Rise in cases of West Nile virus may portend an epidemic

New York Times: The number of West Nile virus cases in the United States is nearly four times what it was a year ago, meaning that a large epidemic may be in store, government researchers are reporting. “It’s certainly a warning sign that we need to be extremely vigilant,” Dr. Lyle Petersen, the director of the division of vector-borne infections at the Centers for Disease Control and Prevention, said yesterday. “The worst is yet to come.”

The virus, carried by mosquitoes, causes a mild, flu-like illness in 20 percent of those infected, and no symptoms in about 80 percent. In about 1 percent of cases, the illness progresses to a brain infection that can be fatal. Last year, 4,269 cases were reported in the United States, including 1,495 brain infections, and 177 people died. The risk of severe illnesses increases with age. So far this year, 122 cases have been reported, with the most in California and the Dakotas. At this time last year, there had been only 33.

The reported cases are just the tip of the iceberg, researchers say. Many infections are never diagnosed because they were mild and the patient did not see a doctor, or was not tested for the virus.

This year, there have already been 42 brain infections and 3 deaths. This is early in the season, since 90 percent of the cases usually occur in August and early September. It is impossible to predict whether the trend will continue, Dr. Petersen said, adding that it may be related to “a lot of weird weather events,” including both the heat waves in the West and unusual storm patterns in the Midwest. If people keep getting infected at the current rate, he said, “we could see the largest epidemic ever.”

The first known case of the disease in the United States occurred in New York City in 1999, and since then the virus has spread to every state. In cases in the past, the virus was transmitted by transfusions and organ transplants, but tests are now done to protect the blood supply. This year, the tests have found 23 potential blood donors who were infected.