercal nodule sectioned to reveal coiled adult worms cut at different angles. x15. C Sections of female Onchocerca volvulus. There are microfilaria in the paired reproductive structures and surrounding acute and chronic inflammation. x39. (B, C from Marty and Andersen 1995).