The West Nile Virus, a disease found in birds that mosquitoes can transfer to humans, is spreading throughout the county much faster this year than in years past and county officials are encouraging the public to take precautions to prevent mosquito breeding and bites.
By mid-July, 96 dead birds tested positive for the virus, as compared with only six by the same time last year. Eleven of those birds were found in Carmel Valley, Del Mar, Solana Beach and Rancho Santa Fe.
A mosquito pool tested positive in Oceanside earlier this month, with dead birds also testing positive in the area. One horse died from the virus in Blossom Valley. No human cases have been reported in San Diego County this year.
"I have never seen it ramp up this fast," said Chris Conlan, vector ecologist with County Vector Control. "Anytime we see elevated activity it's a definite concern. All residents should be taking precautions."
The most important action residents can take is to prevent mosquitoes from breeding in their own backyards.
Any standing water, even a few tablespoons, could become a breeding ground after a few days. Residents are advised to dump out water collected in anything from potted plants and saucers, rain gutters, buckets and trash cans, to children's toys, garden tools and wheelbarrows. Water in small bird fountains should be completely flushed once a week, not just topped off.
Mosquito fish, while in tight supply, are still available for free to control larger water sources such as ponds, fountains, horse troughs and unused swimming pools.
Last year, backyard-breeding sources were linked with all 15 human cases of locally acquired West Nile Virus, Conlan said.
Residents can protect themselves from mosquito bites by wearing insect repellent, staying indoors when mosquitoes most active at dusk and dawn, and repairing any holes in screen doors and windows.
West Nile Virus, common in Africa, West Asia and the Middle East, first appeared on the East Coast in 1999 and spread across the nation appearing in California in 2003. Since 2004, 2,320 human cases of West Nile have been reported throughout the state; of those, 76 were fatal.
There is no treatment for the virus; however, less than one percent of individuals infected develop severe symptoms such as disorientation, coma, convulsions, vision loss, numbness and paralysis.
About 80 percent of humans infected with the virus will not even know they have it, experiencing no symptoms.
The remainder experience mild symptoms that last for a few days, such as fever, headaches, nausea, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms typically appear three to 14 days after being bitten by an infected mosquito.
People over 50-years-old, and individuals with a preexisting health conditions or a compromised immune system are at the greatest risk of getting sick from the virus.
Mosquitoes can pass the virus to any other animal that they bite, including cats, dogs and horses. However, cats and dogs are very resistant to the virus and rarely become ill. A vaccine is available for horses.
Many species of wild birds are infected, but can survive the virus. However, crows, ravens, jays and hawks are highly susceptible and make up the bulk of the reported bird deaths.
Residents can report dead birds to County Vector Control, and personnel will come collect the bird. Birds must have died within the past 24 hours, be intact and not covered in ants. Though the virus cannot be spread through contact with the bird, residents are advised to not touch or move the bird unless necessary.
In addition to collecting and testing birds, County Vector Control monitors and treats major mosquito breeding grounds around large bodies of water. During mosquito season from April through October or November, helicopters treat almost 1,000 acres with larvacide once a month to knock the population down "to a dull roar," Conlan said.
Pockets within the San Dieguito River Valley Park, Carmel Creek, the Polo grounds and the San Elijo Lagoon are a few of the locations treated with the larvacide, which is not harmful to the environment, other animals or humans, Conlan said.
"Any place you have slow moving water where mosquitoes can breed you have that potential (for West Nile Virus)," said Doug Gibson, executive director of the San Elijo Lagoon Conservancy. "Some of our goals in our restoration plans are to remove plants that stagnate water and bring tidal flow into the area."
Mosquitoes breed during the warmer months, and the hotter it is, the faster they hatch. Only a few types of mosquitoes carry the West Nile Virus, and while mosquitoes don't usually travel far, birds do. That's why all residents, no matter where they live, need to take warnings about West Nile seriously, Conlan said.
"The flu kills more people, but this is a very preventable disease by taking certain precautions," he said.
The virus has not been around long enough to determine any patterns associated with it, Conlan said. However, the number of birds testing positive this year is already poised to surpass last year's total, 118, which were mostly reported in August and September.
"Normally, I am not very alarmist," Conlan said. "We have to go with prepare for the worst, hope for the best."
To report a dead bird or request mosquito fish call County Vector Control at (858) 694-2888. For more information, go to