The malaria parasite Plasmodium vivax continues to be the most prevalent form of malaria in Asia and South America. Although not as virulent as the Plasmodium falciparum parasite, P. vivax causes debilitating symptoms, and affects more than 400 million people each year. In the past, malaria could once be treated effectively using drugs such as chloroquine, but the emergence and spread of drug resistance worldwide presents a major challenge to eradicating the disease. In an advance that has the potential to help in controlling P. vivax, a reliable method for evaluating the parasite’s invasion of red blood cells has now been developed by Bruce Russell and co-workers at the A*STAR Singapore Immunology Network.
Malaria spreads when mosquitoes carrying the P. vivax parasite transfer it to humans when feeding on human blood. The blood-stage form of the parasite (the merozoite) then attacks the host’s red blood cells (see image). “Merozoite invades red blood cells through a single cell surface receptor, making it an attractive vaccine target,” explains Russell. “Until now, however, we have lacked a reliable way of assessing the ability of candidate vaccines to block infection.”
The newly developed protocol for routine culturing of P. vivax involves minimal equipment and is highly practical for use in countries where the parasite is endemic, but resources are limited. It uses immature red blood cells (reticulocytes) which are the main targets of the P. vivax parasite. Because reticulocytes are short-lived and represent a tiny fraction of blood cells circulating the body, the researchers used blood extracted from umbilical cords. The cells were than concentrated and mixed with matured parasites that were pre-treated with the trypsin enzyme.
“We isolated and enriched parasites from blood samples taken from infected patients attending a clinic in northwest Thailand, saving the difficult task of having to culture them in the laboratory,” says Russell.
After incubation, the researchers were able to assess parasite invasion under the microscope, and to evaluate antibody performance in inhibiting the parasite. They confirmed the reliability of the protocol using a large panel of P. vivax isolates freshly collected from patients in Thailand.
“We are indebted to our collaborator François Nosten of Oxford University, UK, who has devoted his life to combating malaria in East Asian refugee camps for over 20 years,” says Russell. “Without him, this work would have been far harder to undertake.”
The researchers are now working to automate the technique for use in the field, and hope that it will assist in the development of an effective vaccine against this debilitating disease.
The A*STAR-affiliated researchers contributing to this research are from the Singapore Immunology Network.