The virus that stalks urban neighborhoods » Yale Climate Connections

Each summer, as mosquitoes begin to buzz, the risk of West Nile virus quietly rises — and cities aren’t immune.

In 2012, it burned through the Park Cities, two wealthy municipalities surrounded by the city of Dallas, causing 225 cases of West Nile fever, 173 cases of the more serious neuroinvasive form of the disease, and 19 deaths.

In the U.S., the West Nile virus is most frequently spread by Culex mosquito species, such as Culex pipiens and Culex quinquefasciatus. About 80% percent of people infected with West Nile virus don’t develop any symptoms. But about one in five will develop a fever with flu-like symptoms, and about one in 150 people will develop a severe neurological illness such as encephalitis or meningitis, which can lead to paralysis and death.

Although many people may think of mosquito-borne illnesses as a concern in tropical regions or rural landscapes, cases of West Nile virus have been identified in every state in the U.S., with the highest infection rates in the Great Plains states. And major cities like Los Angeles, Chicago, and Dallas-Fort Worth see the most total cases, according to the National Oceanic and Atmospheric Administration.

What’s helping virus-infected mosquitoes survive and thrive in urban areas? Yale Climate Connections spoke with Dr. Robert Haley, an expert in infectious diseases at the University of Texas Southwestern Medical Center, about the conditions that fuel West Nile outbreaks in urban areas, the public health measures that can stop them, and how warmer climates are expected to increase outbreaks.

This interview has been edited for clarity and length.

Yale Climate Connections: How is West Nile spread?

Dr. Robert Haley: There are several species of Culex mosquitoes. West Nile is spread [in Dallas] by a certain species of mosquito called the Culex quinquefasciatus mosquito – otherwise known as the [Southern] house mosquito. The Culex quinquefasciatus lives around houses – they breed in bottle caps full of water, drains, overwatered lawns, and any kind of anything holding water around your house – and they don’t go very far.

It produces epidemics in birds, and birds and mosquitoes pass it back and forth, and if you have all the right conditions to have a big year, the bird-mosquito epidemic will get very large and spill over into humans.

YCC: What are the conditions that contribute to these bigger years?

Haley: We’ve looked at the number of cases of West Nile neurological disease over about a 12-year period, and we find that the biggest determinants are years where the winter is particularly warm with very few hard freeze days and wet with a lot of rain. Warm, wet winters lead to the amplification of the virus early in the summer so that by midsummer you start getting mosquitoes biting humans, and you start seeing cases in the mid-to-late-summer.

We find that [Culex quinquefasciatus] tends to be [responsible for] urban outbreaks, not rural outbreaks, and you will have a hot spot in different urban areas. We have a hot spot right in the center of the most affluent residential area in Dallas.

[Park Cities are] just north of downtown Dallas. The downtown area, like many downtown areas, is just confluent buildings and concrete, and so it’s what’s called a heat island. It’s always two, three, or four degrees warmer in downtown Dallas than it is in the outlying areas where there are more woods, homes, and lawns. And in the summer, when the transmission is happening, the prevailing winds are from the south and southeast, and so that moves that hot air from downtown Dallas up into that northern central part of the city, which is this affluent neighborhood [of Park Cities]. And so that makes it actually even warmer than the surrounding areas. Chicago, Los Angeles, and Sacramento have hot spots that we know about where they have recurrent problems.

YCC: I live in the northwest corner of Chicago, and I had West Nile meningitis about eight years ago. I spend a lot of time in the community garden here and a lot of time outside, but I think the perception is we worry about these mosquitoes when we’re going for a hike at the forest preserve, or we’re doing something like that, but not necessarily as much around home. So this being an urban problem is important for people to hear, because I think most of our audience doesn’t think about these outbreaks happening in cities. For example, everyone I’ve ever mentioned it to was like, “Oh, in Chicago, where did you travel? Did you go to some exotic location?”

Haley: [The northwest corner of Chicago] is a hot spot. And people think that it’s going to be the poor people who get it, the low-income neighborhoods and people, because many diseases do cluster in people with lower income, but West Nile is [also] a disease of wealthy people because it happens in these areas of high housing density, with high property values, big homes, but very little open space in between.

It’s also been coming to the fore just recently. West Nile never occurred in this country until it came to New York in 1999, and it came to the Midwest – to Chicago and Dallas – in 2002. We’ve had a few cases each year, and then in 2006, we had what we thought was a big epidemic [in Dallas], but then in 2012, we had the mother of all epidemics. It was quite a tragedy.

YCC: What are your recommendations as a doctor for how people can protect themselves?

Haley: Individuals can try to avoid being bitten by mosquitoes, but that’s only so helpful. You can wear mosquito repellent and avoid going out in the early evenings and in the early mornings because that’s when the Culex mosquito is more likely to bite humans.

Editor’s note: Other recommendations include wearing long-sleeved shirts and long pants when you go outside, draining standing water around your home, and keeping doors and windows screened or closed to keep mosquitoes outside.

Haley: But the best way is to have a good public health system in the area because a good public health system can perform mosquito surveillance. They can put mosquito traps all around the city, as we have done for decades in Dallas, and trap mosquitoes.

From mosquito surveillance, the health department can calculate what we call mosquito abundance. Some years, there are lots of mosquitoes, so is it a big year or a small year in the number of mosquitoes out? And the other is the mosquito infection rate – how many mosquitoes have it? Neither one of those predicts epidemics. But if you multiply [the number of mosquitoes by the mosquito infection rate], you get an estimate of the number of infected mosquitoes. And we found that this statistic – the vector index – predicts very well and is the key to forecasting epidemics. When the number of mosquitoes – and the amount of infected mosquitoes – goes up to a certain level, we know that we’re set up for an imminent outbreak. And then you can spray the areas where the mosquitoes live with safe insecticides, and you can interrupt – or even preempt – an epidemic.

Aerial spraying is clearly effective. We compared the number of West Nile cases in areas that were sprayed to those that were not sprayed. And you see that the epidemic lasts longer in the ones that were not sprayed. And the spraying just cuts it right off – you don’t see any more cases after you spray. Aerial spraying kills a high percentage of mosquitoes, but they multiply and come back again within two weeks, but there are too few infected ones to sustain the epidemic among birds and people. So spraying doesn’t get rid of all the mosquitoes, it just tamps them down temporarily. But it’s enough to end the epidemic.

Ground spraying out of trucks is the same spray, and it’s safe, but Culex [quinquefasciatus] mosquitoes live in the tops of trees. They’re primarily eating their blood meals from birds – and that’s where the birds are – so the mosquitoes live with the birds in the tops of the trees. And so, ground spraying, you don’t get the tops of the trees.

It’s been found to be extremely safe. It will kill mosquitoes only; it does not kill bumblebees or honeybees and does not [harm] people.

YCC: As the climate warms, how do you anticipate the risk of West Nile increasing?

Haley: Increasing warmth in any biological system speeds up a lot of the processes. For example, in warmer weather, the virus grows faster in mosquitoes, birds, and humans. And in warmer weather, mosquitoes bite more often and more fiercely. So as the weather warms, those conditions will become more common, and we will also have fewer hard freeze days, which means these epidemics will become more common.