10/21/2013 04:51 EDT | Updated 01/23/2014 06:58 EST

Silver lining to the sucky summer: lack of heat keeps West Nile infections low

TORONTO - It turns out there is an upside of having to endure a sucky summer.

West Nile virus case numbers are relatively low in North America this year, a nice contrast to the explosion of infections racked up in 2012.

In Canada, only 105 people had tested positive for West Nile virus infection by Oct. 5, down from more than 400 at the same time last year. And in the United States, case counts too are about a quarter of what they were in 2012, one of the most active years on record.

All the pieces in the complicated interplay that makes for a bad, a mild or a so-so West Nile virus year have not yet been identified. But in the 14 years since the virus was first detected in North America, it has become clear that one factor, more than all others, is associated with large numbers of human infections.

That factor is heat. And not just heat at any time. Hot days in September after a cool summer do not pose the risk that sweltering days in the spring and summer will, experts say.

"We had a fairly cool spring this year, pretty much across the country. And we feel that's definitely a factor. Because mosquitoes, in order to breed and feed, they need warm temperatures," says Michael Drebot, director of zoonotic diseases at the Public Health Agency of Canada.

"In order for the virus to replicate it needs heat. In order for the mosquitoes to be active, they need warm temperatures. And I think the slow start from a seasonal point of view to both the spring and the summer probably led to less viral circulation and the risk to people of being exposed, because maybe they weren't outside to the same degree."

If you like heat and sun, 2012 was a summer for the ages in many parts of North America. In others, it was an unbearable scorcher. All that heat was reflected in the West Nile case count tables published by the Public Health Agency of Canada and the U.S. Centres for Disease Control.

"It's probably no accident that we had this huge outbreak in 2012 and it was the hottest year on record," says Dr. Lyle Petersen, director of the CDC's division of vector-borne diseases, based at Fort Collins, Colo.

Last year the U.S. reported 6,377 cases (including 703 blood donations that tested positive for the virus), and 286 deaths. As of Sept. 24 this year, there were 1,383 cases (including 248 positive blood donations), and 44 deaths. U.S. surveillance numbers are a bit behind Canada's because of the 2 1/2 week government shutdown that was resolved last week.

In 2012, Quebec had its worst West Nile year on record, with 131 cases. That's more than three times higher than its previous worst year, 2011, when the province had 38 cases. Many years Quebec has reported case counts in the single digits.

Ontario, meanwhile, had 382 cases in 2012. This year it has recorded just 50, says Dr. Doug Sider, the medical director for communicable disease prevention and control at Public Health Ontario.

Sider notes that a few more cases could come to light in the province — the same is true for other jurisdictions — before public health officials can draw a line under the 2013 season. But even if that happens, the numbers won't come close to last year's.

So why is heat so important? It's all about the birds and the bugs, Petersen explains.

West Nile virus is found in the blood of infected birds. Mosquitoes — which prefer biting birds to biting people, you might be surprised to learn — become infected when they take a blood meal from birds carrying the virus. And they infect more birds as they take additional snacks over the course of the spring and summer.

At some point in the season, there are simply enough infected mosquitoes around that they are going to start passing the virus on to people. Typically in our part of the continent, that amplification process as it is called starts to produce human cases around late July onward.

Hot weather speeds up the whole process.

"Heat makes mosquitoes infectious faster and it makes them more infectious," Petersen says. "And that's really important for transmission."

That's because it takes a while for mosquitoes to go from getting infected to being infectious, he explains. The virus has to build up in their systems — a process that is accelerated by heat.

"If you raise the temperature, that period gets shorter. And dramatically shorter, actually. Depending on the mosquito and the temperature, it could go from three weeks down to five days," Petersen says.

The more hot days early in the summer, the faster this whole scenario runs.

There are undoubtedly other factors at play. Even in an active West Nile season, some areas will see a sharper uptick in cases than others.

Saskatchewan experienced an explosion of cases in 2007 — 1,285. It is the highest number of cases reported in any year by any Canadian province — and it's more than the entire country recorded in many years. Texas — the Dallas region in particular — had a terrible outbreak in 2012, with nearly 2,000 cases.

"Temperature kind of raises the thermostat of transmission. But there are also local factors which are hard to define which influence how far up you're going to rise the wave, let's say. So these outbreaks still are fairly focal in nature," Petersen says.

Whatever is behind the difference, it's in the hands of nature, not people, Sider says. Despite the fact that public health officials have pushed "fight the bite" messages hard in a bid to combat West Nile virus, they don't think drops in cases are attributable to increased use of DEET-laced mosquito repellents and more people donning long sleeved garments.

Says Sider: "I can't believe that between 2012 and this year that people are all of a sudden doing a much better job of protecting themselves."