Mosquito behavior may be immune response, not parasite manipulation

"Normally, after a female mosquito ingests a blood meal, she matures her eggs and does not take another one until the meal is digested," said Lauren J. Cator, postdoctoral fellow in entomology and a member of the Center for Infectious Disease Dynamics, Penn State. "If infected, however, mosquitoes will wait to eat until the parasites developing within the gut mature and migrate to the salivary glands."

It was thought that fasting until malaria could be transmitted was beneficial to the malaria parasite because if the female mosquito was not feeding, she was not being swatted. The return of hunger seemed to correlate with the migration of parasites to the salivary glands. The hungrier the mosquitoes are, the more they feed and the more chances to find new hosts.

Cator and colleagues who included Justin George, postdoctoral fellow; Simon Blanford, research associate; Courtney C. Murdock, postdoctoral fellow; Thomas C. Baker, professor of entomology; Andrew F. Read, professor of biology and entomology and alumni professor in biological sciences; and Matthew B. Thomas, professor of entomology, used a mouse model and showed that indeed female mosquitoes behaved in this way.

It was unclear if the malaria parasite caused the mosquitoes' response or if something else was in play. The researchers also looked at how the infected mosquitoes searched for meals and how they responded to the smell of humans. Although the mosquitoes used were biting mice, they also look to humans for a meal.

George ran the experiments testing the mosquito's sense of smell during various stages of parasite maturity and found that the mosquitoes responded to human smell much more readily once the parasites were ready to transfer to their hosts. The same was found of the meal-seeking behavior of the mosquitoes.

The researchers dissected the insects to determine the exact stage of the parasite in each mosquito they tested and what they found surprised them. They published their results in the Proceedings of the Royal Society B today (May 22).

"There were mosquitoes that took an infected blood meal, but didn't get infected or fought off the infection," said Cator. "These mosquitoes behaved in the same way as the infected mosquitoes."

The researchers then injected mosquitoes with killed E. coli to see the response. While the degree of fasting and food seeking was smaller, the noninfected, E. coli-challenged mosquitoes behaved in the same way. They fasted for about the same time and then went searching for a meal. Their responses to human smell and meal searching behavior also mirrored that of malaria-infected mosquitoes.

"Recently, a group from the Netherlands published in PLOS and while they were only looking at the mature parasites in the salivary glands, they found the same response to human odor," said Cator. "This supports that the response is a generalized response to a challenge rather than a manipulation by the malaria parasite and that our findings are probably relevant for human malaria transmission."

The National Institutes of Health supported this work.