West Nile Infection Baffles Officials

As the mosquito season winds down, Los Angeles County health officials said they still don’t know how a local 31-year-old woman developed what appears to be the only West Nile case acquired in California.

The woman’s case has baffled public health experts since they announced her infection in early September. She hadn’t traveled outside the region and didn’t remember being bitten by a mosquito, the route of transmission in nearly all cases. She has recovered.

No other locally acquired cases have been reported in California or in any adjacent state. And health authorities haven’t found any signs of infection in local mosquitoes, birds or chickens, which are typical indicators that the virus has arrived in a region.

“There’s always the theory that a mosquito got off a plane at LAX and infected this woman,” said Dr. Frederick Murphy, a professor at the UC Davis School of Veterinary Medicine. As time goes by and no other evidence is found, that theory “becomes more and more logical.”

Dr. Laurene Mascola, chief of acute communicable disease control for Los Angeles County, described the lone local case as a “big question mark.” But she said her office has given up trying to figure out its origin, saying such knowledge wouldn’t help protect residents.

“We all know it’s going to come,” Mascola said of the virus. The delay “will give us another year to be even more prepared” for when the virus is firmly entrenched in the region.

A spokesman at the U.S. Centers for Disease Control and Prevention said officials there know of no other instance domestically in which West Nile has shown up in a human but could not be found in mosquitoes or birds nearby. They did, however, point to a human infection that could not be linked to a local source last year in the Cayman Islands.

Cases of West Nile acquired elsewhere in the U.S. continue to pop up among visitors to California. Orange County health officials reported a probable case Tuesday in a 70-year-old resident who had traveled to Nebraska and later became ill. He was hospitalized and released.

This year, authorities in 34 states and the District of Columbia have reported 146 deaths and 2,768 infections attributable to West Nile. Federal health officials suspect that tens of thousands of people are infected, but the vast majority of patients display no symptoms.

In about one in five cases, people experience mild symptoms, including fever, headache and nausea. One in 150 require hospitalization. West Nile infections, which can lead to death, can cause meningitis, an irritation of the brain’s membranes, or encephalitis, an inflammation of the brain.

West Nile first appeared in the United States in 1999, killing seven people in the New York area. This year, faced with so many new cases, researchers learned that the virus can be spread through organ transplantation, blood transfusions and breast milk.

Federal health officials have said they are trying to quickly find a test that can detect the virus in blood donations, as well as create a vaccine for it.

Californians who see birds that have been dead for less than 24 hours are asked to report them to health authorities at (877) 968-2473. Authorities will test for the presence of the virus. Los Angeles County has also set up a general information hotline: (800) 975-4448.