Bloomberg News

West Nile Virus Logs Deadliest Year After Hotter U.S. Summer

June 27, 2013

The West Nile virus that swept through the U.S. last year was the deadliest on record, killing 286 people as the warmer-than-average summer may have helped the mosquito-borne virus spread, U.S. health officials said.

The U.S. had 5,674 cases in 2012, the largest number of infections since 2003, the Centers for Disease Control and Prevention said. Cases were reported in every continental U.S. state except for Oregon, including the first-ever instance of the virus recorded in Maine, the CDC said in a report today.

The U.S. next week enters the typical peak season for West Nile, where warm weather makes infection more likely. While there are only six reported cases of the virus this year through June, according to the CDC’s website, more than 90 percent of infections from last year occurred between July and September.

“West Nile virus is going to be a factor in the U.S. every year now,” Marc Fischer, a medical epidemiologist with the CDC’s arboviral diseases branch, said in a telephone interview. “People need to take precautions and protect themselves.”

The reasons for last year’s surge in infections and deaths are unclear due to the ecological factors that drive the virus’ spread, according to the CDC analysis in today’s Morbidity and Mortality Weekly Report. The report notes the number of birds and mosquitoes that spread the virus, as well as weather and human behavior as factors that influence disease severity.

Hot Weather

The first eight months of 2012 in the lower 48 U.S. states were the warmest start to any year in records dating to 1895, according to data released in September by the National Climatic Data Center in Asheville, North Carolina. June to August of 2012 was the third-warmest summer on record as temperatures averaged 74.4 degrees Fahrenheit (23.6 Celsius) or 2.3 degrees warmer than the average in the 20th century, according to the center.

There is no vaccine for West Nile, first detected in the U.S. in 1999, which kills almost 10 percent of people who are hospitalized for the infection and can also cause paralysis or neurological damage.

While four out of five people who develop the virus never develop symptoms, less than 1 percent of cases result in symptoms such as high fever, disorientation, and convulsions. In more severe cases, the virus can invade the central nervous system and cause maladies such as meningitis, encephalitis, or acute flaccid paralysis.

The rate last year at which people contracted the neuroinvasive form of the virus -- 0.92 for every 100,000 people -- is triple the median rate from 2004 to 2011 and approaches the peak incidence in 2002.

The median age of patients with the virus was 56, and the median age of those who died from it was 77.

To contact the reporter on this story: Samuel Adams in New York at

To contact the editor responsible for this story: Reg Gale at

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