Mosquito Dangers Thrive In Autumn's Climate

Trick or Deet!

Mosquitoes typically vie for prominence at a Fourth of July picnic. That’s because the annoying bugs flourish in the hot, wet days of summer. But early autumn is even more problematic. That’s because the mosquitos that bite then are the most dangerous kind, and it’s just about the same time bug repellent is cast aside for pumpkins and Halloween candy.

“Mosquitoes that tolerate the dry and cooler fall weather tend to be the mosquitoes that transmit disease, like West Nile virus,” says researcher Lyric Bartholomay, an Iowa State University assistant professor who tracks mosquitoes.

What’s more, this year is worse than most. The bumper crop of late-season mosquitoes on the prowl this year isn’t simply your imagination. Extreme weather in 2010 gave rise to record populations breeding in puddles, swamps, and ponds in the upper Midwest where researchers have measured mosquito numbers. The pools of water left over from late summer and fall downpours provide the stagnant breeding ground disease-carrying mosquitoes love. 

“Mosquitoes are the most important disease vectors,” Bartholomay says. The insect transmits a range of illnesses that can be deadly to humans, including West Nile virus, St. Louis encephalitis, eastern equine encephalitis, LaCrosse encephalitis, and dengue fever, among others.

So far just this year (as of October 26, 2010), according to the Centers of Disease Control (CDC), West Nile virus infected 832 people in the United States, and dengue fever infected 45 in Florida and 8,436 in Puerto Rico.

Global warming could make this problem worse. Global warming leads to more bouts of extreme weather, including heavier downpours, creating the perfect breeding ground for mosquitoes. In the United States, 2010 is likely to be one of the hottest years on record. Higher temperatures in late summer or early fall increase the odds that mosquitoes carry West Nile virus. 

West Nile virus is currently the most common of mosquito-borne diseases affecting humans in the United States. The spread of West Nile virus began in 1999 when the first cases of showed up unexpectedly in birds and then people. Since it was first discovered in the United States, the CDC estimates 54,000 people have been affected. The CDC reports that 1,196 people have died from confirmed cases of West Nile virus in the United States since 1999.

Additionally, mosquitoes that carry dengue fever acquire the disease by biting humans and become infectious after an incubation period of eight days. With an increase in temperature, the disease replicates itself in just five days. In either case, a bloodsucker will continue to transmit the disease for the rest of its life (about a month).

Bartholomay recently worked with a team of scientists to map the genome of the Culex mosquito, or southern house mosquito, the bug that carries West Nile virus. She says her new surveillance work will also include being on the lookout for diseases that could come to the United States from tropical climates (i.e., carried here by travelers) and be made worse by climate change. She’s especially on the lookout for Rift Valley fever (RVF), which makes animals extremely sick and is also deadly to humans. Outbreaks have been reported recently all over Africa, including South Africa. About 1 percent of humans die from the disease, and between 1 and 10 percent of those affected by RVF suffer permanent vision loss.

The bottom line is that, with global warming pushing mosquito prevalence later into the year, and with mosquitos becoming an ever greater health threat, you need to take measures to protect yourself this fall. After all, West Nile virus isn’t the sort of trick you want this Halloween.

Background Information

Bartholomay,L. C., R. M. Waterhouse, G. F. Mayhew, C. L. Campbell, K. Michel, Z. Zou, J. L. Ramirez, S. Das, K. Alvarez, P. Arensburger, B. Bryant, S. B. Chapman, Y. Dong, S. M. Erickson, S. H. P. P. Karunaratne, V. Kokoza, C. D. Kodira, P. Pignatelli, S. W. Shin, D. L. Vanlandingham, P. W. Atkinson, B. Birren, G. K. Christophides, R. J. Clem, J. Hemingway, S. Higgs, K. Megy, H. Ranson, E. M. Zdobnov, A. S. Raikhel, B. M. Christensen, G. Dimopoulos, M. A. T. Muskavitch. 2010. Pathogenomics of Culex quinquefasciatus and Meta-Analysis of Infection Responses to Diverse Pathogens. Science, 330: 88 DOI: 10.1126/science.1193162

Centers for Disease Control and Prevention. 2009. Dengue Frequently Asked Questions. Accessed October 28, 2010 at

Centers for Disease Control and Prevenction. 2010.Rift Valley Fever in South Africa accessed October 28, 2010 at

Kilpatrick, A.M, M. A. Meola, R. M. Moudy, and L. D. Kramer. 2008. Temperature, Viral Genetics, and the Transmission of West Nile Virus by Culex pipiens Mosquitoes, PLoS Pathogens 4:e1000092 doi: 10.1371/journal.ppat.1000092. available at

USGS Disease Maps accessed October 28, 2010: West Nile Virus cumulative positive test results reported in 832 human tests for the United States at; Dengue Fever positive human test results reported for 8,436 in Puerto Rico and 45 in Florida at

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