An international team of researchers has found that a malaria parasite, Plasmodium vivax, not only circulates in the blood but also takes up residence in the bone marrow.

P. vivax is the malaria parasite responsible for the vast majority of infections outside sub-Saharan Africa. The findings, published in the journal mBio, may help explain why many P. vivax infections go undetected. Based on analysis of P. vivax infections in people and non-human primates, the study revealed bone marrow as an important -- and previously unstudied -- reservoir for parasite replication and transmission.

People with infections who show no signs or symptoms may host undetected parasites in their marrow, the researchers said. "People with infections who are not sick are transmitting the disease before they have symptoms," said co-lead researcher Nicanor Obaldia III from Harvard T.H. Chan School of Public Health in Boston, Massachusetts.

The researchers showed that P. vivax gametocytes, cells that are essential for transmission, mature rapidly in the bone marrow and can be transmitted to mosquitoes before the onset of illness. P. vivax was once considered a 'benign' malaria because it's often asymptomatic, is characterised by low numbers of detectable parasites in the blood and is less lethal than its parasite cousin, P. falciparum, which causes 90 per cent of malaria deaths worldwide.

But P. vivax can cause severe and even fatal symptoms, and more than a third of the world's population is at risk. Tens of millions of people are infected annually, and the parasite poses a significant health care burden in many regions including Southeast Asia and South America.

In the bone marrow, the parasite may remain undetectable by blood tests, and as a result, public health workers may not be able to accurately estimate the parasite burden, said senior study author Matthias Marti from Harvard T.H. Chan School of Public Health. "If there is a large reservoir in the bone marrow in the absence of detectable parasites in the blood, then you may underestimate the number of people that are able to transmit the disease," he said.