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04/29/2003
The first case of avian (crow) WNV in Canada for 2003 has been reported, a month earlier than reported last year. The most significant aspect of the finding is that WNV apparently can over-winter in most North American environments, meaning new outbreaks at most locales do not rely on reintroduction through birds or other sources.
Thomas J. Roffe, PhD, DVM
Northern Rocky Mountain Science Center
USGS-BRD
FWP Bldg, 1400 S. 19th Ave.
Bozeman, MT
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Supporting Documents:
Date: Mon 28 Apr 2003
From: ProMED-mail <promed@promedmail.org>
Source: National Post, Mon 28 Apr 2003 [edited]
<http://www.nationalpost.com/home/story.html?id=DE486F82-2A03-490C-9B0C-46A0
CEF49300>
Dead Crow Suggests Over-wintering of West Nile Virus in Canada
The discovery of the first West Nile-infected crow in Canada this year all but proves the virus survives the Canadian winter, Health Canada acknowledged yesterday. This raises the stakes in efforts to contain West Nile virus, as it means future Canadian outbreaks will arise locally from an ever-present source.
The crow was found dead last week at a busy intersection in Newmarket, north of Toronto. It was sent for testing, which was positive but has not yet been confirmed. "It's a bigger risk if the virus can overwinter and stay within the province," said Dr. Harvey Artsob, chief of the health department's viral zoonotics and special pathogens division. "If it couldn't, then it has to be reintroduced every year, say by migrating birds or from down south."
The crow's death comes almost a month earlier than the first avian case of last year. In 2001, it was not until August that a bird tested positive, marking West Nile's introduction to Canada. Human cases normally arise in late July or August, once the virus has spread to the species of mosquitoes that prey on humans.
A positive result so early after the spring thaw does not suggest this year's cases among humans will occur earlier; it still depends on prolonged warm weather. But it is good evidence that the virus is "seeded" in hibernating mosquito populations all across Ontario, Dr. Artsob said. The crow was among the first few to be tested, he said, which suggests it is not an oddity, but that the virus is present in other crows, waiting to be found. Reports are not available from other monitoring programs in regions where West Nile has recently been introduced, such as the northern United States, Manitoba, and Quebec, because they have not yet started testing for the year.
Manitoba, for example, will start on the first of May. "It's just an opinion, but I think if they were looking [for West Nile virus-infected crows] in some of these areas, they would be finding them," Dr. Artsob said. Dr. Michael Drebot, Health Canada's head of viral zoonosis, said one way the West Nile virus can survive the winter is in _Culex pipiens_ mosquitoes, which prey on birds but not humans, and which hibernate as adults on the walls of dank places like sewers and drainage ditches. "As daylight hours increase and temperatures rise during the spring, these mosquitoes emerge from the sewers and look for a blood meal," he said. This species does not prey on humans, but humans are put at risk when other mosquito species prey on infected birds and then on humans. "We still think it takes quite a while for the virus to build up in nature, and get into other species of mosquitoes that feed on humans," Dr. Artsob said.
Hanif Kassam, medical officer of health for York Region, where the crow was found, said the discovery "puts our plan in action." The region is a part of Health Canada's coordinated effort to track West Nile-infected birds. Most people who become infected with West Nile show no symptoms at all; most others who are healthy experience an illness akin to the flu, including fever, headache, and body aches. Health Canada advises that the elderly, the very young, and those with weakened immune systems are at risk of the more dangerous complications of encephalitis or meningitis, the swelling of the brain or its lining. Clinical evidence gathered since the 2002 epidemic, however, suggests that healthy, middle-aged people will suffer some of the most debilitating and long-lasting effects of the next outbreak. Patients who have survived the initial onslaught of the disease have often developed "acute flaccid paralysis," or a weakening of limbs as the virus attacks a region of the brain stem.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://biology.usgs.gov/cro/WNV04-25-03.htm
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