The patient experienced conjunctivitis after contact with infected dairy cattle in the state's Central Valley.
Idaho's agriculture department says dairy herds are quarantined in four of the state's counties.
California has reported 63 West Nile virus infections this year, including many in the Central Valley.
The latest local dengue case in Los Angeles county isn't related to the cluster reported earlier this month.
At least one household member and six healthcare workers who encountered the index case-patient are being investigated after showing symptoms of viral infection.
The scientists examined outbreaks in European fur farms, South American marine mammals, and US dairy cattle.
California now has 40 avian flu outbreaks in dairy herds, and the US total has reached 238.
In other developments, CDC and Ohio partners launched a serosurvey among bovine veterinary workers who attended a recent professional conference.
The source of the girl's infection isn't known, though H9N2 has been reported in the country's poultry before.
California now has the nation's second most dairy farm outbreaks.
