Bite back: Understanding West Nile virus in urban populations

Holly Whetstone - November 20, 2014

Edward "Ned" Walker, MSU professor of microbiology and molecular genetics, is examining how emerging infections behave in time and space.

Ned Walker, MSU professor of entomology and microbiology and molecular genetics

Of the major events of 2014, no other earned the media coverage that the Ebola outbreak in Guinea, Sierra Leone and Liberia sparked. While many media outlets gave much attention to following the virus’ movement throughoutthe world, the Centers for Disease Control and Prevention quietly reported on other infectious diseases also affecting people around the world. Polio, avian influenza, Middle East respiratory syndrome coronavirus, Chikungunya and enteroviruses caused thousands to become ill.

These infectious diseases, and several others, challenged health infrastructures, chipped away at public health budgets and claimed lives all over the world.

Edward “Ned” Walker, Michigan State University (MSU) professor of microbiology and molecular genetics, is examining how emerging infections behave in time and space. He will use this information to further basic biology and to develop predictive models that guide community and public health responses to emerging diseases.

“Humanity seems to be facing a constant march of emerging infections,” Walker said. “Some of these diseases arrive so quickly, and many of them create social issues in addition to health challenges. When these emerging infections arrive, my colleagues and I want to know where they set up, what will be high- risk years, and if it’s possible to predict how they will behave.”

Walker selected West Nile virus (WNV) to study. This virus, which is part of the Flaviviridae family, appeared in New York City in 1999 and very rapidly spread across the continental United States, reaching the West Coast by 2004. Even though it’s not discussed in the media as often as it once was, Walker said that the disease still causes serious epidemics in the United States.

WNV is transmitted from birds to mosquitoes, which in turn pass the disease on to humans. Symptoms include fever, headache, body aches, skin rash and swollen lymph glands. Severe side effects can include stiff neck, sleepiness, disorientation, coma, tremors, convulsions and paralysis.

“This is a nasty virus, and it’s costly to treat,” Walker said. “Many people don’t recover, but those who do often experience many long- term health problems. Unfortunately, the virus is doing well in eastern, urban portions of the United States.”

Walker’s research — which involves collaborators from MSU, the University of Illinois and the University of Wisconsin, and is supported by the National Science Foundation — focused on the metropolitan Chicago area as the primary study site and metropolitan Detroit as the secondary site.

The group first observed that WNV was only in certain locations — it was not widespread across metropolitan areas. They gathered data on the number and location of confirmed human cases of WNV and found that those locations correlated with areas where mosquito infections were also the highest. The group then categorized the urban landscapes by demography, density of houses and types of buildings.

When Walker combined all of this information, he could further delineate where, how and why the virus set up among human populations.

“The highest areas for human infection are what we call post-World War II neighborhoods,” he explained. “The risk isn’t as high in the inner city, and it’s not as high in the newer suburbs — it’s highest in those neighborhoods that fall right in between. In the Detroit area, this would be communities such as Dearborn Heights, Southfield, Westland, Livonia and Allen Park.”

These communities share a set of common characteristics that encourage mosquito infestations: closely spaced houses on fl plains, lots of structures that encourage bird roosting (ranging from trees to bird feeders) and drainage catch basins along city streets.

“These WNV-carrying mosquitoes are the result of human activity,” Walker noted. “One of their major larval habitats is street catch basins, put along residential streets to collect stormwater. At the bottom of these grates are sumps that gather the material that would normally clog pipes. Water and organic material accumulate in this space and make wonderful mosquito nurseries.”

Another important observation was that the increases in mosquito infection rates correlate with increases in temperature and precipitation events.

Walker’s team examined data across eight years and found that they could produce predictive models that take into account these two weather factors. They used degree- week as a simple measure of temperature; it accounts for the accumulation of heat above a specifi base temperature on a weekly basis. This highlighted a pattern: the risk of contracting WNV was higher during years when the summers were warmer and drier.

“We’re hopeful that our findings will guide public health decisions,” Walker concluded. “They should guide messages from the media and help citizens understand the risks of disease during a given year. It should also provide a model for other diseases, either conceptually or as a model for approaching an emerging infection. So often we are reactionary when it comes to a disease outbreak. I hope this research, and more like it, will help us to become more anticipatory in the future.”