Loiasis or calabar swelling is a parasitic disease characterized by swelling in the subcutaneous tissue of affected human hosts. These swellings are generally called calabar swellings, and they can last for over two weeks in infected human hosts before subsiding. The disease is most prevalent in tropical countries especially in the West African sub region. Loiasis is a blood-borne filarial disease caused by filarial worms that inhabit the subcutaneous tissues of infected human hosts. The disease which can also be called “eye worm” infection is usually characterized by swellings around the affected areas of the body especially at the eyelid, conjunctiva or the arms. The causative agent of loiasis or calabar swelling (i.e. filarial worms) is transmitted to humans by certain species of blood-sucking daytime or diurnal flies which bites and takes a blood meal from the human host. The filarial worms are migratory in nature; and they are known to wander around the subcutaneous tissue from one part of the body to another while producing series of hypersensitivity reactions and localized swellings known as calabar swellings at the affected body sites.
Loiasis is cause by Loa loa, a filarial eye worm known to be endemic in the rain forest areas of West and Central Africa. L. loa or eye worm as it is often called is actively motile, and the filarial worm migrates at maximum rate throughout the subcutaneous tissues including the conjunctiva where their migratory activity produces eye irritation and other inflammatory reactions.
Type and morphology of Loa loa
Loa loa, an eye worm is a nematode (roundworm) that affects the subcutaneous tissues of the body. It is a tissue-infecting filarial nematode that exist mainly as sheathed microfilaria (in blood), infective larva (in its insect vector) and as adult male and female worms (in subcutaneous tissues).
Vector, reservoir and habitat of Loa loa
The insect vector for loiasis is day-biting flies from the genus: Chrysops. Species of insects in this genus that serve as vectors for Loa loa include C. silacea and C. dimidiata. Deer flies or Tabanid flies of the Chrysops genus transmit the parasite to humans during daytime blood meal. Unlike Brugia species and W. bancrofti whose insect vectors are nocturnal in nature, the insect vectors of L. loa (i.e. flies in the genus Chrysops) are diurnal i.e. they are daytime blood feeders.
Clinical signs and symptoms loiasis
Loa loa infection could be overt or covert depending on a variety of factors such as the endemicity of the disease, host immune response, number of infecting filarial worms and the duration of the infection. However, the disease is usually characterized by a remarkable swelling known as calabar swelling at the body site; and this is usually followed by intense itching, pain and fever. Worm migration produces inflammatory allergic reactions in subcutaneous tissues, and there is intense production of tears and pain in the eyes when the worms migrate through the eye conjunctiva i.e. the white part of the eye. There are no serious complications associated with the disease but loiasis in non-immune people can cause serious hypersensitivity reactions.
Pathogenesis of Loa loa infection
Loiasis is caused by the transmission of infective larva of L. loa via the bite of Chrysops flies especially the tabanid or deer flies. Blood-sucking diurnal Chrysops flies are the main intermediate host or insect vectors of L. loa. During blood meal by the insect vector which normally takes place in the daytime and at the arm region or other exposed parts of the body, the infective larva of L. loa enter the body via the wound created by the insect bite on the skin as it takes a blood meal. Infective larva of L. loa penetrates the subcutaneous tissues; and then develops into adult worms which migrate within the affected subcutaneous tissues and even under the eye surface especially the conjunctiva where inflammatory reactions occur. In most cases, people infected with L. loa are asymptomatic; and the disease is usually characterized by the formation of swellings and other associated allergic reactions at affected body sites, and the swellings usually last for only some few weeks before disappearing. Migration of filarial worms of L. loa through the eyes does not cause blindness but instead, the worm migration in the conjunctiva of the eye causes pain, irritation, itching and inflammatory reactions. Mature female worms produce sheathed microfilariae which eventually enter the bloodstream and are taken up by female Chrysops flies during the next blood meal when transmission of the parasite to susceptible human host will occur (Figure 1).
Figure 1: Life cycle of Loa loa. 1. During a blood meal, an infected fly introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound. 2. The larvae develop into adults that commonly reside in subcutaneous tissue of the host. The female worms measure 40 to 70 mm in length and 0.5 mm in diameter, while the males measure 30 to 34 mm in length and 0.35 to 0.43 mm in diameter. 3. Adults produce sheathed microfilariae (that measures 250 to 300 μm by 6 to 8 μm). The microfilariae have diurnal periodicity, and they have also been recovered from spinal fluids, urine, and sputum specimen of infected human hosts. During the day they are found in peripheral blood, but during the non-circulation phase, they are found in the lungs. 4. The Chrysops fly ingests microfilariae during a blood meal from an infected human host. 5. After ingestion, the microfilariae lose their sheaths and migrate from the fly’s midgut through the hemocoel to the thoracic muscles of the arthropod. 6. There the microfilariae develop into first-stage larvae. 7. The first-stage larvae subsequently develop into third-stage infective larvae. 8. The third-stage infective larvae migrate to the proboscis of the insect vector where it stays until the next blood meal. CDC
Laboratory diagnosis of Loa loa infection
L. loa infection is diagnosed in the laboratory by the identification of microfilariae (Figure 2) in the blood sample of infected persons. Since the parasite is diurnal in nature and microfilariae can only be found in the blood during the day time, blood samples for loiasis investigation should only be collected at the daytime period in order to get optimum result even though microfilariae detection could often be difficult in some diseased cases.
Figure 2: Sheathed microfilariae of L. loa in thin blood smear stained with Giemsa. Microfilariae of L. loa are tapered at the tail region; and they also contain nuclei. CDC
Treatment of Loa loa infection
Loiasis is usually not treated since the infection in most of the cases is subclinical or asymptomatic, benign and self-limiting in most of the cases. However, diethylcarbamazine (DEC), a cidal antiprotozoal agent could best be used for treating loiasis. Filarial worms can also be removed from the body surgically, and this is usually carried out in heavy infections.
Control and prevention of Loa loa infection
Chrysops flies are diurnal insect vectors that only feed on human blood during the daytime. Thus human infection with L. loa filarial worms can be prevented by wearing protective body clothing especially when engaging in daytime outdoor activities. Insecticides should be used to control the insect vectors of the parasite, and human settlements should be sited away from forest areas and bushes where the Chrysops flies normally reside in endemic areas.
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