Loiasis

Organism Loa loa, a nematode worm. The life cycle of the parasite is essentially the same as W. bancrofti, except that the vectors are Tabanid flies.

Clinical features The disease is characterized by Calabar swellings (named after a town in Eastern Nigeria), which are transient, itchy and found anywhere on the body. Fever and eosinophilia suggest they have an allergic aetiology. L. loa is often confusingly called the eye worm (to be differentiated from O. volvulus), as the worm is sometimes seen migrating across the conjunctiva, but produces no pathology in the eye.

Diagnosis L. loa is diurnally periodic and diagnosis is made by examining daytime blood in which the microfilaria (Fig. 15.9) will be found. Mansonella ozzardi, M. per-stans and M. streptocerca are also commonly found in blood and skin smears in the same area and need to be differentiated from L. loa as well as W. bancrofti and O. volvulus.

Transmission The vector is Chrysops, a large, powerful fly which inflicts a painful bite, attacking either within the forest or forest fringe.

Incubation period Although microfilariae may appear in the blood after about 6 months, the first symptoms may take years.

Period of communicability Like O. volvulus, the adult can live for up to 17 years, producing microfilariae all this time. L. loa takes 10-12 days to produce infective larvae in the fly.

Occurrence and distribution Loiasis is found in the West and Central African rain forests, especially the Congo River basin.

Treatment Both adults and microfilariae are killed by DEC, but caution needs to be exercised as allergic reactions can be profound. Low dosages of 0.1mg/kg can be used to initiate treatment, gradually building up over 8 days to 6 mg/kg, which is continued for 3 weeks. Steroid cover may be required in those with more than 30 microfilariae/ mm3. Ivermectin will reduce the microfilar-ial stage and produces less reaction, so is more suitable for mass control programmes. However, reactions do still occur, especially in those in whom the worm is seen crossing the eye. So a useful preliminary examination is to show people a picture of the worm in the lower eye and exclude those in which it has been seen.

Control and prevention Extensive control measures are generally not warranted, the main preventive action being against the bites of Chrysops with protective clothing and repellents. Clearing the forest canopy, oiling of pools and mass treatment (with ivermectin) are methods that have been practised in areas of high transmission.

Surveillance Surveys for Calabar swellings or a history of them will indicate the area in which to take a blood smear survey.

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