Most dengue fever cases are asymptomatic – but they could still be a major driver of viral spread

Dengue fever infects about 100-400 million people every year, mostly in tropical and subtropical regions but people infected with the mosquito-borne virus often don’t have symptoms.

Unfortunately when they do, they can be very serious: the virus causes more than 20,000 deaths each year.

A study published in PNAS Nexus has suggested that the asymptomatic infections are key to understanding dengue outbreaks. Asymptomatic infections are those that don’t have any symptoms.

Drawing on six years of health data from Iquitos, a city in the Peruvian Amazon, the researchers found that 8% of locations that showed up in the city were “super spreader” spaces, responsible for more than half the infections in a dengue fever outbreak.

Nearly three quarters (74%) of the infections linked to these super-spreader spaces were asymptomatic.

“Our findings show that any public health intervention that focuses on responding to symptomatic cases of dengue is going to fail to control an outbreak,” says first author Associate Professor Gonzalo Vazquez-Prokopec, an environmental scientist at Emory University, US.

“Symptomatic cases represent only the tip of the iceberg.”

Iquitos, which has around 370,000 inhabitants, is a dengue hotspot. The researchers drew data from a contract-tracing study which ran in the city from 2008 to 2014, based in two city neighbourhoods, each with around 2,400 participants.

Doctors and nurses visited the homes of participants three times a week to survey them on their movement and check for dengue symptoms.

Read more: What can we learn about malaria from Timor-Leste and West Timor?

“For diseases that are directly spread from one person to another, like COVID-19, what matters is who you were near,” says Vazquez-Prokopec.

“But in the case of dengue, what matters most is where you went.”

When they spotted a symptomatic dengue fever case, everyone who had been in the same place as that case was tested for dengue via PCR. More than half of the participants who tested positive for dengue didn’t have any symptoms.

In this analysis, the researchers mapped “human activity spaces” – places like homes, churches and schools – that could be linked to the spread of dengue fever.

They then tried to figure out what made some spaces spread more dengue fever than others.

“We found that the mosquito numbers in a location alone is not a predictor of the risk of infection,” says Vazquez-Prokopec.

While insect numbers played a role, high numbers of asymptomatic people and lots of visitors who weren’t immune to that strain of dengue were also needed to make a location become a super-spreader space.

“That’s the complicated nature of this virus,” says Vazquez-Prokopec. “We have underestimated the role of asymptomatic cases in spreading dengue.”

Asymptomatic cases, according to this study, were major drivers of dengue fever transmission.

“The lesson is that we need to focus on prevention of dengue outbreaks,” says Vazquez-Prokopec.

“The interventions for dengue for decades have been reactive. Simply reacting by closing a net around reported cases of the disease, however, will fail to contain an outbreak because that’s missing the super spreaders.”

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