West Nile virus turns up in Baja
http://www.signonsandiego.com/news/metro/20040102-9999_7m2ni...
By Sandra Dibble
January 2, 2004
TIJUANA ? Epidemiologists have found the first evidence of West Nile virus in Baja California, tracing antibodies for the disease to 16 horses across
the state and six birds in the Mexicali area.
Although six human cases of the potentially deadly disease have been confirmed in Mexico, none has been in Baja California. The virus, carried by
migratory birds, is transmitted to humans and horses by infected culex mosquitoes.
Because mosquito populations are down for the winter, the danger to humans at this time is "greatly reduced," said Dr. Jorge Acu?a, an epidemiologist
with the federal social security hospital in Mexicali.
But that scenario could change dramatically as spring approaches.
Some researchers are predicting a West Nile virus epidemic in Mexico because warm weather and tropical climates make many regions ideal breeding
grounds for the mosquito.
But others say it is unclear how the disease will play out. They suggest that the presence in Mexico of another mosquito-borne disease, dengue fever,
may have helped Mexicans build antibodies that will protect them from West Nile virus.
So far, the effects of the disease have been relatively benign in Mexico.
Since it was first identified in November 2002 in horses in the northern border states of Coahuila and Tamaulipas, federal health officials say just
two horses and 10 birds have died from the disease.
In California, the first evidence of the virus was confirmed in mosquitoes near the Salton Sea in Imperial County in August. The state has since
confirmed three human cases of the disease. The first was in Riverside County, the second in Imperial County and the third, confirmed Dec. 24, a
61-year-old man in Los Angeles County.
The discovery of animals carrying the disease antibodies in Baja California is "not surprising at all," said Dr. Steve Waterman, a medical
epidemiologist with the U.S. Centers for Disease Control. "It confirms what people expected."
Dr. Alma Rosa M?rquez of the Baja California Health Secretariat said the six birds that tested positive for the virus were migratory, so it is unclear
whether they became exposed to the virus in Baja California or elsewhere.
More details are expected later this month. The secretariat is awaiting the test results on native birds, although sentinel chickens placed around the
state to detect the disease have shown no signs of being exposed, M?rquez said. Tests on mosquitoes also have been negative, she said.
With a statewide horse population estimated at 12,000, Baja California agriculture officials are closely following the virus. Eleven of the horses
that tested positive for West Nile are in the Mexicali area, said Dr. Marco Antonio Vargas, head of animal health for the Baja California office of
Mexico's federal agriculture secretariat. The remaining five are in Tijuana and Ensenada.
"The situation is as we had been expecting," Vargas said. "We have the same mosquitoes in Baja California as in California."
The agriculture secretariat has been urging horse owners to vaccinate their animals against the disease. Some of the horses could have tested positive
because they had already received the vaccine, even though health officials have been trying to limit their tests to unvaccinated animals, Vargas
said.
Baja California state health officials say they share information with California's Office of Binational Health. But Paula Kriner, an Imperial County
epidemiologist, said local officials communicate only informally on the topic of West Nile virus.
"I'm hoping we can develop an exchange system," she said.
With the discovery of the Baja California cases, all six states on Mexico's northern border now show evidence of the virus, according to Mexico's
National Center for Epidemiological Surveillance. Across the border from Texas, there were four human cases in Chihuahua and one in Nuevo Leon as of
Dec. 25. A sixth case has been identified in the state of Sonora, but in a location far from the border.
In southern Mexico, 564 horses in the state of Chiapas had tested positive for antibodies to West Nile virus as of Dec. 25, more than in any other
state in Mexico. But none had died as a result of the disease.
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