Officials across the United States on Thursday launched a spraying campaign to kill mosquitoes, as the country faces the biggest spike in West Nile Virus in years.

The virus has led to some 700 cases of the disease so far this year, 26 of which resulted in death. According to the U.S. Centers for Disease Control and Prevention, this is the highest number of cases reported through the end of July since 2004.

Almost 80 per cent of the cases have been reported in Texas, Mississippi, and Oklahoma, CDC says.

In Texas alone, 10 people have died, prompting the mayor of Dallas to declare a state of emergency. The city will be sprayed with a fine coating of insecticides.

Officials blame the hot, dry summer for allowing more mosquitoes, which transmit the virus, to breed. In the United States, most people are infected from June through September, and the number of infections usually peaks in mid-August, according to CDC.

Four in five infected adults will show no symptoms, but the virus can cause fever, headache, nausea, vomiting and rashes.

Less than one per cent of those who contract the virus develop a serious neurologic illness such as encephalitis or meningitis, an inflammation of the brain, CDC said.

There is no vaccine to prevent West Nile virus infection and no medications to treat it.

In milder infections people usually recover on their own although symptoms may last for several weeks.

In some extreme cases, the virus can cause permanent brain damage or death. About 10 per cent of people who develop a neurologic infection due to the virus will die.

Here in Canada, more infected mosquito pools are being found in Quebec, Manitoba and Ontario, most likely also due to the heat.

But unlike the United States, Canada has just a handful of confirmed human cases, likely because municipalities have been aggressively spraying places where mosquitoes breed with larvicides.

The widespread problems in the U.S may be a sign that public health officials didn't take enough precautions.

“You have to balance the health risks of the spraying versus the health risk of the disease, and that is the situation you get to when you have enough disease that is causing significant health problems,” said Dr. Ian Gemmill of Queens University.

Canada began surveillance in 2002, and the worst year has so far been 2007, with more than 2,200 cases, including 12 deaths, according to the Public Health Agency of Canada.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip