Five birds with WNV were discovered before the start of May, according to the county Department of Environmental Health.
The total of 29 for the year is the highest in the state. The county didn't find 29 WNV-positive birds last year until late October.
"These are signs that 2015 might be another tough year in our county for West Nile virus, so people should remember to protect themselves and their families," said DEH Director Elizabeth Pozzebon. "Get rid of standing water around properties where mosquitoes can breed, wear insect repellent when outdoors and report dead birds."
Last year, 11 San Diegans contracted WNV and two died. In a recent prior years, no local cases were reported.
Statewide, more than 800 people were diagnosed with the virus in 2014 and 31 died. No one has caught the illness in California this year, according to state health officials.
West Nile virus is spread to birds and humans by mosquitoes, and the county has an eradication program in place. Residents are also encouraged to help out by dumping water out of outdoor pots and saucers, wheelbarrows, old tires and rain gutters.
Four out of five people bitten by a mosquito carrying WNV won't have symptoms. Of those who do get sick, most will have a mild headache, fever, nausea, skin rash or swollen glands.
The symptoms turn life-threatening in one out of 150 cases, mainly for people over 50 years old or those with weakened immune systems.