The mosquito trackers

Apprehending the insects spreading dengue, chikungunya and Zika

In 2016, Stanford infectious disease expert Desiree LaBeaud, MD, sent teams of schoolchildren to hunt for mosquito larvae and pupae around their homes in coastal Kenya. In particular, they were looking for immature Aedes aegypti mosquitoes.

The kids would say, ‘We found tons. They’re in all these piles of trash at the end of every block,’” said LaBeaud, associate professor of pediatrics at the School of Medicine.

Desiree LaBeaud studies how mosquitoes, people and viruses interact. (Brian Smale photo)

LaBeaud remembers thinking, “Oh, God, now what are we going to do?”

The black-and-white-striped mosquitoes don’t spread malaria, the most famous mosquito-borne disease, but they spread several others, including dengue, chikungunya and Zika, which cause millions of human infections annually throughout the world.

And they adore garbage. Unfortunately, the rural Kenyan communities lacked trash collection and recycling programs. Much of the accumulated litter consisted of discarded, open plastic containers that hold water, where more than 80 percent of the mosquito breeding was taking place, the children and scientists discovered.

For the past several years, LaBeaud’s team has been studying diseases spread by Aedes aegypti and working to reduce outbreaks around the world. Dengue kills about 20,000 people every year; Zika can cause pregnancy loss and serious birth defects; and chikungunya produces debilitating, long-term arthritis in many people. Drugs and vaccines against the viruses are lacking, so there is a pressing need to understand how mosquitoes and humans interact in order to predict and prevent outbreaks. This is what LaBeaud has set out to do.

LaBeaud, trained as a pediatric infectious disease specialist, said the work requires her to be “half ecologist, half anthropologist.” A variety of factors — such as trash collection practices, household water sources and neighborhood violence levels — can all influence local risk for the viruses in the developing world, her team is learning.

Mosquito-borne illnesses are transmitted by intimate chains of insects and humans: An infected mosquito bites a person, who gets sick and is bitten by other mosquitoes, which get infected and bite more people. Most people recover eventually, but the mosquitoes don’t; an infected insect is thought to keep spreading disease until it dies.

Viruses on the move

LaBeaud’s interest in mosquito-borne diseases began on a 2002 trip to Laos. She had recently finished medical school and was completing pediatrics training at Rainbow Babies & Children’s Hospital in Cleveland, where her residency program included an international health track with rotations in developing countries. Her two-month rotation to Southeast Asia overlapped with monsoon season and a large dengue outbreak.

“I treated a lot of children with dengue and saw a lot of children die from dengue,” said LaBeaud. Although many people make a full recovery, dengue hits hard in vulnerable populations, including babies and kids. It can cause life-threatening hemorrhagic fever — with low blood platelet levels and bleeding — as well as dangerous drops in blood pressure. “It’s terrible to have to say, ‘I’m sorry, I can’t help,’ especially being a pediatrician.”

The suffering of her young Laotian patients motivated LaBeaud to study outbreaks of neglected tropical diseases. After her residency, her pediatric infectious disease fellowship took her to Kenya, where she fell in love with the complexity of figuring out how mosquitoes, people and insect-borne viruses interact. “These viruses have both sneaky, insidious transmission and large, overwhelming outbreaks,” she said.

“I treated a lot of children with dengue and saw a lot of children die from dengue. It’s terrible to have to say, ‘I’m sorry, I can’t help,’ especially being a pediatrician.” 

LaBeaud soon learned that insect-borne viruses were on the move. Before 1970, severe dengue had been documented in nine countries. Today it’s in more than 100 countries, putting more than 40 percent of the world’s population at risk. Chikungunya used to be found only in Africa, Asia and India, but is now being reported in Europe and throughout the Americas.

Zika has also become more widespread in recent years, causing a health emergency upon its 2015 arrival in Brazil, for example, where nearly 3,000 infants whose mothers were infected during pregnancy were born with microcephaly and other severe birth defects.

Vaccines for dengue, chikungunya and Zika are in development, but complexities of the viral biology and limited financial support for the research have slowed the work. LaBeaud realized that helping to illustrate the scope of the diseases might improve funding and could assist public health officials in targeting mosquito control efforts.

Hidden outbreaks

In their early stages, both dengue and chikungunya resemble malaria, a parasitic mosquito-borne disease that is widespread in many developing countries. Like malaria, dengue and chikungunya cause high fevers, headaches, chills and muscle aches — and because there are no cheap, accurate, rapid diagnostic tests, in some regions anyone who shows up at a health clinic with these symptoms is automatically diagnosed with malaria.

Since 2014, LaBeaud’s team has been using polymerase chain reaction blood tests to look for genetic material from the dengue and chikungunya viruses and the malaria parasite in blood samples from children treated for fever at

Kenyan health clinics. The scientists’ early data confirm their suspicions that dengue and chikungunya have been hiding in plain sight.

“In some of our communities, 98 percent of the children have a bit of malaria DNA running through their veins,” LaBeaud said, noting that up to 40 percent of the children with a fever-related illness have dengue or chikungunya viruses in their blood. Around a third of Kenyan kids who are sick with fevers actually have viral infections, she estimates. “That’s huge news,” she said. “Before this research, the Kenyan ministry of health didn’t recognize that dengue and chikungunya were endemic in the country.”

“People don’t recognize that there are lots of different mosquito species and all the different mosquitoes have their own little mosquito behaviors.” 

Identifying why children get sick is essential to effectively preventing outbreaks. In the past, mosquito-control efforts in Kenya were targeted only at malaria-carrying Anopheles mosquitoes, which bite at night and breed in vegetated areas such as rice fields and swamps.

But Anopheles-specific prevention measures, such as sleeping under insecticide-treated bed nets, don’t offer protection from dengue and chikungunya, which are transmitted by mosquitoes that bite during the day. Instead, it’s important to target Aedes’ favorite habitat, water containers.

In a paper published this year in the American Journal of Tropical Medicine and Hygiene, LaBeaud’s team showed that children whose households used water storage containers — rather than getting water from a tap or well — were more likely to be infected by the viruses.

“People don’t recognize that there are lots of different mosquito species and all the different mosquitoes have their own little mosquito behaviors,” LaBeaud said.

Purpose matters

Even when mosquitoes’ behaviors are thought to be well-understood, close examination of their interactions with people can yield surprises.

When LaBeaud and graduate student Jenna Forsyth decided to involve Kenyan school children in mapping where Aedes mosquitoes were breeding, they thought they knew what they’d find. They knew that Aedes mosquitoes like to breed in containers. In the rural region where the study was conducted, near Kenya’s Indian Ocean coastline, people keep large household water containers on hand to protect themselves from the unreliability of local taps and wells.

Because the mosquitoes don’t fly very far — traveling within a radius of perhaps a few hundred yards over the course of their lives — the researchers figured that tracking their breeding sites with a house-to-house survey made sense.

“We went in thinking, ‘It’s going to be the prominent containers we know about, the big jerrycans that are 10 or 20 liters,’” Forsyth said. Instead, they discovered that 80 percent of mosquito breeding was happening in “containers of no purpose,” many of which were trash. Other such containers were being kept in a family’s yard “just in case they were needed.”

“It turns out that a lot of the containers used for drinking and cooking don’t sit around long enough for mosquitoes to breed,” said Forsyth. When designing the next steps of the project, they realized they had to target those irregularly used containers. “It’s meaningless if we just say, ‘Dump out your buckets.’”

“We want to use innovation to get the community excited, to incentivize them to clean up the environment and protect their own health.”

The researchers worked with local residents on taking actions that could reduce mosquito breeding, such as storing unused containers upside down. And they challenged 250 children involved in the study to see who could collect the most no-purpose containers. The kids collected 1,000 kilograms of plastic waste, consisting of more than 17,000 containers. They used 4,000 of the containers to sprout native tree seedlings, which were planted around their communities.

“We pivoted our study; the message really became about reducing and reusing plastics,” Forsyth said. The team is repeating the study in an urban region of Kenya and has obtained funding to collaborate with faculty at the Technical University of Mombasa to study how local entrepreneurs can simultaneously reuse plastic waste and alleviate poverty.

“The project goal is to engage entrepreneurs to collect trash for profit, to set up something that will continue without us,” said Amy Krystosik, PhD, a postdoctoral scholar with the LaBeaud lab who has been collaborating on the project. “We want to use innovation to get the community excited, to incentivize them to clean up the environment and protect their own health.”

Feeding on violence

But sometimes the barriers to lowering disease risk have a completely different shape. Krystosik was a graduate student working in Cali, Colombia, when community members told her that local violence might be increasing the spread of mosquito-borne disease.

Cali, a city of 2.4 million, is among the most violent in the world, with homicide regularly ranking as one of its top two causes of death. And Cali’s slums are full of mosquito habitat: Located near lagoons and rivers, they lack basic infrastructure and flood during the rainy season. People throw trash in the waterways, making them even more appealing to container-breeding Aedes mosquitoes.

When Krystosik, then a PhD student at Kent State University, proposed surveying the slums for mosquitoes in 2014, Colombian public health experts told her it would be tricky. She’d have to get help from locals to navigate “invisible borders” between territories controlled by competing gangs. The city’s public health efforts had already been hampered by gang violence; city workers couldn’t check the function of local drains or set up mosquito-control methods that required them to go out into the community.

“I thought, ‘That’s crazy! We have methods to use for vector control, yet we can’t provide these services to communities that need them most,’” Krystosik said.

Her interest piqued, she began a project that continued when she moved to LaBeaud’s lab as a postdoctoral scholar in 2017. Using 2014-2016 data on homicides and cases of dengue, chikungunya and Zika, she mapped the overlap between community violence and illness in space and time.

“Everybody assumed disease risk would be in direct relation to socioeconomic status, but we found, independently, that violence was still a predictor of higher burdens of dengue.” 

The findings, published in 2018 in the International Journal of Environmental Research and Public Health, showed a statistically significant overlap between dengue infections and homicide risk. Homicides clustered in the central-eastern portion of the city, where dengue risk was also highest. The biggest surprise was that the statistical association persisted after controlling for poverty, itself a widely recognized risk factor for mosquito-borne disease.

“Everybody assumed disease risk would be in direct relation to socioeconomic status, but we found, independently, that violence was still a predictor of higher burdens of dengue,” Krystosik said.

Knowing more about the link between violence and mosquito-borne disease could help public health officials better predict outbreaks of disease and gauge the broader health benefit of communities becoming more peaceful. Right now, the combination of local violence and mosquito-borne disease is “a double burden on the population,” she added. “People are not going to be able to perform prevention and protect themselves from these viruses if they’re more interested in daily survival.”

Using climate data

On-the-ground mosquito hunts, as informative as they can be, are challenging to carry out on a large scale. So the research team recently took another approach for predicting where disease outbreaks could occur: building mathematical models that depend on climate data.

Ultimately, the team would like to be able to use remotely sensed data — from weather satellites for instance — to inform when and where mosquito-control strategies such as pesticides would be most effective.

“People don’t want to spray all the time; it’s expensive and labor-intensive,” said postdoctoral scholar Jamie Caldwell, PhD, who is leading the work in collaboration with LaBeaud; Erin Mordecai, PhD, assistant professor of biology at Stanford; and Eric Lambin, PhD, professor of earth system science and a senior fellow at the Stanford Woods Institute for the Environment.

Targeting mosquito control to exactly when and where it’s needed could also reduce the chance that mosquitoes will become resistant to pesticides, as occurred with DDT. Such targeting strategies, Caldwell said, would mean the pesticides’ effects are more likely to last longer. “We’ll get more bang for our buck in lots of ways,” she said.

“These diseases have really exponential spread, so anything you can do to prevent cases a few weeks before an outbreak can save a lot in terms of human health costs.” 

In addition to improving the effectiveness of pesticide use, the models could help spur community education at the right time. There could be TV and radio ads, signs at doctors’ offices or health clinics, and other outreach about what the mosquitoes look like and how to clean up possible habitats, LaBeaud said. Hospitals could also use climate prediction data to prepare for extra cases of illness, making sure they have supplies on hand to provide fluids to patients who become dehydrated, for instance.

“These diseases have really exponential spread, so anything you can do to prevent cases a few weeks before an outbreak can save a lot in terms of human health costs,” LaBeaud said.

Caldwell is building models that incorporate data on ambient temperature, humidity and rainfall, as well as non-climate factors such as degree of urbanization, land use and level of infrastructure. She’s validating the models with data from Kenya and Ecuador, and testing to see whether remote data alone will be enough to drive accurate outbreak prediction there.

The models could also help predict where the diseases will go next.

“In many places, the climate is getting less suitable for malaria, but may be getting more suitable for dengue and chikungunya,” Caldwell said.

That includes the United States. In 2017, the U.S. Centers for Disease Control and Prevention reported 156 cases of chikungunya, including 32 in California, and 437 cases of dengue, with 130 in California.

“We can no longer take a, ‘We are here in America, and the diseases stop at the border’ attitude.” 

Those illnesses were confined to international travelers and don’t seem to have spread to U.S. mosquitoes — yet. But Aedes mosquitoes are here; their presence has been recorded in 220 U.S. counties in 28 states, and the CDC estimates that their potential range “very likely” covers all the Southern states, much of the Midwest and Southwest, and nearly all of California. Local transmission of Zika was reported in parts of Florida and Texas in 2016 and 2017, and although no transmission was documented in the continental United States during 2018, Zika could return.

“Humans can get anywhere in the world in 24 hours, and so can these infections,” LaBeaud said. “They come in us. We go on vacation, the virus gets in our blood, we come home and, if the vectors are there, it’s a perfect storm waiting to happen.

“We can no longer take a, ‘We are here in America, and the diseases stop at the border’ attitude,” she added. “Because of global travel and the way we’re changing our planet, with climate change and extreme weather events, there’s a lot of potential for these mosquito habitats to shift and spread and grow.”

Erin Digitale is the pediatrics science writer for the medical school’s Office of Communication & Public Affairs. Email her at digitale@stanford.edu.

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