Every year about 390 million people are infected with dengue virus through the bite of mosquitoes of the Aedes genus. As per an estimate of these infected people about 300 million are asymptomatic which means they do not have clinical symptoms that are severe enough to be detected by health care systems. Previously, it was assumed that these asymptomatic or mildly symptomatic infections did not have high enough level of viremia (the concentration of the virus in the blood) to infect mosquitoes.
A new study by researchers at the Institut Pasteur in Cambodia, the Institut Pasteur in Paris and the CNRS published in the journal PNAS, have revealed that people infected by dengue virus but showing no clinical symptoms have the ability to infect mosquitoes that bite them.
The results of the study suggest that these asymptomatic people who along with mildly symptomatic patients, represent three-quarters of all dengue infection and these people have a significant role to play in the transmission chain of the virus. The findings of the study put a question mark on the established theories concerning the epidemiology of dengue.
For this study, the researchers from the Institut Pasteur in Cambodia, the Institut Pasteur in Paris, and the CNRS studied the people with few or no symptoms that represent 75% of all dengue infections, and experimented to test if whether these people could actually infect mosquitoes that bite them.
The researchers chose the participants of the study from a human population at risk of dengue infection in Cambodia, in the town of Kampong Cham, roughly 100km northeast of Phnom Penh. Their research strategy depended on their ability to detect dengue infections that had not been identified by traditional health care systems because the participants were asymptomatic. Blood tests were carried out on people living in close proximity – like in the same household or in the immediate neighborhood – to patients with confirmed symptomatic dengue. Those people who tested positive for dengue virus in their blood tests but showed no clinical symptoms were then put into contact with healthy laboratory-bred mosquitoes.
When these mosquitos were later analysed it was found that they were infected and had become capable of transmitting the virus the next time they bit a human. It was also found through the research data that the viremia level has a significant role to play in transmission of dengue virus from a human to a mosquito.
Louis Lambrechts, a CNRS scientist in charge of the Insect-Virus Interactions Group at the Institut Pasteur in Paris opined that the findings of the study are significant. It raises the possibility that asymptomatic people with dengue – the ones that form a majority of people infected by dengue – may have a big contribution to the spread of the virus without realizing it.
Also, people who are virtually or completely unaffected by the virus have a greater chance to be exposed to more mosquitoes during their daily routines than those who are severely ill, bed-ridden or hospitalized.
Veasna Duong, a scientist in the Virology Unit directed by Philippe Buchy at the Institut Pasteur in Cambodia, when this work had been done remarked that these findings should make us review and reconsider our approach to the early management of dengue epidemics. There is also a need to check and adjust the transmission rate estimates to ensure sufficient vaccination coverage for the vaccines currently under development. Meanwhile, in another leg of the study at the Institut Pasteur in Paris, the DENFREE European project, coordinated by Anavaj Sakuntabhai, investigation about the specific biological characteristics of asymptomatic infections is underway.