More than 70 percent of the cases this year have been reported in six states: Texas, South Dakota, Mississippi, Oklahoma, Louisiana and Michigan.
Most people who become infected have no symptoms. In Texas, 89 people found out they had been infected during routine screening for blood donation, Lakey said. A spokeswoman for the American Red Cross said volunteer blood donors are routinely screened for West Nile virus. As of Wednesday, more than 200 would-be donors in 28 states had tested positive this year for West Nile, and the numbers are expected to rise, Red Cross spokeswoman Karen Stecher said. Blood infected with the virus is destroyed or used for research, she said.
Public health experts said it is hard to know for sure what is behind this year's large outbreak, or why Texas — particularly the Dallas area — has been hit so hard. But this summer's heat waves and record temperatures are likely factors, the CDC's Petersen said.
"We know that West Nile virus tends to occur when temperatures are above normal," he said.
The record number of U.S. cases for a full year was reported in 2003, with 9,862 cases and 264 deaths, but the most West Nile deaths were reported in 2002, with 284.
Mosquito-borne outbreaks have always been difficult to predict, experts said. A complex set of environmental factors is involved in West Nile transmission, said Katherine Feldman, Maryland's public health veterinarian. The virus lives in the blood of birds, and mosquitoes spread it to people and horses.
There is no vaccine for humans.
The extent of an outbreak is influenced by the number of mosquitoes and how infectious they are, the population of susceptible bird species, rainfall and temperature. Bringing "all these things together in the right combination at the right time facilitates that the virus go faster and to a greater degree in certain areas of the country," said Roger Nasci, chief of arboviral diseases at the CDC.