Zika, dengue, and yellow fever are about to get much worse

Climate change, urbanization, and changes in human populations have driven many beloved species to the brink of extinction. But one of the deadliest animals in the world — the mosquito — is thriving.

Around 700,000 people die every year from mosquito-borne disease. The biggest culprit is malaria, but other mosquito-borne diseases, like dengue fever, chikungunya, and Zika, have proliferated wildly in recent years, and now make up a substantial share of the global burden of mosquito-borne disease. By some estimates, the number of dengue infections has increased 30-fold in the past 30 years.

The culprit? Climate change, plus urbanization and changes in where human populations are concentrated. And a new study in Nature Microbiology suggests that things will only get worse. Using statistical mapping techniques, they model how two disease-carrying mosquitos, Aedes aegypti and Aedes albopictus, have spread over the last 30 years, and predict how they’ll spread over the next 30.

The results are alarming. These species of mosquito — which carry infectious diseases including Zika, dengue, chikungunya, and yellow fever, though not malaria — are expected to spread throughout most of the United States and Europe, exposing hundreds of millions of people to these diseases.

“Overall,” the study finds, “our predicted expansions will see Ae. aegypti invading an estimated 19.96 million km by 2050 (19.91–23.45 million depending on the climate and urbanization scenarios), placing an estimated 49.13% (48.23–58.10%) of the world’s population at risk of arbovirus transmission.”

The expanding range of disease-transmitting mosquitos is a significant source of harms from climate change. These diseases largely have low mortality, but serious quality-of-life costs (in the case of Zika, brain damage in fetuses). It’s not clear that most countries are ready to address the public health challenge.

What’s driving the spread of mosquitos?

Modeling which parts of the world will be affected by mosquitos is tricky, in part because lots of factors affect their spread.

Climate change, of course, has vastly expanded the range of possible suitable habitats for many species of mosquito. Urbanization has, too. The paper argues that in the past, projections of mosquito spread were overly simplistic: “Those projections assumed that both species can fully infest all areas of predicted newly suitable habitat.”

In reality, it’s more complicated than that. Ae. aegypti, particularly, has a short flight range, and on its own would take centuries to expand the range of its habitat. It has mostly expanded through shipping: Mosquito eggs can catch a ride in tires and potted plants.

That means that urbanization, shipping, and adaptation among the mosquito species to new conditions all play a role in the spread of mosquitos. It’s not sufficient to check whether a location will be habitable to them, argues the study — researchers also need to think about how quickly mosquitos will reach and colonize the area.

The authors solved this with a mathematical model that made use of statistics, especially in the US and Europe, on mosquito spread so far. They found that urbanization, shipping, and human activity will be the most relevant drivers of mosquito spread at first. Later, it’ll be climate.

Moritz Kraemer for Nature Microbiology

Once the mosquitos have infested an area, it’s typically not long before a serious mosquito-borne disease outbreak. “Local outbreaks of these arboviruses have typically followed within 5–15 years of infestation by Ae. aegypti and Ae. albopictus,” the paper notes.

The mosquito-borne illnesses that we’ll be seeing more of in coming years

Neither Ae. aegypti nor Ae. albopictus carry malaria, the deadliest mosquito-borne illness. They do transmit Zika, dengue, chikungunya, and yellow fever. Zika is a virus that was initially regarded as fairly harmless — typically causing no symptoms at all and typically mild even in those who experienced symptoms. That changed in 2016, when a surge of Zika cases in Brazil was linked with a dramatic increase in microcephaly, a birth defect in which the brain doesn’t develop completely.

Chikungunya never appeared anywhere in the Western Hemisphere before 2013, but since then has been cropping up in Central and South America, with a few cases in the United States. It causes debilitating joint pain, though it typically lasts for only about a week and most people recover without any problems.

Dengue fever typically manifests as a rash and a high fever. It, too, typically lasts about a week, though in some cases it progresses into potentially lethal illnesses such as dengue hemorrhagic fever and dengue shock syndrome. There have been outbreaks of dengue fever in Hawaii, Florida, Puerto Rico, and the Gulf Coast states, and a dramatic increase in dengue cases in much of South America.

The paper recommends that countries do everything they can to limit the spread of mosquitos into new territory. They recommend “entomological surveillance,” or looking out for the bugs so that action can be taken quickly when they’re encountered, “particularly around high-risk introduction routes such as ports and highways.” They recommend nations develop “rapid response protocols for vector control to prevent introduced mosquitoes from establishing permanent population.”

In the long run, though, we can’t permanently halt the spread of mosquitos by extensively testing for their presence and responding strongly with localized insect-control measures. Those measures will delay mosquito colonization substantially, but they are unlikely to successfully prevent it, and the poorest countries are least likely to successfully implement them.

On a slightly more optimistic note, it seems possible that the spread of these so-called neglected tropical diseases into the US and Europe might mean they’ll be a little less neglected. If more resources are dedicated to researching vaccinations and effective treatments, the toll of the diseases could be significantly reduced.

From a more ambitious perspective, there are technologies being developed which, if they worked, might solve the problem for good. Some researchers today are working on gene drives that would target the few species of mosquitos that carry diseases that affect humans (most mosquitos do not affect humans, and it’d be a bad idea to target those for eradication). There are lots of concerns about how to responsibly deploy powerful technologies like gene drives, but the increasing toll of mosquito-borne disease might motivate countries to figure out how to address those concerns and move forward.

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