San Diego's Four West Nile Cases Signal a Bigger Risk Than the Number Suggests

San Diego County has confirmed its fourth human case of West Nile virus this year. Health officials are urging residents to guard against mosquito bites as the season of peak transmission approaches.

On its face, four cases may seem small. Ian McHardy, Ph.D., senior director of microbiology and infectious disease at Scripps Health, said the figure looks different once you consider how many infections it takes to produce one symptomatic case. "When you look at the number of people that need to be infected in order for you to actually get a symptomatic case, it actually starts to sound a whole lot more relevant," he said.

Most people who contract the virus never know it. McHardy said about 80% of infections cause no symptoms, while 20% lead to a mild to moderate flu-like illness. Fewer than 1% progress to meningoencephalitis or another neuroinvasive disease, which can be life-threatening when the virus reaches the brain or nervous system.

The virus follows a seasonal pattern, according to McHardy. Cases begin climbing in summer and reach their highest point around August and September, with reports sometimes continuing into October and November. "The vast majority of the season for West Nile is ahead of us," he said. That means the four current cases could mark a significant start to the season, with many more possible.

West Nile is spread by infected Culex mosquitoes, which are most active in the evening, overnight and early morning. Older adults and people with weakened immune systems face the greatest risk of severe illness.

McHardy recommended wearing insect repellent outdoors at dusk and dawn and getting rid of standing water around the home. He noted the hot summer has made breeding sites more likely. "People leaving their pools uncovered and neglected, plant saucers, clogged gutters, buckets, toys, old tires, wheelbarrows," he said, adding that mosquitoes can breed in many places. Residents can also request free mosquito-eating fish from the county for ponds, fountains and unused swimming pools.

Prevention matters because treatment options are limited. "Unfortunately, we don't have any extremely effective antivirals at this time," McHardy said.

Anyone who develops severe symptoms should seek medical care right away. McHardy listed high fever, stiff neck, disorientation, confusion, muscle weakness, paralysis and vision loss as signs of a severe, possibly neuroinvasive infection that requires hospital care.

He stressed that avoiding infection is far easier than dealing with the aftermath. "It's much safer to just put on mosquito repellent, long sleeves, long pants, and prevent the actual infection, rather than going down the path of having perhaps lifelong consequences with West Nile," he said.