Loa loa filariasis overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]
Overview
Loa loa filariasis is a skin and eye disease caused by the nematode worm, loa loa filaria. Humans contract this disease through the bite of a horsefly. The Deer fly and the Mango flyare also vectors for Loa loa. The disease can cause red itchy swellings below the skin called Calabar swellings.
Historical Perspective
The first case of Loa loa infection was noted in the Caribbean (Santo Domingo) in 1770. Localized angioedema, a common clinical presentation of loiasis, was observed in 1895 in the coastal Nigerian town of Calabar—hence the name, Calabar swellings. The association between Loa loa and Calabar swellings was not realized until 1910 (by Dr. Patrick Manson).
Causes
Loiasis is caused by the nematodes (roundworm) Loa loa that can inhabit the lymphatics and subcutaneous tissues of humans. Adult Loa worms are sexual, with males considerably smaller than females at 30–34 mm long and 0.35-0.42 mm wide compared to 40–70 mm long and 0.5 mm wide. Adults live in the subcutaneous tissues of humans, where they mate and produce worm-like eggs called microfilaria. These microfilariae are 250-300μm long, 6-8μm wide, and can be distinguished morphologically from other filariae—they are sheathed and contain body nuclei that extend to the tip of the tail.
Epidemiology and Demographics
Loa loa parasites are found in West and Central Africa. The east-west geographical distribution of the disease extends from southeastern Benin to southern Sudan and Uganda. The north-south geographical distribution extends from about 10°N to Angola. It is estimated that between 3 and 13 million people in West and Central Africa are infected.
Risk Factors
The people most at risk for loiasis are those who live in the high-canopied rain forests of West and Central Africa. The deerflies that transmit the parasite typically bite during the day and are more common during the rainy season. They are attracted by the movement of people and by smoke from wood fires. Rubber plantations create a favorable environment for the flies, as the trees form a dense canopy.
Diagnosis
History and Symptoms
Symptoms include itchy swellings (Calabar swellings) anywhere on the body, that are usually non painful and are often found near joints. Less common symptoms include generalized itching, muscle pain, joint pain, and fatigue. Infected persons may not have any symptoms at all
Laboratory Findings
Diagnosis can be difficult in patients with low levels of larvae in the blood. The diagnostic work-up is usually begun after someone develops eye worm, Calabar swellings, or unexplained elevated levels of eosinophils on blood tests after travel to an affected area. The diagnosis can be made by identification of the adult worm by a microbiologist after its removal from under the skin or eye, identification of an adult worm in the eye by a healthcare provider in a patient with risk factors for infection,identification of the larvae on a blood smear made from blood taken from the patient between 10AM and 2PM and identification of antibodies against L. loa. Unfortunately these tests cannot distinguish between active infection and a history of exposure or past infection and they are not widely available in the United States.
Treatment
Medical Therapy
Treatment of loiasis involves chemotherapy or, in some cases, surgical removal of adult worms followed by systemic treatment. The current drug of choice for therapy is diethylcarbamazine (DEC). The recommend dosage of DEC is 8 - 10 mg/kg/d taken three times daily for 12 days. The pediatric dose is the same. DEC is effective against microfilariae and somewhat effective against macrofilariae (adult worms).[1]
In patients with high microfilaria load, however, treatment with DEC may be contraindicated, as the rapid microfilaricidal actions of the drug can provoke encephalopathy. In these cases, albendazole administration has proved helpful, and superior to ivermectin, which can also be risky despite is slower-acting microfilaricidal effects.[1]
Surgery
Surgical excision of migrating adult worms is an effective treatment for symptoms localized to the migrating worm and provides an opportunity for diagnosis. Systemic therapy would be required to cure the infection unless the patient is infected with only a single adult worm.
Primary Prevention
There are no vaccines available to prevent becoming infected with Loa loa. Diethylcarbamazine (DEC) 300mg taken once a week is effective at preventing loiasis in long-term travelers to affected areas. As the deerflies breed in muddy, shaded areas along rivers and are attracted to smoke from wood fires, avoiding those areas may reduce one's risk of infection. Other prevention efforts include personal protection measures against biting insects. This includes wearing insect repellant such as N,N-Diethyl-meta-toluamide (DEET) on exposed skin, wearing long sleeves and long pants during the day when deer flies bite, and wearing permethrin- treated clothing.
References
- ↑ 1.0 1.1 The Medical Letter – Filariasis. Available online at: http://www.dpd.cdc.gov/dpdx/HTML/PDF_Files/MedLetter/Filariasis.pdf.