While Loa loa migration through the subcutaneous tissue may cause prickling and itching sensations, most infected patients are asymptomatic. Loa loa migrating through subcutaneous tissues are rarely visible except when they pass over the bridge of the nose or the conjunctiva of the eye. Worms moving deeper in the facial tissue may induce shifting aches and pains. Adult worms occasionally cross the conjunctiva (see Figure 1), resulting in reddening, swelling of the eyelids and conjunctiva, watering and photophobia. Worms migrating across the conjunctiva are typically visible for 5 to 30 minutes.

Patches of localized subcutaneous edema known as Calabar swellings of up to 20 cm in diameter may occur. Calabar swellings typically last for 1 to 3 days and are characterized by redness, heat, pain and itching. Occasionally a worm can be seen moving under the skin. Loa loa are not always present in the Calabar swelling when it occurs. Calabar swellings appear to result from an allergic reaction to either dead worms or the metabolic products of live worms. Calabar swellings appear to be partially triggered by local muscular activity and appear most frequently on the back of the hand or on the arm. Swelling is sometimes severe enough to temporarily incapacitate manual laborers.

Clinical manifestations are considerably more severe in short-term visitors to endemic areas than in natives. Infection in endemic individuals is typically asymptomatic, with microfilaremia and few Calabar swellings. Non-endemic individuals are typically afilaremic, have severe allergic reactions, and frequently are incapacitated by Calabar swellings, pruritus, and urticaria. Klion et al. (1991) found that among infected visitors to endemic areas, 10% presented with microfilaremia and 95% exhibited Calabar swellings while 90% of infected residents of endemic areas were microfilaremic and only 16% exhibited Calabar swellings. When compared with the subjects not native to endemic areas, the endemic population presented with reduced levels of peripheral blood eosinophils, parasite-specific IgG, and lymphocyte proliferation to parasite antigens. These findings suggest that observed differences in clinical presentation between expatriate and endemic populations with loiasis might result from differences in immune response to parasite antigen. A longitudinal survey by Garcia et al. (1995) found that among microfilaremic individuals, levels of parasitemia remained constant over time and were not influenced by age, suggesting that genetic factors might influence host defense mechanisms against loiasis infection.

Worms occasionally migrate through ectopic sites, provoking unusual reactions. Loa loa located in the tunica vaginalis and spermatic cord have caused hydrocele and orchitis. An adult worm in the bowel wall has caused obstructions via colonic lesions. Documented complications with infection include endomyocardial fibrosis, retinopathy, arthritis, peripheral neuropathy, pleural effusion, and breast calcifications. However, these complications rarely occur in individuals native to an endemic region.

The image of a Loa Loa worm migrating across the eyeball is from the Parasitological Diagnostics Aid Page