The deerflies (genus Chrysops) that transmit Loa loa bite during the day. If a deerfly takes a blood meal from an infected human and ingests larvae, the larvae will infect cells in its abdomen. After 7 to 12 days the larvae mature into larvae that are infective for humans. These infective larvae migrate to the mouth parts of the fly and enter the wound made in the skin by the fly when it takes a blood meal from a human.
Once larvae enter the human body they usually mature to adult worms in around 5 months. Adult worms live between layers of connective tissue under the skin and between fascial layers overlying somatic muscles. Fertilized females can produce thousands of larvae (microfilariae) a day. The larvae then migrate into the lymphatic system and accumulate in the lung. These larvae are intermittently released into the blood stream, usually around midday. It usually takes 5 to 6 months or longer after initial infection for the larvae to be found in the bloodstream. The larvae can survive up to one year in the human body. Adult worms may live up to 17 years in an infected person and can continue to release new larvae into the body for much of this time.
Most people with loiasis do not experience signs or symptoms of infection, though persons who do not live in areas where the parasite is found, such as travelers to the area, are more likely to experience them. The most common manifestations of the disease are Calabar swellings and eye worm. Calabar swellings are localized, non-tender swellings usually found on the limbs and near joints. They are associated with itching that occurs in the area of the swelling or is generalized to the large areas of the body. Eye worm is the visible migration of the adult worm across the surface of the eye. Eye worm can be accompanied by eye congestion, itching, pain, and light sensitivity. Although eye worm can be very distressing, it usually lasts less than one week (often just hours) and commonly causes minimal damage to the eye. Other manifestations of disease include generalized itching, hives, muscle pains, joint pains, fatigue, and adult worms visibly migrating under the surface of the skin. Increased numbers of eosinophils are usually found on blood tests. Persons with long-term infection might develop kidney damage through immune complex deposition, though progression to chronic kidney disease is not common. Other uncommon manifestations include inflammation of the lymph glands, scrotal swellings, and lung infiltrates along with collections of fluid around the lung. Loiasis may also be associated with scarring of heart muscle.