UK scientists reviewed 26 studies that measured levels of Plasmodium parasites -- which cause malaria -- among blood donors in sub-Saharan Africa between 2000 and 2017 and found that an average of 23.46% tested positive.
Percentages varied greatly across the nine countries included in the study, ranging from 0% to as much as 74% of donors carrying the parasites in their blood.
Overall, there is a high risk that a potential blood donor or bag will contain parasites, said Dr. Philippe Guerin, director of the Worldwide Antimalarial Resistance Network and professor of medicine at Oxford University's Centre for Tropical Medicine and Global Health. "In an ideal world, systematic screening would prevent that" being used on patients.
"Malaria is one of the primary infections that can be transmitted through a blood transfusion in sub-Saharan Africa," said Selali Fiamanya, a research fellow at the Worldwide Antimalarial Resistance Network who also worked on the study.
Blood supplies are typically screened for blood-borne diseases before being made available to recipients, but Guerin believes that screening is not always being conducted systematically and that when it is, current lab techniques are not sensitive enough to spot all malaria parasites, particularly latent infections or when parasites are hiding in people who are infected but symptom-free.
"The technical challenges of diagnosing and removing the Plasmodium parasites from blood banks requires further analysis, but we know already that these findings put the next generation at risk," Fiamanya said.
Blood donors are usually adults, and in regions with high rates of malaria, adults often develop some immunity against the parasite, meaning they could have the parasites in their blood but not feel sick, Guerin said.
Meanwhile, pregnant women and children receive the majority of transfusions in this region, Fiamanya said.
"Blood transfusions are lifesaving interventions," Guerin said. "The point is to assess the risk and put interventions in place to mitigate the risk."
He suggested using tools with increased sensitivity to the parasites, treating blood recipients with antimalarial drugs or using technologies to filter blood before it is given to patients.
The transmission risk of contracting malaria through blood supplies is unknown but is likely to be high, Guerin said. "If you've got parasites circulating, the infection risk is high."
However, in terms of public health at the population level, blood transfusions are rare, he noted. Eliminating malaria from a population "needs a range of interventions to reduce transmission," Guerin said. "It's a puzzle, and blood transfusion is part of that puzzle."
The team now hopes to investigate the likelihood of contracting the disease when infected with Plasmodium parasites through blood transfusion and to enable further research to identify the risk in other countries.
"Both in Latin America and in Southeast Asia, and there is scope for future study," Fiamanya said. "Transmission, resistance and species variation all differ greatly across regions, but it is likely it is also occurring elsewhere."
"It is well-known that malaria can be transmitted through blood transfusion and that ideally, donor blood would be screened for the presence of parasites. But the scale of the problem is not well-known," said Aubrey Cunnington, a clinical senior lecturer and member of the Network of Excellence in Malaria at Imperial College London, who was not involved in the research.
But he highlighted that "there have been enormous changes in malaria transmission intensity in some countries over the period 2000-2017. The date of each individual study is likely to be an important determinant of how common it was to find parasites in blood."
The Plasmodium parasite also has a complex life cycle and can infect either humans or mosquitoes more readily at certain stages.
"We do not know whether the report is referring only to the stage of the parasites which can cause disease or whether it is also referring to the stage which can be transmitted by mosquito bite," Cunnington said.
"The data reported don't tell us directly what the impact on transmission, control or elimination might be. However it does remind us of this problem ... and it highlights the need for more work on this problem."