TORONTO - It's too soon to know what kind of a year it'll be for West Nile virus, says the director of zoonotic diseases at the Public Health Agency of Canada, in the followup to what turned out to be "one heck of a year" in 2007.
Harvey Artsob handles the West Nile file at the agency, and notes that almost all of the more than 2,300 human cases last summer were on the Prairies, where weather conditions were excellent for the mosquito Culex tarsalis - a warm winter followed by above-normal temperatures in spring and a hot, humid summer.
"The single most important factor to consider as risk for West Nile in terms of our experience so far is weather conditions," he said from Winnipeg. "And it's the years where we get very large numbers of mosquitoes of the vector (carrier) species - those were the years where we've had a large number of human cases."
The first year for human infections in Canada was 2002, when more than 400 people, mainly in Ontario, were hit by the virus, with Culex pipiens thought to be the primary vector. The next year as the virus travelled westward, there were 1,481 cases. But activity dropped off to 25 cases in all of Canada in 2004, 224 in 2005 and 151 in 2006.
The files are now bulging with examples of human cases in Canada, but studies of these people are still few and far between.
A major undertaking by Dr. Mark Loeb of McMaster University in Hamilton that has followed patients for years is in the publication pipeline, and the results will be coming soon.
"We've looked at the natural history of what happens to individuals who have neuro-invasive disease - that's meningitis, encephalitis, acute flaccid paralysis - or West Nile fever (the milder form)," he said.
"We do know that we have patients who have acute flaccid paralysis where symptoms persist. Anecdotally, for the patients who have acute flaccid paralysis ... the deficit, although there's variability ... they're the patients who do worse."
A couple of U.S. studies were presented at an emerging infectious diseases conference in Atlanta, Ga., in March.
In one of them, Joy Holbrook of the Utah County Health Department followed up with 16 people stricken in 2005 and 67 patients diagnosed in 2006.
"I did the study because people were asking us questions that we couldn't answer, we couldn't find any information in the literature, and so I thought I'll just figure it out myself," she said from Provo, Utah. "People were really more than willing to help. They wanted to do something."
Patients were interviewed about their health status in three domains: physical, cognitive and functional, which encompassed questions, for example, on ability to do housework, shop and prepare meals.
Only 20 per cent of the 16 patients diagnosed in 2005 achieved marginal recovery in all three domains. All the participants still had fatigue and headaches 18 months after getting sick, Holbrook said.
"There are some very significant long-term effects of West Nile virus," she said, adding that more research is needed.
Beyond the symptoms, Holbrook noted that lives have been changed.
"I had a teenage boy who was a high school athlete, lost his scholarship because he couldn't compete that last season of his senior year, and went into a huge depression because of all that."
She said the teenagers who were afflicted in 2005 all continued to have problems with concentration, memory and depression 18 months after diagnosis.
"I guess my question is how many children weren't recognized as having that, who also could have been having troubles in school because of it," she wondered.
Loeb, meanwhile, is also doing a study looking for genetic differences between people who suffer neuro-invasive forms of the disease and those who have mild infection.
As for treatment, Artsob said there are no antiviral drugs that work, so it comes down to support for the patient - rehydration, help with pain and control of seizures for those with neurological problems.
There is a vaccine for horses. Human vaccines are under development, but there are no reports of progress on these, he said.
An untold number of people have been exposed to the virus, but were asymptomatic and may now have immunity. Health officials are getting an indication of this as people are tested for the virus when giving blood.
"After 2003, when we had outbreaks in the three Prairie provinces, each one of the Prairie provinces did a study of how many people were exposed to the virus," said Artsob.
In an area of western Manitoba, they found that about three per cent of the population tested had antibodies to West Nile, he said.
And in Saskatchewan, which had the greatest burden of cases in 2003, the number of people with antibodies was much higher.
"They were in the double digits," he said. "One area was almost 20 per cent of the people having antibodies. But the overall rate was in the 12 to 15 per cent of all of the areas studied."
"We do believe that there is immunity from West Nile and that it probably can last for a long time," Artsob said.
But can a person just get tested to see if they have antibodies, and therefore might have immunity?
"Theoretically, yes," Artsob said, adding that it's not an easy test to perform and "we can't just offer it on a regular basis to individuals."
Artsob said it would be interesting to do another survey in the aftermath of the 2007 season.
"In all the subsequent years, more and more people might be being exposed to the virus, and so what is the sero-prevalence, or the infection rate, of the population now? We don't have figures for that."
