It’s only a single cell, but the avian malaria parasite is able to quickly detect when a mosquito bites its host and respond by reproducing faster, according to a new study.
This is part of the parasites’ strategy for surviving seasons with no mosquitoes. The parasites “remain dormant, like bears during the winter,” said Dr. Sylvain Gandon, an evolutionary epidemiologist at Université de Montpellier and an author of the study. “Do nothing, wait for better times. And it’s a problem for the parasites to know when the better times are coming.” The avian malaria parasite shares this survival challenge with human malaria parasites that live in places with a mosquito-free dry season.
When mosquitoes bit canaries infected with avian malaria, the parasites reacted by reproducing more within nine days. “These malaria parasites, they’re a lot more sophisticated than they’ve been given credit for,” said Dr. Sarah Reece, a reviewer of the study and a Royal Society Fellow at Edinburgh University. The study was published in PLOS Pathogens in September.
The researchers were interested in relapses, when a bird’s level of parasites spikes after a period of dormancy. Relapses don’t happen in the acute phase just after infection, because the levels of parasites are already too high. So for this study, the researchers waited four months to make sure the twenty canaries were in the dormant phase of the infection, and then divided them into a control group and a mosquito-bitten group. The birds in the latter group were bitten by malaria-free mosquitoes three times, and their blood was drawn every three days for a total of five times.
There were more parasites in the blood of the mosquito-bitten birds than the control birds, even though they had all started out equally infected. The malaria parasites started reproducing more as soon as they sensed that mosquitoes had bitten the birds.
This gave the parasites a better chance of hopping a ride on the mosquitoes to a new bird host. When the researchers dissected each batch of mosquitoes to look for parasites in their gut, they found more parasites in the mosquitoes that bit the birds later in the experiment.
The authors have no experimental evidence yet to suggest how a parasite knows a bird has been bitten, but it’s likely that the parasite can detect molecules that the mosquito injects into the bird’s blood. “Given how quickly it seems the parasites can respond to the presence of mosquitoes, it seems likely that they’re directly responding to something in the saliva,” said Reece.
It remains to be seen whether human malaria also has this ability, but Gandon said that other researchers have noticed, in studying some human malaria epidemics over time, that “often the epidemic starts right at the moment when mosquitoes are around… which is a bit strange, because if the epidemic was driven by the mosquitoes, then there would be a lag between the appearance of the mosquito and the epidemic. Because the mosquito, to mature the infection, to be able to transmit, it needs at least one or two weeks.”
A possible explanation for this, said Gandon, is that human malaria can also sense when mosquitoes bite its host. “I think that we’re probably going to find the same sorts of plasticity in other malaria species as well,” agreed Dr. Dave Shutler, parasite ecologist and an author of an earlier study in rodent malaria whose results conflicted with Gandon’s. “I’m quite pleased to see them get the significant results that we were unable to get,” Shutler said.
Finding out whether this is the case in human malaria is important. Information about this could help us predict relapses, which occur frequently in one of the species, Plasmodium vivax, that causes malaria in humans. “If you’re a medic and you can predict when your patients are likely to have a relapse, then… you can be a step ahead of the parasites and treat people before the parasites have gotten a foothold,” said Reece.
“We’ve underestimated them,” said Reece. “It makes you wonder what else they can do on such a short time scale.”