West Nile Cases May Number ‘Tens of Thousands,’ U.S. Says

“Tens of thousands” of Americans have probably been infected by the West Nile virus this year and the number will continue to grow through the end of September, officials said Thursday.

The last week of August and the first week of September are the peak period of transmission, said Dr. Lyle Petersen of the federal Centers for Disease Control and Prevention in Atlanta.

But he cautioned that the vast majority of those infected have not suffered any significant symptoms other than a mild flu-like illness. Through Thursday, there have been 854 confirmed cases of severe illness--meningitis or encephalitis--caused by the virus in 28 states, and 48 deaths.

Although no cases have been reported in California, Los Angeles County health officials said Thursday they are testing several people, including one young adult woman with aseptic meningitis, a condition sometimes linked to West Nile. Preliminary tests have shown possible exposure to West Nile. Final results are expected next week.

County health officials say they haven’t found West Nile in surveillance of mosquitoes, chickens, horses or dead birds in the area. “For us, the question is not whether it’s going to be in Los Angeles, but when,” said Dr. Jonathan Fielding, the county’s public health chief.

Florida officials said the suspected West Nile infection of a 71-year-old female transplant recipient had been confirmed. That means all four people who received organs from a Georgia accident victim have been confirmed to be infected. Three of them were hospitalized with encephalitis; one died as a result.

The fact that all four organ recipients have contracted the virus “would implicate the receipt of organs as potentially the mode of transmission,” Petersen said.

The accident victim got blood and blood products from as many as 60 donors before she died two days after the accident. Blood samples taken immediately after the crash showed no evidence of West Nile, Petersen said, but a sample taken when her organs were harvested tested positive.

“But that does not rule out the possibility that she could have had a mosquito-borne infection,” he said. She may simply have developed viremia, active viral replication, after being hospitalized.

Mississippi officials also revealed Thursday that a woman who had received 18 units of blood during an obstetrical procedure had been diagnosed with West Nile four weeks later.

But Dr. Ed Thompson of the Mississippi health department noted that the woman also reported multiple mosquito bites during the period and that she lived in an area where the virus is endemic. “There is as strong evidence that the virus was mosquito-borne as there is for anything else,” such as a blood transfusion, he said.

Officials have recalled all remaining blood products from the donors and are testing them and the donors for West Nile.

Nonetheless, “there is absolutely no evidence at this point that transmission by transfusion has occurred in the United States or anywhere else,” Petersen said.

The number of confirmed cases of West Nile infection this year represents a “massive increase” from 2001, and the increase is not just from better reporting, he said. The growth is corroborated by increases in the number of dead birds and by evidence of the virus in larger numbers of trapped animals.

The “tens of thousands” figure comes from the fact that an estimated 150 people are actually infected for every one who develops encephalitis or meningitis.

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Times staff writer Charles Ornstein contributed to this report.