West Nile Virus is alarming example of climate change’s effects

The message gets to the Santa Clara County Mosquito and Vector Control District that a dead bird was found in Sunnyvale. If it’s a crow or a jay, the district sends out a team to collect the bird and bring it back to their in-house lab to test it for West Nile Virus. If the bird tests positive, a team of five technicians in white pick-up trucks goes out and sets 40 mosquito traps in a one-mile radius of the site. At this point, Dr. Noor Tietze, Entomologist is called in for mapping and planning strategies. After one night of trapping, the team returns in their white pick-up trucks to collect the mosquitoes and bring them back for testing.

“If we get any positive mosquitoes, we go to the next step and fog the area,” said Tietze. They spray with Ultra Low Volume (ULV) fog, an insecticide strong enough to knock down the mosquito population by 60-80 percent but mild enough to use in residential areas. In case this brings to mind “Silent Spring” images of children running behind a DDT truck and polluting their lungs, you should know that Tietze’s trucks only put out 1.5 fluid ounces per acre, not enough to affect honeybees or dragonflies, much less larger mammals like us.

“We’re quite busy right now,” Tietze said, chuckling. On May 27, the district tested five positive crows in one day, which led to another fogging in Sunnyvale on June 2 at night.

West Nile is now prevalent in California, and has been for some time. In late April, a dead American crow in Contra Costa County tested positive for the disease. This is the one of the earliest cases of West Nile in the mosquito season that has announced itself in the Bay Area to date.

In 2013, the United States Centers for Disease Control and Prevention reported 176 cases of West Nile Virus in California, six of which were fatal. Two of those 176 cases were right in Santa Clara County — fortunately, they were non-fatal. Still, infectious disease experts warn that if we don’t take preventative measures soon, climate change could make the virus a whole lot worse.

“Fortunately, it’s not a super virulent virus. That is, not most of the people who get West Nile don’t show symptoms,” said Steve Schutz, scientific program manager at the Contra Costa County Mosquito and Vector Control District. “But for the few who do, it can be very serious.”

Recently, with drought and warming temperatures, concern around West Nile has been the highest that it has been since its initial spread to California. We have entered a new era where reports of tropical disease no longer mean you’re safe at home — those reports could be right from your backyard.

A tropical disease in your backyard

West Nile Virus used to be a disease of tropical birds, carried between birds by mosquitoes. The Culex family of mosquitoes has carried West Nile from birds to humans, allowing the virus to spill over into human hosts in a zoonosis — a term that describes the transmission of a disease between species.

West Nile Virus activity across the United States, as of July 8, 2014. (Map courtesy of Centers for Disease Control and Prevention. Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases, and Division of Vector-Borne Diseases)
West Nile Virus activity across the United States, as of July 8, 2014. (Map courtesy of Centers for Disease Control and Prevention. Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases, and Division of Vector-Borne Diseases)

While roughly 75 to 80 percent of those infected with West Nile Virus don’t show symptoms, the rest develop West Nile Fever. They can experience fever, headache and nausea among other symptoms. But the real problem is West Nile Encephalitis — a rare complication that happens in less than one percent of people who get infected. West Nile Encephalitis is a serious disease that affects the brain, with a frightening case fatality level of 4 to 15 percent. The scariest part: there is no known treatment for West Nile Virus.

West Nile first showed up in the United States in New York in 1999. Schutz suspects it spilled over to the U.S. via illegal birds in the illegal pet trade. Other hypotheses pose that mosquitoes may have hitched rides to New York on airplanes with the spread of globalization.

“Once it was here, it turned out that some of our local birds were highly susceptible to it. North American birds had never been exposed and had no immunity,” Schutz explained. Once infected, all it took was several bird migrations for the disease to spread coast to coast.

Climate change now threatens to spread West Nile further. West Nile is, remember, a tropical disease. As the mercury rises, so does the incidence of the disease.

Summer nights make happy mosquitoes

With decreased and more variable water supplies, birds and mosquitoes bottleneck around what few water supplies are available, increasing contact between the two species. Birds are the middlemen between mosquitoes and humans, essentially storing the virus and letting it incubate until a second mosquito bites the bird and ferries the virus along to human hosts. The scarcer the water supply, the more opportunities for contact between birds and mosquitoes, the more the virus can amplify, the greater chance of infection in human hosts.

“In California, there was already background rainfall variability in dry areas,” said William Durham, professor of infectious disease at Stanford University. “If climate change is adding variation to that, birds and mosquitoes will have fewer choices for water sources and standing water anywhere could become a breeding ground for West Nile.”

Higher temperatures make matters even worse.

“Since mosquitoes are cold-blooded, they’re pretty much the same temperature as their surroundings,” said Schutz. “The warmer the mosquitoes are, the faster the virus can replicate inside the mosquito. The virus migrates from gut of the mosquito to its salivary glands and amplifies. The warmer that is, the faster that happens,” he explained.

In addition to allowing the virus to amplify more quickly, warmer temperatures extend the window when mosquitoes can be out biting humans each day. With warmer mornings and evenings, the time when the temperature is mild enough for mosquitoes to be out biting will be longer, increasing the possibility of transmission to human hosts. With warmer summers, the Culex will be able to develop more rapidly, too.

Warmer temperatures have favored the Culex mosquito and have also led to the appearance of new families of tropical mosquitoes that are non-native to the Bay Area. Recently, the vector control districts in San Mateo and Fresno found some non-native tropical mosquitoes that are capable of transmitting Dengue and Yellow Fever.

The mosquitoes likely make it to California on airplanes, and once they’re here, where once they would have perished because of a foreign climate, our warming climate is allowing them to survive, and maybe someday even thrive.

“I don’t see it being an immediate impact,” said Schutz. “But I think over the long term, if we do start having milder winters or warmer evenings, when it doesn’t cool down as much overnight, that potentially is going to increase transmission of West Nile Virus. The warmer it gets, the easier it will be for these tropical mosquitoes to colonize the Bay Area, too.”

Before you grab the bug spray

To protect against West Nile, the public needs to understand all the factors that are pushing infection rates up. And certain human activities are complementing the effects of climate change in alarming and unexpected ways.

“Neglected swimming pools could be a major part of the problem,” said Durham.

The year 2007 marked one of the first years that disease control experts saw a spike in West Nile in the Bay Area, especially in Santa Clara County. One of Durham’s hypotheses to explain the spike is the host of abandoned swimming pools left behind by the subprime mortgage crisis. This is a narrow case of a broader problem—man-made standing water sources.

“One thing that comes up is hoarding water,” said Tietze. “Especially with late rains, people may have collected quite a bit of water in their backyards. If that water isn’t covered properly it could create a breeding ground for Culex pipiens.”

Swimming pools, uncovered buckets that have collected water and birdbaths: the stakes of cleaning, or emptying, these water sources are much higher than those of your typical household chore. Even standing water in clogged gutters could be a threat.

In terms of protection measures, this is a good place to start. Check your backyard. Check your neighbor’s backyard.

Beyond controlling man-made sources of standing water and prudent use of mosquito repellent, the next step to control West Nile is the ULV fog that Tietze and his team spray neighborhoods with. Especially when disease control centers resort to fogging residential areas with insecticide, people will begin to see the high stakes of fighting diseases like West Nile.

“Public concern is high,” said Durham. “When West Nile Virus comes whistling in, public health officials spray. They’ve already sprayed areas of San Jose and Morgan Hill. You start to wonder how much control you have if your kids are going to breathe that stuff.”

“People have questions because it’s something different,” Tietze explained. “We only fog one night. Other areas in the country, they may do it several nights in a row.” For now, it’s our only option.

West Nile Virus is an alarming example of the effects of climate change that we’ve already baked into our system. Reducing carbon emissions isn’t going to retroactively push temperatures back down. The effects of climate change — West Nile among them — are here to stay. And it’s up to us to find creative, sustainable ways to adapt to those effects.

Homepage mosquito photo courtesy of “k yamada” via Flickr.

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