Malaria is caused by Plasmodium parasites that are transmitted by mosquitoes. It kills over a million people very year, many of them children under the age of five. Creating a malaria vaccine is proving difficult, partly because immunity to the parasite is difficult to acquire naturally, and partly because we still do not know exactly what constitutes protective antimalarial immunity in humans.
The idea of using irradiated parasites as a vaccine has been around for some time; irradiated parasites provoke an immune response but are unable to fully develop in the human body. However, this technique is only effective if the patient is bitten by irradiated mosquitoes more than 1,000 times over 5 or more immunisation sessions. This renders the technique rather impractical.
Tests in animals have shown that immunity can be achieved by inoculating them with intact parasites while treating them with chloroquine, a drug that kills the parasites towards the end of their life cycle in the human body. Furthermore, this treatment appears to be far more effective than the irradiated parasite scheme.
In this small-scale study, scientists from France, the Netherlands and Singapore infected 10 healthy human volunteers with malaria parasites while treating them with chloroquine. The volunteers were bitten by mosquitoes infected with malaria parasites on three occasions at monthly intervals. For comparison, five volunteers underwent the same treatment but were bitten by uninfected mosquitoes.
A total of 8 weeks after the last session with the mosquitoes, and 4 weeks after the last dose of chloroquine, all 15 volunteers were exposed to mosquitoes that were infected with malaria parasites. The volunteers who had been exposed to the parasite in the first phase of the study did not become ill, while all five control subjects developed symptoms of malaria and underwent treatment.
The next question for the researchers is to investigate how long this protection lasts; this will be assessed in a follow-up study. In this experiment, all 15 volunteers were treated with standard antimalarial treatments at the end of the study as a precaution.
The scientists concede that even this vaccination method is not practical for use in the general population. Nevertheless, they note that the findings suggest that "the concept of a whole-parasite malaria vaccine warrants further consideration."
The findings are timely; in another article in the same issue of the NEJM, scientists report on the growing resistance of the malaria parasite to artemisinin compounds, which are currently the most effective drugs we have against the disease.
For more information, please visit:
- Radboud University Nijmegen Medical Centre, http://www.umcn.nl
- New England Journal of Medicine (NEJM), http://content.nejm.org
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