West Nile Virus: How Climate Change May Be Contributing To Its Spread

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West Nile virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. The virus can infect humans, birds, mosquitoes, horses and some other mammals. (Photo: Getty Images)

Here’s something else that may suck more because of climate change: mosquitoes. And when mosquitoes suck more they may be more likely to transmit pathogens like the West Nile Virus (WNV).

Mosquitoes sucking more is not the only thing affecting the spread of WNV that may change with climate change. As Slomit Paz summarized in publication in the Philosophical Transactions of the Royal Society B: Biological Sciences, many mosquitoes like it hot and wet. This may sound dirty, because it is. When the temperature and humidity are higher, mosquitoes may not only feed more frequently but also reproduce more. They also may become infectious sooner after acquiring WNV.

Climate change seems to be giving horny mosquitoes what they want. As the NASA Global Climate Change website summarizes, over time there’s been progressive increases in temperature as well as the frequency and intensity of heat waves around the world. Meanwhile, precipitation patterns have changed with average rainfall having gone up in many locations such as the Northern U.S.

All of this could be helping expand the habitats for the mosquitoes that can carry and transmit WNV through biting. A study published in PLOS ONE showed how this could be occurring for the Culex quinquefasciatus, one of the many different mosquito species that can carry WNV. Additionally, increasing the frequency at which mosquitoes bite and have sex (but not at the same time) makes them even more adept at spreading WNV.

The Asian tiger mosquito (Aedes albopictus) is just one of many mosquito species that can carry and transmit the West Nile virus. (Photo: Getty Images)

Has climate change actually helped spread WNV? Unless you have a time machine and a gigantic air conditioner, you may think that proving this definitively could be difficult. However, some historic numbers and observations are raising concerns about this potential impact of climate change.

What we do know is that in just under two decades since its American debut in New York in 1999, WNV has spread quite widely. Take a look at the U.S. Centers for Disease Control and Prevention (CDC) tracking of reported WNV cases since 1999 In the first 2 years, New York, New Jersey, and Connecticut were the only states with reported cases. But 2002 saw an explosion of reported cases across many states. This doesn’t necessarily mean that mosquitoes went overboard with the sexy time and biting that year. Some of the increase in reported cases could be due to a “start spreading the news” increase in awareness. But the problem clearly had expanded significantly since 1999.

By 2003, WNV activity was present in every state except for Washington, Oregon, Hawaii, and Alaska. In subsequent years, cases appeared in the two Pacific Northwest states as well. This year as of September 4, the CDC has logged WNV activity in people, birds, or mosquitoes in 45 states and the District of Columbia. The only states currently missing from this 2018 list are Alaska, Arkansas, Hawaii, Maine, and Wyoming. And you can now scratch the Dirigo state (yes, that’s Maine’s state motto) from the list, because there are reports of horse being infected with WNV in Maine.

It’s true that the total number of reported cases has not consistently increased every year since 2003. As the CDC numbers show, 2003 had 9,862 reported cases, and tallies in recent years have fallen well short of this number. However, this does not mean that the WNV problem hasn’t been getting progressively worse. As Tony Leys reported for the Des Moines Register, 2018 has already been the worst year for WNV in Iowa since 2003. (The worst year for Iowans and not the virus, which apparently is having a good year there.)

And, according to the European Center for Disease Prevention and Control (ECDC), “an unusual early start and high number of locally acquired West Nile virus infections has been observed this year” among countries in the European Union. Locally-acquired means that transmission of the virus occurred within Europe as opposed to someone getting infected while traveling abroad. The ECDC report also indicated that “while the majority of areas affected in 2018 were areas where cases were reported in the previous years, it is likely that the virus spreads to new areas in 2018.”

Moreover, keep in mind that the number of reported cases may not be an accurate measure of the spread of WNV and far underestimate the actual number of cases. That’s because many people may not even realize that they have been infected with most people not even having symptoms. About 20% of those infected develop West Nile Fever, which could be easily mistaken for other disease such as the flu. Typical symptoms of West Nile Fever include several days of fever, headaches, fatigue, body aches, a skin rash, vomiting, or diarrhea. There may be a clear sign that you have Bieber fever, but not West Nile Fever, unless you see a doctor and get tested.

West Nile Virus infections are more likely to be reported when they have progressed to neuro-invasive disease. This is when the virus invades and wreaks havoc in the nervous system. It occurs in less than 1% of those infected. Potential complications include encephalitis, which the brain gets inflamed,or encephalitis, when the membranes that wrap the brain and spinal cord become inflamed. These “itis-es” are medical emergencies and can lead to many complications such as mental status changes, seizures, paralysis, and even death. So far this year, there have been a total of 559 reported WNV cases with 312 of them resulting in neuroinvasive disease and 18 resulting in death.

Contra Costa County Mosquito and Vector Control District technician Josefa Cabada checks for mosquito larvae in a retention pond in Bay Point, California. (Photo by Justin Sullivan/Getty Images)

Speaking of numbers, in a study published in 2015 in Global Change Biology, a team of researchers from UCLA (Ryan J. Harrigan and Thomas B. Smith), The University of Tubingen (Henri A. Thomassen) and the University of Leeds (Wolfgang Buermann) used data on WNV infections and the climate from 2003 to 2011 to develop a mathematical model to characterize the relationship between evolving climate conditions and WNV infection incidence. They showed that the continuing trends of rising temperatures and changing precipitation patterns will foster further spread of WNV such as leading to WNV being established as far north as southern Canada and even northern British Columbia by 2080.

Of course, climate change is not the only reason why WNV continues to spread. After all, besides controlling the mosquito population or telling WNV to stop it, there is currently no way of halting the spread of WNV. There is no vaccine or treatment for WNV available. Typically when a disease spreads, it is a system of factors at work and not just a single cause-single effect. Nonetheless, indications are that climate change certainly isn’t helping.

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