On Top of Everything Else, Climate Change Helps West Nile Disease to Spread

The good news is, widespread immunity can trump a drought's effects.

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Feb 8 2017, 12:01am

In case there weren't enough reasons to be freaked out about climate change, here's another one: a new study shows it can make it easier for West Nile disease to spread. Super.

West Nile disease, which is carried by birds and spread to humans by mosquitoes, was first detected in the US in 1999 and since then, there have been tens of thousands of cases and nearly 2,000 deaths. But predicting when and where a new outbreak might occur has been tricky. So a team of researchers recently gathered a bunch of data on the outbreaks of West Nile to see what role the environment and climate change had on the spread of the disease.

The researchers collected disease incidence data from 1999 to 2013 across the US and compared it with a host of environmental factors, including temperature and precipitation. They ran all these numbers through a predictive model to figure out which environmental factors had an impact on the number of human West Nile case.

They found that drought—which previous research has shown is getting more severe thanks to climate change—increased the number of people infected with West Nile disease, according to the paper published in Proceedings of the Royal Society B. This might seem counterintuitive to those who know that mosquitoes need standing water in order to breed, but Sara Paull, lead author on the study and a researcher at the National Center for Atmospheric Research, told me it's a little more nuanced than just water versus no water.

"We found that drought had no effect at all on mosquito abundance—it wasn't reducing or increasing mosquito populations," Paull said over the phone. "Drought was influencing West Nile by increasing the percent of mosquitoes that are infected with West Nile virus. Basically, if you have a lot of drought, you have a higher number of mosquitoes that are infected."

Paull told me there are a number of theories being studied right now as to why that seems to be the case. One theory is that, with fewer water resources, mosquitoes and birds are more likely to come in contact with one another, making it easier for mosquitoes to pick up the virus. Another theory is that the lack of water causes stress on birds' immune systems, making them more susceptible to West Nile, or keeping them sick for longer. The researchers predict that as we see more regular and severe droughts due to climate change, we could see the rates of West Nile in the US double, at least initially.

But Paull and her colleagues also found that the effects of drought could be trumped by an even more influential factor: immunity. Once someone has had West Nile, their body develops a natural immunity to the disease. If enough people in an area have this natural immunity, the drought can only have so much effect. There just aren't as many fresh victims that haven't already been infected.

Paull told me this understanding could help public health organizations better prepare for possible outbreaks: if your area is heading into an intense drought, and it's never been hit with West Nile before, you might want to launch a new education campaign or invest in vector control (i.e. killing mosquitoes).

But we can't extrapolate these findings to other diseases, even other mosquito-borne viruses like Zika, because there are just too many variables.

"The mosquito is a different species with Zika, it's just totally different biology," Paull said. "But the fact that climate, temperature, and precipitation have an effect on vector-borne diseases, and that those effects aren't entirely nailed down yet, is important for the public to be aware of."

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