As many as 10,000 people in Western Canada may have been infected with the West Nile virus this summer, even though only 2,165 felt ill, said Dr. Harvey Artsob, director of zoonotic diseases at the National Microbiology Lab in Winnipeg.
Because only 20 percent of all cases develop symptoms such as fever, and less than one percent develop serious neurological symptoms, Artsob said that it is reasonable to assume that far more people were infected – up to five times more – but either never realized it or didn’t consider it serious enough to visit a doctor.
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“That’s a rough figure, and it varies depending on the virulence of the strain, but as a generalization, those figures hold,” said Artsob.
What’s more, because West Nile belongs to the same family as dengue and yellow fever, it is likely that those who get it are handed a special prize: immunity lasting for decades even in cases where the patient does not become ill, Artsob said.
Dengue fever, common in the tropics, is in many countries considered a childhood illness that is rarely fatal. According to the World Health Organization website, of the 50 to 100 million cases that occur each year, only an estimated 500,000 result in serious illness.
According to most medical textbooks, immunity to dengue fever is commonly assumed to be virtually permanent.
“We feel relatively confident in generalizing that immunity to West Nile is very long-lived. Whether or not it is lifelong, we don’t know and we don’t claim it,” he said.
Such natural immunity may extend to horses to some degree, according to Wayne Lees, Manitoba’s chief provincial veterinarian, but without testing for the presence of West Nile antibodies, it would be impossible to tell whether a horse has caught a mild case of the bug and gotten over it, or made it unscathed through the summer’s hordes of mosquitoes.
For this reason, it’s probably best to play it safe by administering vaccine to horses on an annual basis, Lees said. The vaccine is available from vet clinics for $25 to $30 per dose.
Needs to be repeated
Because a vaccine is designed to introduce only a small, controlled dose of killed or weakened virus to the immune system with the aim of triggering an effective response, it needs to be repeated each year.
An actual infection involves a much larger amount of the pathogen, and therefore is more likely to result in lasting immunity.
“Horses that had been exposed to West Nile virus and survived would probably have much higher levels of immunity than horses that had been vaccinated, but that immunity tends to drop off over time. That’s why you have to keep revaccinating,” Lees said.
“Otherwise, it’s like Russian roulette. You don’t know which horses are protected and which ones aren’t.”
The latest surveillance data in Manitoba shows only eight horses came down with West Nile this year. Results from past years indicate a steep reduction in infections since 2002, when 236 cases were reported.
The latest human surveillance numbers show 543 cases in Manitoba and 289 in Alberta reported. Saskatchewan had the highest number of any province with 1,307.
