A rodent malaria, Plasmodium berghei, is experimentally transmitted to mice by merely probing of infective mosquito, Anopheles stephensi

We found that infection of a rodent malaria, Plasmodium berghei, occurred when the sporozoites were injected into the skin, the muscle, the peritoneal cavity and the tail end. Mice, which were injected with sporozoites in the tail end and had the site cut 5 min later, did not develop malaria. We also found that mice developed malaria when malaria infective mosquitoes, Anopheles stephensi, were forced not to take blood but only to probe into the skin. Moreover, the mice probed by the infective mosquitoes were protected from malaria infection if the site was treated with Kyu (heat treatment) after the mosquitoes had probed. These findings indicate that malaria infection occurs not only by blood feeding of the infective mosquito but also by probing of the mosquito. Sporozoites injected into the skin remain at the injected site for at least 5 min, then migrate to the blood vessels and invade into the blood stream. At present, the mechanism is not clear, although we propose here the existence of the skin stage of malaria parasites before the liver stage and the blood stage.