A single magic bullet to eradicate malaria around the world is unlikely after scientists in London discovered the impact of super-infected mosquitos.
For the first time, researchers have shown that the number of parasites each mosquito carries influences the chance of malaria infection, saying their findings could have global implications for public health and implications for the development of anti-malaria vaccines.
Scientists at the Medical Research Council's Center for Outbreak Analysis and Modelling say new findings may explain why the only registered malaria vaccine, RTS,S, has had only partial efficacy in recent trials.
They say to determine the intensity of malaria transmission, researchers and international organisations like the World Health Organisation currently rely on a measure called the entomological inoculation rate (EIR): the average number of potentially infectious mosquito bites per person per year.
But they say that measure does not take into account how infectious each of the mosquito bites may be, adding there have been no comprehensive studies using biting mosquitoes, which more accurately reflect real-world scenarios.
In a study funded by the PATH Malaria Vaccine Initiative and the Medical Research Council (MRC), published in the journal PLoS Pathogens, researchers have found that the number of parasites each individual mosquito carries influences whether a person will develop malaria. Some mosquitoes can be 'hyperinfected', making them particularly likely to pass on the disease.
The researchers discovered that the more parasites present in a mosquito's salivary glands, the more likely it was to be infectious, and also the faster any infection would develop.
Studies with mice and human volunteers enabled the research team to explain why the malaria vaccine RTS,S is effective only around 50 percent of the time, and why any protection rapidly drops off after three years.
"The vaccine was less effective when mice or humans were bitten by mosquitoes carrying a greater number of parasites. The researchers think this is because the vaccine can only kill a certain proportion of the parasites, and is overwhelmed when the parasite population is too large," their study says,
Dr Andrew Blagborough, from the Department of Life Sciences at Imperial College London, said: "These findings could have significant implications for public health. We have shown that the concept of relying on the number of bites alone to predict malarial burden is flawed, and has probably hampered the successful use of control measures and the development of effective vaccines.
Study co-author Dr Thomas Churcher, also from Imperial, said: "Vaccine development has come a long way, and this new insight should help future vaccine studies to be tested more rigorously. 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."
