Thursday, July 22, 2004
A dead crow found on an east San Jose lawn has tested positive for West Nile virus, signaling that a worrisome outbreak of the mosquito-borne disease in Southern California has taken a big leap north and landed in the Bay Area.
The discovery was announced Wednesday by Santa Clara County mosquito-control officials after state lab tests showed that the crow, found June 28, had been killed by the virus.
California has been spared a serious outbreak of West Nile virus until this year, but 32 people have come down with it in Southern California since a dead crow tested positive April 1 and the disease was diagnosed in a San Bernardino County woman on June 8.
Sick and dying crows typically presage an outbreak of West Nile virus among humans. California has found the disease in 876 dead birds in six southern counties.
"We have been waiting for quite some months now for West Nile virus to rear its ugly head in Santa Clara County," said Tim Mulligan, program manager of the county's vector-control district. "We are concerned." So far, none of the California patients has died, but a similar outbreak in Arizona has sickened 125 people and claimed two lives.
Last year, 264 Americans died of West Nile virus, the worst year for infections by the virus since it was discovered in New York City in 1999.
Mulligan said his district has been working full-tilt at mosquito abatement ever since West Nile virus turned up in Southern California in 2002, and additional efforts will be directed at the east side neighborhood where the dead bird was found.
The virus is spread by migrating birds, which probably explains how it managed to jump 300 miles from its stronghold in Los Angeles and San Bernardino counties to the Bay Area in a short time, Mulligan said. A dead crow found in Bakersfield tested positive last week, the first sign of a possible breakout along the central valley "flyway." But crows, which are particularly susceptible to the virus, usually have a range of only about 20 miles, indicating that infected mosquitoes that killed the Santa Clara County bird are located nearby.
"We don't want people to panic," Mulligan said. "But it's wise to be aware in the general area of east San Jose there is a general potential of West Nile virus." Mulligan urged residents to "do your citizen-type duty" and check their backyards and neighborhoods for pots and puddles of standing water that allow mosquitoes to breed.
"You don't have to use pesticides or sprays," Mulligan noted. "Just dump the water out." Vicki Kramer, chief of vector-borne diseases for the California Department of Health Services, said a very large portion -- 70 percent to 80 percent -- of the dead birds tested in San Bernardino and Riverside counties have tested positive for West Nile virus, a "red flag" that the disease is highly endemic in those areas.
The disease is particularly lethal to crows, magpies, jays and ravens -- a family of birds known as corvids.
Early testing in a region tends to focus on corvids, but Kramer said the state is stepping up its surveillance in the Bay Area for any dead birds.
Once West Nile is detected in a region, mosquito control officers can focus their resources on that area. In San Bernardino County, spraying against adult mosquitoes in residential neighborhoods has been ordered after less aggressive methods failed to knock down the number of human cases and positive test results in pools of mosquitoes and "sentinel" chicken flocks, whose blood is tested for antibodies to West Nile virus. Kramer acknowledged that callers on the hot line earlier this season were having difficulty getting through because of the high volume of reports.
"There was just one hot line for the entire state," she said.
Additional capacity and voice-mail have been added to ease the telephone bottleneck, she said.
Four out of five people with West Nile virus experience no symptoms at all and develop what is believed to be lifelong immunity to the disease. The rest may suffer headache, fever, a rash and other flulike symptoms.
But West Nile virus can cause serious illness -- permanent nerve damage or a potentially fatal swelling of the brain -- for about one in 150 of those infected. Last year, 264 Americans died from the disease.
West Nile virus originated in Africa and first appeared in the New York City in 1999. Since then, it has rapidly spread across the continental United States. Colorado was hit particularly hard last summer, with nearly 3,000 cases and 63 deaths. So far this year, there have been 12 cases in Colorado and no deaths.
After a region is hit hard by the epidemic in one season, it typically it not as heavily affected the following year. One theory is that the birds and humans develop a level of immunity that makes it harder for the virus to spread than in a "virgin" population never exposed to it.
Nicholas Komar, a research biologist at the Centers for Disease Control and Prevention arbovirus lab, in Fort Collins, Colo., said experiments are under way to determine if the theory of "herd immunity" is the correct explanation.
Komar said it is too early to tell how this season's West Nile virus outbreak will compare with last year's. The number of cases reported so far is higher nationwide than last year, but that could be the result of more screening for the disease.
If the disease follows patterns of the past few years, it will peak in the first or second week of August, but public health officials will not get the reports of those illnesses until mid-September because of the time it takes to test and record the results.
"We are seeing an increase in the number of cases reported every week," Komar said. "The epidemic is on the upswing." Editor Notes: NONE
nlm.nih.gov
c.2004 San Francisco Chronicle |
