Loa loa is a nematode that occurs solely in the rainforest area of Central and West Africa.

Adult Loa loa worms are 3 to 7 cm long and 0.5 mm wide. They live 4 to 17 years. The microfilariae appear 6 to 12 months after infection. They measure 230 to 300 m m by 7 m m. The nuclei are found near the end of the tail. There is a residual egg membrane (sheath), but this cannot always be seen in microscopic preparations. The microfilariae are found in the blood and exhibit periodicity, with the highest density occurring around midday. Transmission occurs via the bite of female Chrysops flies. They are insects with beautiful and often yellowish-gold iridescent eyes when they are alive (chrysos = gold). Chrysops flies belong to the Tabanidae. This genus contains principally insects which suck blood of mammals and which are active during the day. There is no animal reservoir.

The adult worms migrate through the subcutaneous tissues. This migration or the intermittent discharge of large quantities of microfilariae causes transient local oedema: Calabar swellings (Calabar is a place in Nigeria close to the border with Cameroon). There is also local redness and itching. When the worm passes under the conjunctiva, it can be observed and removed (Loa loa is for this reason known as the eyeworm). N.B.: there is no intra-ocular invasion and there is no risk of blindness. The adult worm migrates through the loose-meshed subconjunctival connective tissue. This migration can thus be observed macroscopically, in contrast to the migration of microfilariae of Onchocerca volvulus. Tiredness, fever and joint pain may be present. The subcutaneous passage of the worms can sometimes be perceived as an itchy and rapidly moving linear swelling. Dead worms can calcify and thus be radiologically visible (e.g. in the hands and wrists). In general, loiasis is accompanied by massive hypereosinophilia.

*

Loa loa does not occur in Asia. Worms belonging to the genus Thelazia however can be found subconjunctivally in regions of India, Myanmar and China. Certain Dirofilaria species tend to occur in the eyelids / a periorbital location.

Treatment of loiasis is based on administration of DEC for 3 weeks. The dose of DEC should be gradually build up over the course of 4 days, up to 400 mg/day. DEC is both micro- and macrofilaricidal against Loa loa, although often several treatments are necessary. In extremely severe infections, apheresis may be necessary since otherwise there is the risk of meningo-encephalitis. Apheresis however requires complicated and expensive apparatus and specialised personnel. Before starting with DEC, simultaneous onchocerciasis should be excluded in view of the risk of extremely unpleasant Mazzotti reactions in the patient. Mebendazole at high doses and for prolonged periods is also active against Loa loa. Albendazole is effective as well. Ivermectin causes a marked but transient reduction in microfilaraemia. One week after administration, on average 10% of the original microfilaraemia still persist. With high microfilaraemia (>8,000/ml; especially if >50,000/ml) there is an increased chance of neurological complications (headache, confusion, gait disorders, hypertension, incontinence, encephalopathy, coma). Sometimes, the microfilariae can be found in the cerebrospinal fluid. On autopsy, an inflammatory reaction is observed around dying microfilariae in the brain. Removal of the adult worms during their migration beneath the conjunctiva (local anaesthesia with cocaine or tetracaine) is possible. But if this is not done, the worm creeps on spontaneously and leaves the eye. While extracting the worm from the eye, care has to be taken not to rupture the worm, as this leads to a severe inflammatory reaction.

DEC 5 mg/kg, 3 days per month can be used as prevention in an endemic region. DEC 300 mg per week (dose for adults) is also effective. Vector control is problematical as the breeding sites are very diffuse and widespread and the insects bite out of doors.