Last week a woman in her 60s tested positive for West Nile Virus (WNV), becoming the first human WNV infection in Orange County this year, as compared to two cases at this time last year. As of August 25, 2023, the California Department of Public Health had reported 61 infections of human West Nile Virus statewide this season.

Vector-borne, particularly mosquito-borne disease, should be a consideration in those presenting with fever at this time of year. While WNV is endemic in Orange County, locally acquired malaria has been reported in other parts of the United States: CDC Health Update Important Updates on Locally Acquired Malaria Cases Identified in Florida, Texas, and Maryland.

• Health care providers should consider WNV infection in patients with prolonged fever or acute neurologic illness.
• Clinical Description: The incubation period is usually 2 to 6 days after a bite from an infected mosquito. Most symptomatic WNV cases have an acute febrile illness. Symptoms also include headache, malaise, arthralgia, or myalgia, and occasionally nausea, vomiting and/or a maculopapular rash. About 1 in 150 WNV-infected persons develops neuroinvasive disease, which can manifest as encephalitis, aseptic meningitis, acute flaccid paralysis, and/or transverse myelitis.
• Testing for WNV: Testing for WNV should include serum WNV IgM and IgG, with CSF IgM when a lumbar puncture is performed. Because serum WNV IgM may be negative early in the course of disease, repeat serology may be indicated if initial testing was done within 8 days after the onset of illness. Viral cultures and tests to detect viral RNA (e.g., reverse transcriptase-polymerase chain reaction [RT-PCR]) can be performed on serum, CSF, and tissue specimens that are collected early in the course of illness and, if results are positive, can confirm an infection. However, the likelihood of detecting a WNV infection through molecular testing is fairly low.
• Surveillance of West Nile Virus in the community: Report all WNV infections to Orange County Public Health by phone (714-834-8180) or fax (714-564-4050) within 1 working day. For more information on mosquito surveillance and control activities, please visit the Orange County Mosquito and Vector Control District website at http://www.ocvector.org/.
• Avoiding mosquito bites is the best way to prevent WNV infection:
• Eliminate standing water which can serve as mosquito breeding sites around the home or workplace.
• Ensure windows and door screens are in good repair.
• Use an insect repellent containing active ingredients registered with the U.S. Environmental Protection Agency when outdoors. Active ingredients include DEET, picaridin, and oil of lemon eucalyptus. Natural bug sprays may not be effective against mosquitoes. For more information, visit http://www.cdc.gov/westnile/faq/repellent.html.