Researchers have identified the dose of dengue virus in human blood that is required to infect mosquitoes when they bite.

Mosquitoes are essential for transmitting the virus between people so the findings have important implications for understanding how to slow the spread of the disease.

By defining the threshold of the amount of virus needed for transmission, the research also provides a target that experimental dengue vaccines and drugs must prevent the virus from reaching in order to be successful at preventing the spread of disease during natural infection.

Recent estimates indicate that there are 390 million infections of dengue across the globe each year and with no vaccine or specific treatment available, current measures to prevent the spread of disease are focused on controlling the mosquito vector.

In research funded by the Wellcome Trust, scientists and doctors at the Oxford University Clinical Research Unit at the Hospital for Tropical Diseases in Vietnam studied the factors that influence the transmission of dengue viruses from dengue patients to the mosquitoes that feed on them.

Their findings reveal that mosquitoes that feed on dengue patients with very high levels of virus in their blood are more likely to be infectious to other humans two weeks later.

"Our findings suggest that focused public health intervention strategies to prevent transmission from these `high risk` spreaders of the virus could have a major impact in slowing the spread of disease," Professor Cameron Simmons, a Wellcome Trust Senior Fellow at the Oxford University Clinical Research Unit in Vietnam said.

The study is published online in the journal Proceedings of the National Academy of Sciences.