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37-34: Loiasis

Philip J. Rosenthal

ESSENTIALS OF DIAGNOSIS

ESSENTIALS OF DIAGNOSIS

  • Subcutaneous swellings; adult worms migrating across the eye.

  • Encephalitis, which may be brought on by treatment.

  • Microfilariae in the blood.

GENERAL CONSIDERATIONS

Loiasis is a chronic filarial disease caused by infection with Loa. The infection occurs in humans and monkeys in rainforest areas of West and central Africa. An estimated 3–13 million persons are infected. The disease is transmitted by chrysops flies, which bite during the day. Over 6–12 months after infection, larvae develop into adult worms, which migrate through subcutaneous tissues (eFigure 37–57), including the subconjunctiva (leading to the term “eye worm”). Adults can live for up to 17 years.

eFigure 37–57.

Life cycle of Loa loa. The vector for L loa filariasis is flies from two species of the genus Chrysops: C silacea and C dimidiata. During a blood meal, an infected fly (genus Chrysops, day-biting flies) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound

image. The larvae develop into adults that commonly reside in subcutaneous tissue
image. The female worms measure 40–70 mm in length and 0.5 mm in diameter, while the males measure 30–34 mm in length and 0.35–0.43 mm in diameter. Adults produce microfilariae measuring 250–300 mcm by 6–8 mcm, which are sheathed and have diurnal periodicity. Microfilariae have been recovered from spinal fluids, urine, and sputum. During the day they are found in peripheral blood, but during the noncirculation phase, they are found in the lungs
image. The fly ingests microfilariae during a blood meal
image. After ingestion, the microfilariae lose their sheaths and migrate from the fly's midgut through the hemocoel to the thoracic muscles of the arthropod
image. There the microfilariae develop into first-stage larvae
image and subsequently into third-stage infective larvae
image. The third-stage infective larvae migrate to the fly's proboscis
image and can infect another human when the fly takes a blood meal
image.

A flowchart of the life cycle of loa loa.
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CLINICAL FINDINGS

A. Symptoms and Signs

Many infected persons are asymptomatic, although they may have high levels of microfilaremia and eosinophilia. Transient subcutaneous swellings (Calabar swellings) develop in symptomatic persons. The swellings are nonerythematous, up to 20 cm in diameter, and may be preceded by local pain or pruritus. They usually resolve after 2–4 days but occasionally persist for several weeks. Calabar swellings are commonly seen around joints and may recur at the same or different sites. Visitors from nonendemic areas are more likely to have allergic-type reactions, including pruritus, urticaria, and angioedema. Adult worms may be seen to migrate across the eye, with either no symptoms or conjunctivitis, with pain and edema. The most serious complication of loiasis is encephalitis, which is most common in those with high-level microfilaremia and microfilariae in the ...

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