West Nile virus a North American fixture
Canada, Mexico, U.S. differ on reactions, results
By Christy Oglesby
CNN
(CNN) -- It's probably faster to list where the West Nile virus isn't than where it is. The mosquito-borne malady has nearly circled the globe since its 1937 discovery in Africa. North America is one of its latest conquests.
Within two years of West Nile's 1999 introduction to North America in New York, the virus was in Canada. The following year, horses in Mexico had it.
In the previous 60 years, evidence of the virus had manifested in humans and animals, with cases in the Middle East during the late 1950s, and subsequent reports in the following decades from Europe, western Asia and Australia.
Today, in Canada, health officials "are really keen to get across the message to people that West Nile virus is first of all here to stay. It will continue to occur in places where it's already occurred and don't be fooled into thinking that low numbers one year, means it will be low numbers the next year," said Dr. Paul Sockett, at the Public Health Agency of Canada.
Translation: West Nile virus is a North American fixture.
And while Canada, the United States and Mexico had different reactions to the disease's arrival with surprisingly disparate results, their collaborative efforts generate data on the virus's ecology and allow for a sharing of educational and preventative measures.
A tale of three countries
After West Nile's confirmed presence in New York, national health officials in Canada and the United States quickly adopted defensive and preventative measures.
The U.S. Centers for Disease Control and Prevention "immediately started working with New York City health departments to educate the public," said Emily Zielinski-Gutierrez, a CDC behavioral scientist. "We had clean up campaigns to get rid of potential breeding sites and bring a lot of attention to using repellent and helping people avoid mosquito bites."
Recognizing that migrating birds, carriers of the virus, don't respect municipal boundaries, Canada moved into battle formation with hens on the front lines before there was evidence of the virus in the country.
"We actually set up our first surveillance in 2000. ...We decided we couldn't wait, and we set up ... programs that included looking for dead birds and monitoring dead birds, and the sentinel chickens," said Sockett, director of the foodborne, waterborne and zoonotic infections division at the Public Health Agency of Canada.
The beaked border guards, tested weekly for West Nile antibodies, served as the nation's first warning system. That part of the plan had the dual benefit of generating public and media interest, which helped with education efforts.
Mexico waited. When the virus reached Texas, it became an agricultural concern for Mexico because of the importance of horses to ranching.
Years later, the United States with all its educational and prevention efforts reported nearly 10,000 human cases in 2003, and Canada logged about 400 that same year. Mexico had hardly any human cases, and still doesn't. The virus mainly afflicts horses there, with one CDC study showing West Nile antibodies present in 22 percent of the horse sample from the country.
"That's one of the real mysteries about Mexico, that there are very few documented human cases if any, and the virus has been there for three years," said Dr. Scott Weaver, director for tropical and emerging infectious diseases at the University of Texas Medical Branch.
"We don't see evidence of people in Mexico developing neurological diseases like encephalitis at the same rate that they do in the United States, and we're really not sure why," said Weaver, whose research efforts in Mexico have tracked various forms of the disease in horses. "One theory is that people in Mexico are exposed to more related flavivirus, which is a different virus of the same genus, and there may be some cross protection when a person ... develops antibodies that protect them to some extent against West Nile infection."
A combination of weather patterns, topography and natural immunities have a significant effect on how far, how fast and among whom the virus spreads, health officials said.
At attention
While West Nile has reached each of the states of the continental United States, Hawaii is hoping its isolation might protect it. But health officials realize that when it comes to West Nile, no state's an island.
Officials at the Hawaiian health department think a West Nile stricken bird couldn't make it that far on its own, but someone could import one.
 We do know ... once West Nile has shown up in an area, it doesn't go away. 
-- CDC's Emily Zielinski-Gutierrez
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Consequently, "The Department of Health currently works at and around the major ports of entry to reduce mosquito populations, so that if West Nile virus is introduced via ship or plane, there is a lesser likelihood of the disease cycle becoming established," said Shokufeh Ramirez, the West Nile virus coordinator for the Hawaii Department of Health.
And the U.S. Postal Service implemented an embargo a couple of years ago. Up until that point, birds could be shipped via the postal service, which meant that there was no inspection at the state level, Ramirez said.
Those precautions might spare the remote Hawaii, but what else might be ahead? Where will the virus venture?
Weaver has some ideas.
"Evidence from antibodies shows that West Nile has reached as far as Colombia and Venezuela in South America," said Weaver, who also is involved in research there.
"What is fairly predictable at this stage is that the virus will continue to spread down through South America. What we really don't know is what the impact on human disease and veterinary disease will be because the ecology may be completely different, and there may be much less exposure of people because people in Latin America are exposed to a lot more flavivirus so the question of cross protection from antibodies comes into play."
As for other regions or mapping the next big West Nile wave, the CDC's Zielinski-Gutierrez avoids prognosticating.
"The only sure thing about making predictions is that you'll be wrong," she said. "However, we do know and what we do say, is that once West Nile has shown up in an area, it doesn't go away."
