Loa loa filariasis or loiasis is a skin and eye disease caused by the nematode worm Loa loa. Humans contract this disease through the bite of a deer fly or mango fly (Chrysops spp.), the vectors for Loa loa. The adult Loa loa filarial worm can reach from three to seven centimetres long and migrates throughout the subcutaneous tissues of humans, occasionally crossing into subconjunctival tissues of the eye where it can be easily observed. Loa loa does not normally affect vision but can be painful when moving about the eyeball or across the bridge of the nose. Loiasis can cause red itchy swellings below the skin called "Calabar swellings". The disease is treated with the drug diethylcarbamazine (DEC), and when appropriate, surgical methods may be employed to remove adult worms from the conjunctiva. Loiasis belongs to the group of neglected tropical diseases, and there is a call for it to be included in the high-priority listing.
Signs and symptoms A filariasis such as loiasis most often consists of asymptomatic microfilaremia in peripheral blood. Some patients can develop lymphatic dysfunction causing lymphedema. Episodic angioedema (Calabar swellings) in the arms and legs, caused by immune reactions, are common. Calabar swellings are 3–10 cm (1.2–3.9 in) in surface area, sometimes erythematous, and not pitting. When chronic, they can form cyst-like enlargements of the connective tissue around the sheaths of muscle tendons, becoming very painful when moved. The swellings may last for one to three days and may be accompanied by localized urticaria (skin eruptions) and pruritus (itching). They reappear at referent locations at irregular time intervals. Subconjunctival migration of an adult worm to the eyes can also occur frequently, and this is the reason Loa loa is also called the "African eye worm". The passage over the eyeball can be sensed, but it rarely causes permanent eye damage. Eyeworms affect men and women equally, but advanced age is a risk factor. Eosinophilia is often prominent in filarial infections. Dead worms may cause chronic abscesses, which may lead to the formation of granulomatous reactions and fibrosis. Loiasis is often thought to be a mild disease. It can sometimes cause unusual problems in the kidneys, heart, brain and nerves, lungs, eyes, and skin. In the human host, Loa loa larvae migrate to the subcutaneous tissue, where they mature into adult worms in approximately one year, but sometimes up to four years. Adult worms migrate in the subcutaneous tissues at a speed of less than 1 cm/min, mating and producing more microfilariae. The adult worms can live up to 17 years in the human host.
Cause
Transmission Loa loa infective larvae (L3) are transmitted to humans by the deer fly vectors of the tabanid genus Chrysops—C. dimidiata and C. silacea. These carriers are blood-sucking and day-biting, and they are found in rainforest-like environments in western and central Africa. Infective larvae (L3) mature to adults (L5) in the subcutaneous tissues of the human host, after which the adult worms—assuming the presence of a male and female worm—mate and produce microfilariae. The cycle of infection continues when a non-infected mango or deer fly takes a blood meal from a microfilaremic human host, and this stage of the transmission is possible because of the combination of the diurnal periodicity of microfilariae and the day-biting tendencies of the Chrysops spp.
Reservoir Humans are the primary reservoir for Loa loa. Other minor potential reservoirs have been indicated in various fly-biting habit studies, such as hippopotamuses, wild ruminants (e.g. buffalo), rodents, and lizards. A simian type of loiasis exists in monkeys and apes which is transmitted by the deer fly vector Chrysops langi. There is no crossover between the human and simian types of the disease.
Vector Loa loa is transmitted by several species of tabanid flies. The two most prominent vectors are from the tabanid genus Chrysops—C. dimidiata and C. silacea. These species exist only in Africa and are popularly known as deer flies, mangrove flies or mango flies. Chrysops langi is also a competent vector species. Chrysops spp. are small (5–20 mm, 0.20–0.79 in long) with a large head and downward-pointing mouthparts. Their wings are clear or speckled brown. They are hematophagous and typically live in forested and muddy habitats like swamps, streams, reservoirs, and rotting vegetation. Female mango and deer flies require a blood meal for the production of a second batch of eggs. This batch is deposited near water, where the eggs hatch in 5–7 days. The larvae mature in water or soil, where they feed on organic material such as decaying animal and vegetable products. Fly larvae are 1–6 cm (0.39–2.36 in) long and take 1–3 years to mature from egg to adult. When fully mature, C. dimidiata and C. silacea assume the day-biting tendencies of all tabanids. The bite of the mango fly can be very painful, possibly because of the laceration style employed; rather than puncturing the skin as a mosquito does, the mango fly (and deer fly) makes a laceration in the skin and subsequently laps up the blood. Female flies require a fair amount of blood for their aforementioned reproductive purposes and thus may take multiple blood meals from the same host if disturbed during the first one. Although Chrysops dimidiata and C. silacea are attracted to canopied rainforests, they do not do their bite there. Instead, they leave the forest and take most blood meals in open areas. The flies are attracted to smoke from wood fires and they use visual cues and sensation of carbon dioxide plumes to find their preferred host, humans. A study of Chrysops spp. biting habits showed that C. dimidiata and C. silacea take human blood meals approximately 90% of the time, with hippopotamus, wild ruminant, rodent, and lizard blood meals making up the other 10%.
Morphology Adult Loa worms are sexually dimorphic, 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 wormlike eggs called microfilariae. These microfilariae are 250–300 μm long, 6–8 μm wide, and can be distinguished morphologically from other filariae, as they are sheathed and contain body nuclei that extend to the tip of the tail.
Lifecycle
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