Loa loa is a parasitic worm that infects people in West and Central Africa and is spread by the bite of a deerfly. Adult worms (macrofilariae) live under the skin and cause symptoms such as swellings, itching, and hives. Smaller worms (microfilariae) are found in the bloodstream. Diethylcarbamazine (DEC), the recommended medication for Loa loa infection, can produce very serious side effects, especially in people with high numbers of parasites in the blood. Researchers are investigating new treatments for Loa loa that have fewer or less serious side effects. Researchers believe that a certain kind of blood cells called eosinophils, which increase in the blood after DEC treatment, may be one of the causes of the side effects seen with DEC treatment. Reslizumab is a drug that lowers eosinophils in the blood. Giving reslizumab before DEC treatment might prevent the eosinophils from increasing and reduce some of the side effects from DEC.