The Los Angeles County Department of Health announced Thursday that tests conducted by the Centers for Disease Control had determined that a young woman who had been hospitalized in August who is now recovered indeed had the virus, although officials were still puzzled over how she had become infected.
"We assume that the woman was bitten by a mosquito, the typical way in which a person is exposed to the virus," said Public Health Director Dr. Jonathan Fielding. "However, mosquito surveillance units in Los Angeles and throughout the state have not detected any West Nile virus activity."
The agency said that an apparent second case of West Nile was being investigated, but the victim had recently arrived in Los Angeles from Texas and was exposed to the virus in the Lone Star State. The California unnamed female patient in the earlier case had not been out of the state and did not recall being bitten by a mosquito.
The latest victim in Illinois was hospitalized with fever, headache and tremors on Aug. 29 and died Tuesday of West Nile encephalitis. He was the 346th Illinois resident to contract the virus, state health officials said.
Thirty-three more people have tested positive for the mosquito-borne illness: 11 in Chicago; 17 in suburban Cook County; three in DuPage County and one each in Crawford and McHenry counties. The youngest person believed infected this summer was a 3-month-old infant, who recovered and has been released from the hospital.
Although hot summer temperatures were starting to cool off, health officials expected the rate of infections to continue because of the three to 14-day period in which symptoms can appear.
"West Nile virus can be deadly, but it is no match for common sense," said Dr. John Lumpkin, state public health director. He recommended that people use insect spray, wear long-sleeved shirts and long pants, and stay indoors at dawn and dusk when mosquitoes are most active.
Michigan Department of Community Health Chief Medical Executive Dr. David R. Johnson confirmed 15 new confirmed cases in Michigan as well as the deaths of two elderly Oakland County women, which raised the state's West Nile death toll to eight.
"Confirming these additional cases is not surprising as we will continue to see cases of West Nile virus in Michigan until after we experience a hard frost," said Johnson. "Regardless of what county or community a person lives in, it is crucial that they continue to take common sense precautions to minimize exposure to mosquitoes."
Illinois leads the nation in West Nile cases and fatalities, followed by Louisiana, which has reported 16 new cases -- including one death -- bringing the state's death toll to 11, the second highest in the country.
Louisiana Department of Health and Hospitals officials, however, were optimistic that the outbreak in the state is waning.
"This downward trend in new cases is continued evidence that our personal responsibility campaign, our disease surveillance efforts and local spraying activities have all been effective in combating this virus," said DHH Secretary David W. Wood.
The National Centers for Disease Control and Prevention has confirmed 1,201 West Nile virus cases in the United States and 46 deaths to date.
(Phil Magers in Dallas and Hil Anderson in Los Angeles contributed to this report)