West Nile cases jump 25 percent in a week, CDC says West Nile virus cases, which are spread by mosquitoes, continue to rise in the U.S., health officials said. West Nile cases in the U.S. continue to climb, jumping 25 percent in a week, with 1,993 cases nationwide and 87 deaths in the country's worst outbreak for this time of year since the virus was detected here in 1999, health officials said Wednesday. Texas continues to log the most cases, with at least 888 reported illnesses and 35 deaths, according to figures from the Centers for Disease Control and Prevention. But Dr. David L. Lakey, commissioner of the Texas Department of State Health Services, said his latest figures actually show that Texas has reported 1,013 cases and 40 deaths. "Officially, this is the worst week ever for West Nile in Texas," Lakey said. The nationwide numbers were up from 1,590 cases and 65 deaths reported Aug. 28, said Dr. Lyle R. Petersen, director of the CDC's division of vector-borne infectious diseases. "We expect this increase to continue for the next several weeks, probably until October," said Petersen, who added that he was infected with the virus himself a few years ago. Many cases have not yet been logged because of the lag in reporting time, he said. More than half the cases -- 54 percent -- are the serious neuroinvasive variety, which can lead to encephalitis or meningitis. The vast majority of West Nile infections, some 80 percent, are so mild that people don't know they're infected. About 20 percent develop symptoms and about 1 percent may develop serious, neuro-invasive illnesses. West Nile has been detected in 48 states, with human cases reported in 44 states, Petersen said. Five other states -- Missississippi, South Dakota, Oklahoma, Louisiana and California -- also continue to report high numbers of cases. The outbreak is still far shy of the worst year ever for West Nile virus infections and deaths in the U.S. In 2003, 9,862 cases were reported and 264 people died, according to the CDC's records. The disease is spread by infected mosquitoes, which breed in water. Officials have said a mild spring, hot weather and heavy rains may have contributed to the epidemic this year, but they're not sure. Other factors include the type and number of infected mosquitoes, the number of birds that become infected and other factors, he said. "There's a very complicated ecology to the transmission of these viruses in nature," Petersen said. Why the outbreak is so bad in Texas is a matter of speculation, he added. The outbreak may have begun to peak, said health officials, who hope they're seeing a decline in cases -- and in mosquitoes that test positive for the virus, Lakey said. Sprayed pesticides appear to have reduced the population of mosquitoes, he added.