Vaccination to prevent transmission of Plasmodium yoelii malaria.

It was possible to block the transmission of infection of the rodent malaria parasite Plasmodium yoelii nigeriensis to Anopheles stephensi mosquitoes by immunizing mice with a vaccine containing formalin-fixed gametes. Both intramuscular and intravenous routes were effective, immunity was achieved with a single dose and the immunity persisted for 6 months at least. Transmission-blocking immunity was found to reside in a serum factor, probably antibody, and to be directed against extracellular gametes, acting on them in the gut of the mosquito, while gametocytes in the circulation of the vertebrate host remained unaffected. The gamete vaccine afforded partial protection against the disease, but immunization with asexual parasites alone showed that this protection was due largely to the presence of asexual forms as contaminants and that anti-gamete immunity is stage specific.