They believe three adults and one child have been affected.
Three are from the Visalia area, and one is from Tulare.
It comes as officials confirmed the Saint Louis Encephalitis Virus, also known as SLEV, has been detected in mosquitoes in Tulare County.
Symptoms of SLEV infection are similar to that of West Nile.
Mosquitoes transmit both illnesses.
Most people don't even know they're infected and show no signs of symptoms.
But a small percentage do get symptoms, and they can even be deadly in less than one percent of individuals.
Report mosquito presence (anonymously, if you like) by calling the Delta Vector Control District, or your local vector control agency, toll-free, at 1-877-732-8606 or by reporting online at www.DeltaVCD.com, or on its Facebook page (www.facebook.com/DeltaVectorControlDistrict), Twitter (@DeltaVCD), or Instagram (@DeltaVCD) accounts.
Apply an EPA-registered insect repellent that contains DEET, picaridin, oil of lemon eucalyptus, or IR3535 according to label instructions, especially in the early morning and evening.
Wear long-sleeved shirts and pants if outside in the early morning and evening.
Make sure that doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens that have holes.
Eliminate standing water and containers that can hold water from around the home.
Find out about mosquito reports all over the state of California by logging on to www.westnile.ca.gov. A Tulare County map of virus activity for the DVC area is viewable on the Delta Vector Control District homepage at http://www.deltavcd.com/ by clicking on the tab "Current Mosquito Collections."