In the battle against malaria, science is taking a true 360-degree approach. There are techniques that render mosquitoes immune to the parasites (and give them oddly glowing red eyes) so that they can't pass the disease on to humans; a breath test to diagnose the disease in humans; and an effort to make malaria-carrying mosquitos infertile. But oddly, according to researchers at Imperial College London, one major component to malaria research has been absent – comparing the number of parasites transmitted in a mosquito's bite to disease transmission, rather than just looking at the number of bites themselves. So that's exactly what they set out to study.
And sure enough, it turns out that parasite concentration in a mosquito's blood is a much more important factor in malaria transmission than being bitten multiple times, which might explain why the one and only registered malaria vaccine – RTS,S – only worked some of the time in trials.
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It seems that some mosquitoes are simply "super infected" and those are the ones that are likely to pass along malaria.
"It is surprising that the relationship between parasite density and infectiousness has not been properly investigated before, but the studies are quite complex to carry out," said study co-auhtor Dr Andrew Blagborough, from the Department of Life Sciences at Imperial.
"Researchers have long wondered whether the more malaria parasites in a mosquito's mouthparts, the more likely they are to infect a host with the disease," said Dr Morvern Roberts, programme manager for global infections at the Medical Research Council who funded the research. "No one has been able to demonstrate this until now but the authors of this paper have shown that this is the case in both mouse models and in humans."
In their tests, the researchers subjected sedated mice to bites from mosquitoes with varying levels of parasites in their bodies and found that only the mosquitoes with lots of parasites could transmit the disease. They then moved on to human volunteers and saw the same thing. Study co-author Dr Thomas Churcher, from the MRC Centre for Outbreak Analysis and Modelling at Imperial, told New Atlas that at this time, however, the precise numbers of parasitic transmission are a bit of a mystery.
"In this study we only tested a single dose of the vaccine and the precision of our measure of parasite dose is quite uncertain," he told us.
"What we do know is that in this study only volunteers given a vaccine who were bitten by mosquitoes with more than 1000 parasites in their salivary glands following blood-feeding developed infection," he continued. "This suggests that the vaccine was providing protection against lightly infected mosquitoes but not heavily infected mosquitoes. This is similar to what we saw in mice. Unfortunately we do not know exactly how many parasites were injected into the person as this is currently impossible to tell in human volunteers."
Churcher adds, though, that having this new knowledge and being able to focus on parasitic concentrations rather than on number of bites, as groups such as the World Health Organization currently do, could help dramatically advance the research in the field.
"Vaccine development has come a long way, and this new insight should help future vaccine studies to be tested more rigorously," he said. "However, in the end, it is unlikely that one magic bullet will eradicate malaria, and we should continue to seek and apply combinations of strategies for reducing the burden of this disease."
The group's work will be published today in the journal PLoS Pathogens.