All of the immune genes that were involved in limiting infection by the malaria parasites were also important for the resistance to bacterial infection. However, several immune genes that were essential for resistance to bacterial infection did not affect Plasmodium infection. According to the authors, the findings add to the understanding of mosquito immunity, and could contribute towards the development of malaria-control strategies based on blocking the parasite in the mosquito. The study is published in the June 8, 2006, edition of PloS Pathogens.

"Mosquitoes that transmit malaria can kill large portions of Plasmodium parasites, and some mosquito strains are totally resistant to Plasmodium. However, our observations suggest that mosquitoes have not evolved a highly-specific defense against malaria parasites. Instead, they employ factors of their antimicrobial defense system to combat the Plasmodium parasite," said George Dimopoulos, Ph.D., senior author of the study and assistant professor with the Bloomberg School's Malaria Research Institute. "The degree of mosquito susceptibility to Plasmodium, and thereby its capacity to transmit malaria, may therefore partly depend on the mosquito's microbial exposure, which can differ greatly between different geographic sites. Potentially, we could boost the mosquito's capacity to fight the malaria parasite by exposing it to certain microbes or compounds that resemble the microbe molecules responsible for immune activation."

In this study, the investigators also analyzed the immune responses of Anopheles gambiae mosquitoes to infection with different Plasmodium parasite species, one that causes malaria in humans and another that only infects rodents. The study revealed that mosquitoes mostly employ the same immune factors in defending against the two different Plasmodium species. Only a few immune genes were more important in the defense against either one of the two species.

"The mosquito's immune system appears to employ a variety of antimicrobial defense factors (genes) against the malaria parasite, and can therefore significantly limit infection. The parasite, on the other hand, is capable of evading these defenses to a degree that allows its transmission by the mosquito. Now we have to figure out how to make the mosquito's immune system more effective in killing malaria parasites at multiple stages that would render the development of evasive mechanisms impossible for the parasite," said Dimopoulos.

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Materials provided by Johns Hopkins University Bloomberg School of Public Health. Note: Content may be edited for style and length.