In this Aug. 16, 2012 file photo, mosquitos are sorted at the Dallas County mosquito lab in Dallas. U.S. health officials say there's been an alarming increase in the number of West Nile cases. (LM Otero/AP)

Since West Nile virus arrived in New York in 1999, it has spread to nearly every state. This year’s outbreak is on pace to be the worst yet, according to the Centers for Disease Control and Prevention.

As of Sept. 25, the CDC reported 3,545 cases of West Nile illness this year, resulting in 147 deaths. Because of the lag in reporting to the CDC, the number of new cases is expected to increase through November — or until the first hard freezes kill the mosquitoes that transmit the virus.

About half of reported cases are “neuroinvasive,” meaning the virus gets into the brain, spine or nerves, where it can cause potentially fatal brain swelling and temporary or permanent paralysis.

Texas has been hardest hit this year. A few cases have been reported in the Washington area, including three in the District.

CDC experts estimate that 400,000 to 1 million Americans have become ill from the virus. Most of them never knew it because the symptoms were so mild or because they never got a diagnosis.

There is no vaccine and no treatment for West Nile virus. Drug companies developing vaccines pulled back their efforts in recent years, according to Laura Kramer of the New York State Department of Health. “They felt there was no money in it, just not enough cases to warrant the expense of putting a vaccine to market,” she said.

Development of drugs to treat the virus has also halted, with no studies of West Nile drugs currently underway in the United States.

Brian Vastag