Loiasis aetiology and management

Loiasis aetiology and management

Loiasis aetiology and management

Loiasis is a filarial infection characterized clinically bytransient soft tissue swellings at sites of adult Loa loa wormdeath or migration of adult worms across the eye, subconjunctivally.Chrysops flies are the vectors.

Aetiology Loa loa adults measure about 60 x 0.5mm (female) and30 x 0.4mm (male). They migrate freely in subcutaneoustissues. Six months after infection adults have developedand fertilized females start to shed microfilariae,which are found in peripheral blood during the daytime.Chrysops flies ingest microfilariae in blood meals. Developmentto adult worms takes place in subcutaneous tissues,and shedding of microfilariae begins by 90 days after infection.Adults live for up to 15 years.

Epidemiology The endemic areas cover the tropical rainforestregions of West and Central Africa, extending as far eastas the southern Sudan and Uganda. The vectors live in theforest canopy and descend to lower levels to feed. Theprevalence in some populations is 100%. Animal reservoirsdo not play a part in the cycle of transmission.

Clinical features Transient itchy soft tissue swellings up to 7cm across, onthe limbs or less often the face, are a common presentation;these are called 'Calabar swellings' and are most oftenseen in people who have lived in endemic areas for a relativelyshort time. In indigenous residents of endemicareas a more typical presentation is migration of an adultworm across the globe of the eye beneath the conjunctiva,causing local irritation, a feeling of movement and some alarm.Occasionally a worm dies immediately beneaththe skin, producing a linear swelling in the skin.Problems can arise in patients who have large numbersof microfilariae in the peripheral blood (over 25-50/mm3),beginning a short time after starting treatment with DEC.Encephalitis or encephalomyelitis can occur, probablybecause of dead microfilariae occluding vessels of the brainand spinal cord. Cerebral oedema and granulomatousreactions around microfilariae are found in fatal cases.

Diagnosis The diagnosis is confirmed by finding typical sheathedmicrofilariae in peripheral blood samples taken during theday. 3 When there are few microfilariae, millipore filtration of blood may show parasites. Eosinophiliain the peripheral blood is usual and a positive serologicaltest, e.g. filaria indirect fluorescent antibody test, providesonly indirect evidence of infection.

Management DEC is the most active drug in the treatment ofloiasis. It kills microfilariae well and adult worms moreslowly. While the patient is on DEC, Calabar swellings mayappear. Also, worms dying close to the skin may be seen.When microfilariae counts are above 25-50/mm3 DEC cannot be given alone because of the danger of precipitating cerebral complications. Removal of microfilariae fromthe circulation is recommended. Whole blood exchange transfusion, or apheresis using a blood separator, can beused to remove large numbers of microfilariae. This makesit safe to give DEC. Prednisolone in doses of 40-60 mg/day,beginning 24 hours before starting DEC and continuing tillthe top dose is reached, is usually more convenient.

Prevention and control Measures for the control of loiasis have not been successful.Vector control is difficult. Mass chemotherapy is notappropriate because of the need for individual supervisionof treatment in people with heavy infections, and becauseof the coexistence of onchocerciasis in the same endemic areas. Other measures include protection from biting byclothing and the use of insect repellants.

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