As humans and animals develop immunity to the West Nile virus, the number of confirmed human cases of the mosquito-borne disease has seen a dramatic decline in California, a trend that is expected to continue, health experts said.
The state is believed to have hit its peak last year with 935 human cases, including 28 deaths. So far this year, 68 cases have been logged, with one death in Butte County announced this week.
The downward trend is already in its second year in Los Angeles County, which peaked at 309 human cases in 2004, with 14 deaths. That compares with 43 cases last year and no fatalities, and two cases so far this year.
"What's occurring here now is the same thing that's occurring in other areas: You have exposed a virus to a naive population and have had a large die-off," said Dr. Jonathan Fielding, the county's health officer. "Those who survived are developing antibodies to fight off the virus. It's Darwin 2006."
San Bernardino and Orange counties have also shown a dramatic drop in human infections largely attributed to adaptation in the bird population. San Bernardino County reported 35 human cases in 2005, down from 197 the previous year; Orange County reported 17 cases in 2005, down from 64 the prior year, and one case so far this year.
Although hotter temperatures increase mosquito activity, state health officials said the number of people and animals testing positive for the virus is expected to remain low even through West Nile's peak season in August and September.
"Indeed, in Southern California particularly, there has been a pattern where counties will be hit pretty hard at first, less so the next year and certainly even less activity the next year," said Vicki Kramer, chief of vector-borne diseases for the state Department of Health Services.
First detected in Africa and long common in the Middle East, Asia and parts of Europe, West Nile virus is transmitted to humans through mosquito bites. The insects acquire the virus by feeding on infected birds.
The vast majority of infected people do not get sick, and those who do generally exhibit symptoms similar to the flu. In extreme cases, meningitis and encephalitis occur, sometimes resulting in death.
The virus was first detected in the United States in 1999, spreading from the East Coast and increasing in severity. In 2003, Colorado reported the highest number of human cases, with 2,947 infections and 63 deaths. The virus quickly spread to Arizona and California.
In L.A. County, people who tested positive for the virus tended to live near the county's main "crow routes," clustered around the hills and mountains of eastern Los Angeles County and the lower areas of the San Gabriel Valley, researchers found.
But since the 2004 outbreak, a significant decline in human cases has occurred in hard-hit areas partly because of increased mosquito control efforts but mostly because humans and animals were developing immunity, experts said.
William Reisen, a UC Davis researcher who has been studying the evolution of West Nile in California birds, said antibody rates rose after 2004. The next year, L.A. County reported little West Nile activity until late summer, when new and more susceptible birds began to hatch.
Not all parts of California have seen a decline in West Nile cases, however. In Kern County, officials reported 60 confirmed human cases in 2004 and 68 last year. So far this year, 20 cases have been reported, the highest number in the state so far.
On Thursday, state health officials announced that a Butte County woman in her 80s had died from complications of the virus. She is the first person in California to die from the disease this year.
"For most people, the risk of serious illness from West Nile virus is low, but this woman's death reminds us that we must take precautions to protect ourselves and our families from mosquito bites," said Dr. Mark Horton, a state public health officer, in a prepared statement.
Although humans are developing an immunity, health officials warn that it is a much longer process than in birds because humans are infected at a significantly lower rate.
Dr. Lyle Petersen, who heads the Division of Vector-Borne Infectious Diseases at the Centers for Disease Control and Prevention, estimated that 1.5 million Americans have been infected with West Nile.
"You can imagine how long it will take to get to the rest of them," he said.
In countries like Egypt and Uganda, where West Nile was first detected, people became fully immune to the virus by the time they reached adulthood, federal health officials said.
Times staff writers Rong-Gong Lin II and Mai Tran contributed to this report.