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Microfilaria worms found in tissue and skin The main species of microfilariae found in the skin and tissue are Onchocerca volvulus and Mansonella streptocerca. Microfilariae of Onchocerca volvulus and less often, Mansonella streptocerca migrate through the dermis causing itching and skin texture changes and occasionally arrive in the eye where they cause blindness. Detection of these microfilariae is from skin snips or nodule biopsies. When high numbers of microfilariae are present, they can occasionally be found in the blood and urine.
Oncocerca volvulus Introduction
Oncocerca volvulus is mainly found in West Africa and Central and South America. Oncocerciasis,
also known as river blindness, is a major public health problem, especially in West Africa, there an eradication program has been established. Onchocerca volvulus is transmitted by the species Simulium
or black fly whose breeding habitat is by fast flowing rivers or streams. Life cycle
Morphology The whitish adult worm lies coiled within capsules in the fibrous tissue. The female can measure up to 50 cm while the males are shorter measuring up to 5 cm. The microfilariae of O. volvulus are unsheathed and are usually found in the dermis. They measure between 221 - 287m long.
Head of O.volvulus showing spatulate head
Tail of O. volvulus free of nuclei Clinical Disease Clinical manifestations are due to microfilarie in the epidermis. Light infections may be asymptomatic or cause pruritis. This leads to scratching which can result in infection. Lyphadenopathy may also be a feature of early infection. After months or years, onchodermatitis results in secondary stage of thickening due to intradermal oedema and pachydermis. There is a loss of elastic fibres resulting in hanging groin, hernias and elephantiasis of the scrotum. There is finally atrophy of the skin resulting in loss of elasticity. There is mottled depigmentation of the skin. Ocular lesions are related to the intensity of the microfilariae in the skin. Ocular lesions include sclerosing keratitis, secondary glaucoma and cataract, coroidoretinitisnand fluffy corneal opacities. If untrated, blindness can occur Laboratory diagnosis
1. Analysis of Skin Snips
Small amounts of skin are collected by using a needle to raise the skin and then to slice about 1 mg of skin to a depth of 0.5mm. Snips are collected from several sites, usually the shoulders or the buttocks
and sometimes the chest and calves. The snips are placed immediately in 0.5 ml normal saline in a microtitre plate and left for 4 hours to allow the microfilariae to migrate out of the tissues. After 4
hours, the wells are examined using an inversion microscope. The microfilariae should still be moving and can be identified from the table below. The microfilariae can also be collected by filtration or
centrifugation and the deposit containing microfilariae can be stained with Giemsa at pH 6.8. 2. Analysis of Biopsies
Biopsies of tissue nodules can be dabbed on to a slide to produce impression smears and then stained with Giemsa stain at pH 6.8 for the presence of microfilariae. Mansonella streptocerca Microfilaria of M. streptocerca
were first reported in the skin of a West African patient in 1922. These microfilaria are primarily found in the skin but have been also reported in the blood. Life cycle
The life cycle is the same as that of the blood Mansonella species. Clinical Disease Infection is characterised by pruritic dermatitis and hypopigmented macules.
Laboratory diagnosis M. streptocerca can be diagnosed by demonstrating the microfilaria in a skin snip. Snips are
collected from several sites, usually the shoulders and buttocks and sometimes the chest and calves. The snips are placed immediately in 0.5ml of 0.9% sodium chloride in a microtitre plate and left for 4
hours to allow the microfilaria to migrate out of the tissues. After 4 hours, the wells are examined using
an inversion microscope. The microfilaria should still be moving and can be identified by staining with Giemsa at pH 6.8
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