SINGAPORE – Contracting dengue fever begins with getting bitten by an infected Aedes mosquito. But its saliva also plays a key role, a study has found. 

Not only does it act as an anaesthetic that hides the sensation of being bitten, but the saliva of dengue-carrying mosquitoes also contains a protein which suppresses the human immune system.  

This makes victims more susceptible to haemorrhagic fever, which affected 32,173 people in Singapore in 2022, according to the National Environment Agency (NEA). 

Dr Mariano A. Garcia-Blanco, principal investigator of the study, said the discovery could help explain why the disease is so easily transmitted, and lead to ways to prevent infection.

“It is remarkable how clever these viruses are – they subvert mosquito biology to tamp down our immune responses so that infection can take hold,” he added.

“There is no doubt in my mind that better understanding of the fundamental biology of transmission will eventually lead to effective transmission-blocking measures.”

Dr Garcia-Blanco expects that similar immune-dampening substances will be found in other mosquito-borne infections such as Zika, West Nile and yellow fever. “The specific molecules here are unlikely to apply to malaria, but the concept is generalisable to viral infections.”

The study, funded by the Agency for Science, Technology and Research and the National Institutes of Health’s National Institute of Allergy and Infectious Diseases, was published in the scientific journal Plos Pathogens on March 30. 

The scientists involved discovered that the infected mosquitoes’ saliva contained not just the expected dengue virus, but also molecules known as flaviviral subgenomic RNA.

Stored in tiny bubbles within the saliva, these molecules were found to be specifically responsible for suppressing the immune system of hosts. 

“By introducing this RNA at the biting site, dengue-infected saliva prepares the terrain for an efficient infection and gives the virus an advantage in the first battle between it and our immune defences,” said the researchers in the paper. 

Ms Tania Strilets, a PhD graduate student and co-first author of the paper, said: “It’s incredible that the virus can hijack these molecules so that their co-delivery at the mosquito bite site gives it an advantage in establishing an infection... These findings provide new perspectives on how we can counteract dengue virus infections from the very first bite of the mosquito.”

On March 24, NEA said there had been more than 2,000 dengue cases reported since the beginning of 2023, and 39 active clusters.

It added that weekly cases since the start of 2023 had remained above 100, and that a dengue outbreak could be expected with the warmer months of June to August approaching, due to an expected surge in the Aedes aegypti mosquito population detected in the community.