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West Nile Mosquitoes Emphasize the Local in Global Warming

A hotter world will indeed be a sicker world, but how that happens will be a local affair.
The proboscis of a female mosquito pierces the epidermis and dermis to allow it to feed on human blood from a capillary. (PHOTO: PUBLIC DOMAIN)

The proboscis of a female mosquito pierces the epidermis and dermis to allow it to feed on human blood from a capillary. (PHOTO: PUBLIC DOMAIN)

Scientists have realized for some time that a hotter world will be a sicker world. Some of the reasons are relatively intuitive—more heat deaths, for example (which may, or may not, offset deaths from the cold). Some are a little less straightforward—more kidney stones from people not drinking enough water in hotter climes.

Falling along the intuitive side will be the relentless spread to the poles and up the mountainsides of so-called vector-borne diseases historically kept corralled by traditional climate—cold nights, dry summers, snow. Think of hordes of insects—mosquitoes overladen with dengue fever and malaria pathogens, ticks slavering with Lyme disease bacteria—trooping forward with anthropomorphic menace. That’s actually how a lot of the mosquito-borne threats have been presented, especially as places like Britain, which has never known the bite of malaria, and Southeast Asia, which can’t easily handle more disease, are impacted.

As Princeton epidemiologist Andrew Dobson of Princeton University said a decade ago when an important analysis in Science focused on this growing threat: "Climate change is disrupting natural ecosystems in a way that is making life better for infectious diseases. The accumulation of evidence has us extremely worried. We share diseases with some of these species. The risk for humans is going up."

In a special issue of the journal Science last month, some of Dobson’s collaborators on that analysis grappled with the difficulty of knowing where that risk will appear:

Human mosquito-borne diseases, such as malaria and dengue fever, are frequently proposed as cases where vector and disease expansion into the temperate zone could follow from climate warming. However, some researchers have argued that ranges will shift with warming, rather than expand, and that the best predictors of infection risk are economic and social factors, especially poverty. Controversy has also arisen over which climatic variables are most important in delimiting the distributions of these diseases. Detecting impacts of climate change on human vector-borne diseases remains difficult, in part, because active mitigations, such as vector-control, antimicrobials, and improved infrastructure can complicate detection of a climate signal.

They noted other complications in making what seems like an easy call, complications made evident in new research appearing in the Proceedings of the National Academy of Sciences. University of Arizona geographers Cory Morin and Andrew Comrie studied the likely spread of one species of West Nile carrying mosquito across the southern United States, from California to the Carolinas. They determined that while climate change is increasing the species’ range and season, as was fully expected, how that plays out in your neighborhood is not be clear cut at all.

As Morin explained in a release: "The mosquito species we study is subtropical, and at warmer temperatures the larvae develop faster. However, there is a limit – if temperatures climb over that limit, mortality increases. Temperature, precipitation or both can limit the populations, depending on local conditions."

Across the bug-arrific Southeast, for example, hotter and drier temperatures may mean fewer “mosquito days per month,” while moister and cooler areas you might expect to be mozzy-resistant may have many more skeeters. Time and location, not just temperature, will be key variables for public health officials to pore over. (The weather site AccuWeather, for all practical purposes, already produces a periodic West Nile forecast.)

This finding contradicts the often-held assumption that projected warmer conditions always favor mosquitoes and clarifies some of the uncertainty in complex feedbacks involving climate and climate change influences on vectors and virus transmission, enabling more targeted public health action by using location-specific knowledge of vector responses to climate.

Morin and Comrie’s Dynamic Mosquito Simulation Model wasn’t meant to be the final word on West Nile. Their study looked at the ecology of but one species of West Nile-prone mosquito (Culex quinquefasciatus if you must know) and didn’t look at birds, which play a large role in the West Nile milieu.

The public health component of their work is the most important takeaway from the study, as more and more researchers—here’s a recent example from ecologist Xavier Rodó—try to get a handle on how to pinpoint where, and how, to intervene in an epidemiological terra incognita.

But there’s another takeaway, this one related to one of the reasons the preferred nomenclature switched from “global warming” to “climate change.” Warming won’t be uniformly or evenly experienced, but change will be. Don't let the variability bug you.