With large numbers of virus-infected culex tarsalis mosquitoes remaining in parts of the Prairies, West Nile could deliver a hefty late season punch, according to the Public Health Agency of Canada.
As of Aug. 16, 106 cases of human infection had been found in Manitoba, 82 in Saskatchewan (40 unconfirmed), 37 in Alberta and two in British Columbia that were thought to have been acquired during travel on the Prairies.
Two people have died from the virus in Manitoba and 14 of that province’s total were found through routine screening of blood donors. In the past week, 48 new cases have been added to the total.
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Last year, Canada had 151 cases, compared to 225 in 2005 and 25 in 2004.
But in 2003, 1,481 cases were reported. Saskatchewan accounted for 947 that year, including seven deaths.
“In that year we had 66 cases reported up until Aug. 4,” said health agency spokesperson Alain Desroches.
“This year, if you look at the same period, we have 78. The last time we saw so many cases early in the season was in 2003 and we ended up with a very nasty season.”
He said one explanation for the late season rise in the number of reported cases could be the time lag from onset of symptoms – two to 15 days – and the time before a visit to the doctor can be arranged, a diagnosis made and the findings reported to the agency’s West Nile surveillance program.
Another reason could be that a greater percentage of late season mosquitoes are of the virus-carrying type, he added.
Robin Lindsay, a research scientist at the National Microbiology Laboratory in Winnipeg, said this year’s mosquito trap counts are comparable to those in 2003.
“It was a very good year for mosquitoes in the Prairies,” he said. “We had very good conditions, which allowed them to build up very high numbers early on and they are really percolating the virus.”
A pool of mosquitoes is tested by putting up to 50 in a vial and checking for the virus.
Of the 5,000 samples taken from trapped mosquitoes in 37 communities in Manitoba, 754 pools have tested positive for West Nile. In 2003, the number was 290 for the entire season.
Possibly because of a shortage of standing water in recent weeks, the traps have been catching up to 90 percent culex tarsalis. In many areas few of the roughly 36 species of nuisance mosquitoes do not carry the virus.
Hot spots for disease-carrying mosquitoes are the southwestern corner of Manitoba and the Red River valley, both typically warmer areas.
Although annual vaccinations have proven effective at protecting horses, Saskatchewan has reported the highest number of equine West Nile infections this summer on the Prairies, with 24 confirmed positive test results, said Robert Kerr, Manitoba’s chief provincial veterinary officer. No equine infections were reported last year, but 133 cases were reported in 2003.
Alberta has reported three equine cases, one confirmed and two suspected. The confirmed case was a horse that exhibited neurological symptoms and was put down, according to chief provincial vet Gerald Ollis.
Manitoba has had two confirmed equine cases of the disease this year, compared to 47 in 2003. The previous year saw 236 infections.
