Chronic filarial infection is associated with downregulation of immune responses to both helminth and non-helminth antigens. Praziquantel (PZQ) treatment of schistosomiasis is associated with a dramatic interleukin-5 (IL-5)-dependent increase in eosinophilia that is correlated with resistance to reinfection. We hypothesize that chronic filarial infection with Mansonella perstans (Mp) may attenuate post-treatment eosinophilia and thus impact resistance to reinfection. To address the first part of this question, we plan to compare post-PZQ reactions and reinfection rates in 20 subjects with schistosomiasis and Mp infection to those in 20 subjects with schistosomiasis and no evidence of filarial infection in an area coendemic for both infections in Mali. Signs and symptoms, complete blood counts, intracellular and serum cytokine levels, and markers of eosinophil activation will be assessed at baseline, 4 and 8 hours, and 1, 2, 3, 5, 7, 9, and 14 days post-treatment and compared between the two treatment groups. Schistosoma haematobium reinfection rates will also be compared at 1, 3, and 6 months post-treatment.