ANN ARBOR—There are hundreds of millions of new cases of malaria annually, but much of what scientists believe about preventing the spread of malaria in urban settings is mistaken, a new University of Michigan study suggests.

Factors such as house construction and use of anti-mosquito coils aren’t as important as where someone spends the night, says study co-author Mark Wilson, professor of epidemiology at the U-M School of Public Health.

Wilson and former Ph.D. student Jose Siri studied malaria risk in Kisumu, Kenya, and found that the greatest risk factor for a child living in an urban area was whether the child spent at least one night a month in a rural area. Those children were nine times more likely to contract malaria.

“We found that factors like house construction and mosquito coils weren’t important, whereas traveling to rural areas was,” said Wilson, who is also a U-M professor of ecology and evolutionary biology. “That probably relates to the lack of the use of bed nets in those rural areas.”

The study, which was recently published in the American Journal of Tropical Medicine and Hygiene, suggests that travelers should carry bed nets with them if they travel from the city to rural areas.

“One takeaway message is that using the legal residence as the location of risk might be misleading,” Wilson said. “This is important because if the disease is transmitted primarily during the night, you need to consider where people are during the night. People usually acquire infection a week or more before having disease symptoms.

“It’s very important to know where infection originates because the only way to interrupt transmission is by focusing on people and places at highest risk. Also, without pinpointing where or when people were infected, one might be taking the wrong precautionary measures.”

The study results show that special information and prevention efforts are necessary for the growing group of urban residents who travel to rural areas.

Foreign tourists are at relatively lower risk when staying in the city because hotels will already have screens and mosquito nets, and tourists are very likely taking anti-malarial drugs, Wilson says. However, if they embark upon a more exotic or adventurous vacation, they should be sure to bring along a treated mosquito net, he adds.

Other studies on the effects of socioeconomic and environmental factors on severe malaria show inconsistent results, and rarely have they evaluated malarial patterns in urban settings. Wilson is now studying urban malaria epidemiology in Blantyre, Malawi, in collaboration with Don Mathanga, another former Ph.D. student. Their investigations, along with colleagues at Michigan State University and other major research institutions, are part of a recently awarded seven-year NIH grant establishing an International Center of Excellence for Malaria Research in Malawi.

The University of Michigan School of Public Health has been working to promote health and prevent disease since 1941, and is consistently ranked among the top five public health schools in the nation. Whether making new discoveries in the lab or researching and educating in the field, SPH faculty, students, and alumni are deployed around the globe to promote and protect our health.

Mark WilsonU-M Department of EpidemiologyU-M School of Public Health