Platelets -- found in the blood -- are the first line of defence in patients with malaria, killing up to 60 per cent of the malaria parasites circulating in the bloodstream, finds a study.

The study found that platelets bind to and kill parasites in patients infected with each of the major malaria parasite species which infect and kill humans -- Plasmodium falciparum, P. vivax, P. malariae and P. knowlesi. "These are important findings and are the first direct evidence of protection by platelets in any human infectious disease," said the lead author, doctoral student Steven Kho from the Menzies School of Health Research, an Australia-based non-profit organisation.

"We found that platelets may kill around 20 per cent of circulating Plasmodium parasites in clinical malaria, and in P. vivax this may be as high as 60 per cent," Kho added. The study, published in the journal Blood, involved 376 people, with and without malaria, from Papua, Indonesia and Sabah, Malaysia. The process of killing the parasites is triggered by a toxic platelet peptide called PF4. The platelets bind to the human red cells, containing the malaria parasites, and kill the parasites by releasing into the red cell PF4.

The findings suggest PF4-based peptides could be potential candidates for malaria treatment in the future, the researchers said. "Given platelets show activity in the test-tube in killing many other microbes that infect humans, and low platelets are a risk for infection in other human diseases, we should consider platelets to be an important first-line defence in how humans protect themselves from microbes," said Brendan McMorran, Associate Professor at The Australian National University. "Previous studies in laboratory mice infected with malaria parasites have shown conflicting results, but the findings in human malaria are now clear - platelets kill parasites," the researchers noted.