Loa loa is an arthropod-borne filarial nematode (roundworm) that causes Loa loa filariasis. Loa loa actually means "worm worm", but is commonly known as the "eye worm", as it localizes to the conjunctiva of the eye. Loa loa is commonly found in Africa. It mainly inhabits rain forests in West Africa and has native origins in Ethiopia. The disease caused by Loa loa is called loiasis and is one of the neglected tropical diseases. L. loa is one of three parasitic filarial nematodes that cause subcutaneous filariasis in humans. The other two are Mansonella streptocerca and Onchocerca volvulus (causes river blindness). Maturing larvae and adults of the "eye worm" occupy the subcutaneous layer of the skin – the fat layer – of humans, causing disease. The L. loa adult worm which travels under the skin can survive up to 10–15 years, causing inflammations known as Calabar swellings. The adult worm travels under the skin, where the female deposits the microfilariae which can develop in the host's blood within 5 to 6 months and can survive up to 17 years. The young larvae, or microfilariae, develop in horseflies of the genus Chrysops (deer flies, yellow flies), including the species C. dimidiata and C. silacea, which infect humans by biting them. After bites from these infected flies, the microfilariae are unique in that they travel in the peripheral blood during the day and migrate into the lungs at night.
Morphology
L. loa worms have a simple structure consisting of a head (which lacks lips), a body, and a blunt tail. The outer body of the worm is composed of a cuticle with three main layers made up of collagen and other compounds which aid in protecting the nematodes while they are inside the digestive system of their host. Juveniles have a similar appearance to adult worms, but are significantly smaller. Male adults range from 20 to 34 mm long and 350 to 430 μm wide. Female adults range from 20 to 70 mm long and can be about 425 μm wide. They vary in color.
Lifecycle The human is the definitive host, in which the parasitic worms attain sexual maturity, mate, and produce microfilariae. The flies serve as intermediate hosts in which the microfilariae undergo part of their morphological development, and then are borne to the next definitive host. Two species of Chrysops deerflies, C. silacea and C. dimidiata, are the main vectors for this filariasis.
A fly bearing third-stage filarial larvae in its proboscis infects the human host through the bite wound. After entering the human host, the larvae mature into adults, commonly in subcutaneous tissue. Adult females measure about 40 to 70 mm in length and 0.5 mm in diameter. Males measure some 30 to 34 mm by 0.35 to 0.43 mm. The adult female produces large numbers of microfilariae, about 250 to 300 μm in length and 6-8 μm in width. She continues to do so continuously for her lifetime, which typically spans several years. Microfilariae tend to reside within spinal fluids, urine, and sputum; by day, they also circulate in the bloodstream. Apart from their presence in bodily fluids, however, microfilariae in the noncirculating phase also occur in the lungs. The vector fly ingests microfilariae while feeding on the host's blood. Once inside the vector, the microfilaria sheds its sheaths and escapes through the walls of the midgut into the fly's haemocoel. It then migrates through the hemolymph into the wing muscles in the fly's thorax. In the thoracic muscles, the microfilaria develops successively into a first-stage larva, second-stage larva, and finally into the infectious third-stage larva. The third-stage larva migrates to the fly's proboscis. Once the larva is established in the proboscis and the fly takes its next human blood meal, the cycle of infection continues.
Disease
Signs and symptoms Usually, about five months are needed for larvae (transferred from a fly) to mature into adult worms, which they can only do inside the human body. The most common display of infection is the localized allergic inflammations called Calabar or Cameroon swellings that signify the migration of the adult worm in the tissues away from the injection site by the vector. The migration does not cause significant damage to the host and is referred to as benign. However, these swellings can be painful, as they are mostly found near the joints. Although most infections with L. loa are asymptomatic, symptoms generally do not appear until years, or even more than a decade, after the bite from an infected fly. However, symptoms can appear as early as four months after a bite. These parasites have a diurnal periodicity in which they circulate in the peripheral blood during the daytime, but migrate to vascular parts of the lungs during the night, where they are considered non circulatory. Therefore, the appearing and disappearing characteristics of this parasite can cause recurrent swelling that can cause painful enlargements of cysts in the connective tissue surrounding tendons. Additionally, chronic abscesses can be caused by the dying worms. The most visual sign of an adult worm infections is when the worm crosses the sclera of the eye, which causes significant pain to the host and is usually associated with inflammation and less likely, blindness. Eye worms typically cause little eye damage and last a few hours to a week. Other tissues in which this worm can be found includes: the penis, testes, nipples, bridge of the nose, kidneys, and heart. The worms in these locations are not always externally visible.
Risk factors People at the highest risk for acquiring loiasis are those who live in the rainforests of West or Central Africa. Furthermore, the probability of getting bitten by a deerfly increases during the day and during rainy seasons. The flies are also attracted to smoke from wood fires. These flies do not commonly enter houses, but they are attracted to the houses that are well lit, so will congregate outside. Travelers can be infected in less than 30 days after arriving in an affected area, although they are more likely to be infected whilst being bitten by multiple deerflies over the course of many months. Men are more susceptible than women due to their increased exposure to the vectors during activities such as farming, hunting, and fishing.
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