It’s not just the mosquitos spreading dengue among our communities. Healthy mosquitos can catch it from us before they pass it on to our friends and our families.

Why is that so important to understand?

Because our dengue surveillance initiatives and vaccination programs need to understand how we infect mosquitos, as well as how mosquitos infect us, if they’re going to truly succeed. “Dengue vaccine programs need to look at all the people that are contributing to dengue transmission and how the vaccine is changing that,” reveals Professor Thomas W. Scott, Director of the Vector-Borne Disease Laboratory in the Department of Entomology at University of California, Davis (UCDavis).

Mild dengue can be transmitted too

However, until recently, research into how mosquitos catch dengue from us only focused on how healthy mosquitos can become infected by biting people with severe dengue symptoms. It ignored the idea that everyone who catches dengue – however mild – can contribute to the spread of the disease.

“Up to three-quarters of people with dengue don’t become sick enough for it to disturb their daily routine,” comments Professor Scott. “If you want to significantly reduce the spread of dengue, those people are important.”

Professor Scott is leading a five-year research program that is examining the risk of healthy mosquitos catching dengue from an infected person. It’s looking at how our behavior when we’re sick with dengue – whether mild or severe – influences the likelihood of healthy mosquitos catching dengue from us.

The research is closely aligned with the World Health Organization’s (WHO) integrated approach to vector management (IVM). IVM advises that you need a complete picture of every aspect of the virus and how it is transmitted to combat the disease.

Tracing contacts and feeding mosquitos

Professor Scott’s team is made up of researchers from UCDavis and U.S. Naval Medical Research Unit No. 6 (NAMRU-6). They’re examining how the gravity of our illness and the length of time we’re ill affects how infectious we are to mosquitos and the chances of them biting us.
The program is made up of three projects. The first will examine how people’s infectiousness to mosquitos changes over the course of their illness. For this, the team must first go door-to-door to find people infected with dengue.

They’ll then go to the homes that person has visited in the previous two weeks to find other people who are infected. The initial person will have symptoms, but their contacts will often have an illness too mild to even merit a visit to the doctor.

Having found people with all different types and severities of symptoms, the team will feed mosquitos on them to look at how the severity of their illness affects mosquitos’ ability to catch dengue from them. As Professor Scott explains, “The mosquitos will be examined to see if they have become infected and their saliva tested to assess whether they’re able to transmit the disease.”

Comparing severity of illness to mosquito contact

The second project will compare how a person describes how sick they are and how sick blood tests shows they are with the things they do that might increase or decrease their contact with mosquitos.

“Very sick people stay at home. They get bitten by fewer mosquitos than someone who is feeling OK and is out and about, but still has the virus in their blood,” says Professor Scott.

For the third project the team will then take the data from project one and project two and try to link how sick we are with how often we’re getting bit by – and infecting – mosquitos. This project will then create mathematical models of how dengue is transmitted. These models will allow them to understand how the severity of our illness impacts the chances of us spreading dengue to mosquitos.

“The models will help us make sense of some of the patterns of transmissions that don’t make sense if you only look at the people who get sick enough to go to a clinic,” adds Professor Scott.

Learn more about the dengue vaccination program launched in Brazil

Gearing up for the transmission season
The research is still in its early stages. Researchers are currently working out how to feed mosquitos on people’s blood for project one; working on ways to gather the data on illness for project two; and, for project three, they’re using the data that’s already available to develop the models.

“We’re working in Iquitos in Peru where the transmission season is from September until March or April,” reveals Professor Scott. “We’re gearing up for the transmission season later this year.”

Working closely with the community

The team has been working with the local community for over 16 years. Davis faculty member Amy Morrison (seen in the photo opposite) lives in the Peruvian city of Iquitos. The trust developed means the project team is having no problem recruiting people to participate – its acceptance rate to date is around 90%.

“There are people who have questions,” states Professor Scott. “There are people who choose not to participate. But by and large, we have a very good working relationship with the people in the community.”

Even with ample participants, the study may still face significant challenges if transmission is low during a particular season. Unlike laboratory studies where the number of infected people is known in advance, the total number of participants recruited throughout the study will depend on transmission dynamics during the study period.

The study will therefore probably run for a couple of years to ensure its findings aren’t influenced by any abnormal transmission patterns in any one year.

A large collaborative project brings unique opportunities

Professor Scott, however, believes the biggest challenge – and one of the most exciting things about the project – will be coordinating all the different parts of the large collaborative project. The 12 different academic institutions involved in the study must interact efficiently and effectively.
“Each member of the team brings complementary expertise and is outstanding at what they do,” he notes. “We have social scientists collaborating with clinicians, for example, so it is very different to what they would normally do.”

We hope this clear picture on how dengue is transmitted to and from our families and friends will make surveillance initiatives and vaccination programs more effective, giving them the upper hand in the battle against dengue.

Do you know of any other initiatives like this? Tell us about it!