According to the Centers for Disease Control, there have been over 1100 reported cases of West Nile virus disease in the US this year, including 42 deaths. If these numbers seem high, they are - in fact, it's the highest number of reported cases since West Nile was first detected in the US in 1999, and West Nile season has just begun. Given that the peak of West Nile epidemics generally occurs in mid August, and it takes a few weeks for people to fall ill, the CDC expects that number to rise dramatically. But why now?

Though the CDC doesn't have an official response to that question, the director of the CDC's Vector-Borne Infectious Disease Division said that 'unusually warm weather' may be to blame. So far, 2012 is the hottest year on record in the United States according to the National Climatic Data Center, with record-breaking temperatures and drought a national norm. It's likely no coincidence that some of the states hit hardest by West Nile are also feeling the brunt of the heat. More than half of cases have been reported from Texas alone, where the scorching heat has left only 12% of the state drought-free. Fifteen heat records were broken in Texas just last week on August 13th.

The heat waves, droughts and other weather events are the direct effects of climate change say leading scientists. As NASA researcher James Hansen explained in a recent Washington Post editorial, "our analysis shows that, for the extreme hot weather of the recent past, there is virtually no explanation other than climate change." He says that the European heat wave of 2003, the Russian heat wave of 2010 and catastrophic droughts in Texas and Oklahoma last year are all the repercussions of climate change. Confidently, he adds that "once the data are gathered in a few weeks’ time, it’s likely that the same will be true for the extremely hot summer the United States is suffering through right now."

The fact that the worst US West Nile epidemic in history happens to be occurring during what will likely prove to be the hottest summer on record doesn't surprise epidemiologists. They have been predicting the effects of climate change on West Nile for over a decade. If they're right, the US is only headed for worse epidemics.

What Is West Nile Virus?

To understand the connection between climate change and disease, you first have to understand West Nile. First discovered in Uganda in 1937, it is what epidemiologists call a zoonotic disease, that is, one that is transmitted from animals to humans, not from humans to humans. West Nile virus mainly infects birds, which are the virus' true hosts. We humans (as well as livestock and other animals) are accidental casualties of a bird-mosquito disease. West Nile travels via mosquitos that pick up virus particles when they bite infected birds. These particles stay in the mosquito's salivary glands, and are transmitted into the next host when the mosquito feeds. Humans and other mammals don't have high enough numbers of viruses in their blood for mosquitos to pick up the infection from them, which is why we are considered "dead end hosts".

As anyone who hangs out outdoors is aware, mosquito populations pick up in the summer, when environmental conditions are just right for feeding and breeding. So, too, does West Nile. Though it sounds scary, West Nile is a fairly mild infection. Around 80% of people infected show no symptoms at all. Most of the remaining 20% will present with fever, headache and body aches, and nausea which can last for a few days or several weeks, much like getting the flu. Of course, like any illness, West Nile has the potential to be very serious. Less than 1% of infections result in a condition called West Nile meningitis or encephalitis, where the virus infects the spinal cord and even brain. Severe symptoms include high fever, headache, neck stiffness, a decreased level of consciousness (sometimes approaching near-coma), tremors, convulsions, weakness, numbness and paralysis. West Nile meningitis or encephalitis can last several weeks, and the neurological effects can be permanent. As with the flu, the elderly and anyone with a compromised immune system is at a higher risk of severe symptoms and death.

Currently there are no vaccines or antivirals with which to prevent and control West Nile virus in humans. The best offense in the case of West Nile is a good defense. To protect against illness, don't get bitten by mosquitos in the first place. Wear protective clothing, use nets and screens, and wear insect repellent whenever you are in an area where people have gotten sick. The CDC has updated maps on where cases have occurred, though if you're unsure if your area is suspect, caution is better than regret. Of course, if you look at that map, only one state has no West Nile activity: congrats, Vermont, on so far eluding the epidemic.

Turning Up The Heat

Higher temperatures bolster the chances of infection on many fronts. Temperature has a profound effect starting at the source: the mosquito. Studies have found that mosquitos pick up the virus more readily in higher temperatures. Higher temperatures also increase the likelihood of transmission, so the hotter it is outside, the more likely a mosquito that bites an infected bird will carry the virus and the more likely it will pass it along to an unwitting human host.

In the United States, epicenters of transmission have been linked closely to above-average summer temperatures. In particular, the strain of West Nile in the US spreads better during heat waves, and the spread of West Nile westward was correlated with unseasonable warmth. High temperatures are also to blame for the virus jumping from one species of mosquito to a much more urban-loving one, leading to outbreaks across the US.

Though you might think that the droughts associated with heat waves would slow down mosquitos, it turns out to be the exact opposite. That's because the main mosquito now involved in West Nile transmission, city-loving Culex pipiens, actually thrives in drought conditions. C. pipiens tends to breed underground in water that sits in city drains. During a drought, these pools become rich in organic material that C. pipiens needs to survive, whereas rainfall flushes the drains and dilutes the nutrients in the standing pools. Drought also has a negative effect on C. pipiens' predators like frogs and dragonflies - and where there are less predators, there are more mosquitos. To add to the problem, drought tends to cause birds to cluster around water resources, making them easy pickings for hungry mosquitos and upping transmission rates.

The Real Inconvenient Truth

I can't sum it up any better than Paul Epstein did in his 2001 review of climate change and West Nile:

"We have good evidence that the conditions that amplify the life cycle of the disease are mild winters coupled with prolonged droughts and heat waves—the long-term extreme weather phenomena associated with climate change."

While some politicians are hesitant to accept the scientific consensus of climate change, West Nile is more than happy to reap the rewards of our poor environmental choices. It's not the only disease benefitting from our flagrant CO2 emissions; epidemiologists predict that many vector-borne diseases, including deadly ones like malaria and dengue fever, will increase in incidence world wide as global temperatures rise.

While the CDC is hesitant to blame this year's West Nile outbreak on climate change directly, the science is clear. Record-breaking incidences of West Nile are strongly linked to global climate patterns and the direct effects of carbon dioxide emissions. Climate change isn't just going to screw with the environment, it will continue to have devastating public health implications. In addition to better mosquito control and virus surveillance, we need to focus our efforts on reducing and reversing climate change if we want to protect our health and our well-being.

Image credits: Disease collage by Visual Mozart / purchased from ImageZoo; Transmission cycle image from the CDC website; mosquito danger image from petrafler / 123RF Stock Photo