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Summer Bugs: Do Warm Winters Lead To More Insect Carried Diseases?

06/04/2012 09:41 EDT | Updated 08/04/2012 05:12 EDT
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Winter haters in some parts of Canada got a major break this year, with one of the mildest seasons in memory.

But as we head towards summer, there may be a price to pay for the lack of the frigid temperatures a few months back. While it's not a given, the mild winter may lead to an increase in cases of diseases spread to people by insects this summer, experts concede.

West Nile virus, Lyme disease, and even heartworm in pets — all may flourish as a consequence of the mild winter.

More ticks — some of which carry the bacterium that causes Lyme disease — may have survived the winter than would normally be the case. And an earlier start to the cycle of mosquito breeding and biting could potentially lead to more West Nile cases in people and heartworm in pets this summer, they say.

They stress the conditional, however. With these diseases, a complex interplay of factors establishes the risk of infection and that changes from year to year.

"The warm winter might contribute to changed risk from these diseases," says Nick Ogden, a research scientist with the Public Health Agency of Canada who specializes in this area.

"It's too early to be certain, because it's not just what happens over winter that causes the risk, but also what happens this summer for West Nile virus and also over subsequent years for Lyme disease."

In veterinary circles, there has been some concern about heartworm, a disease dogs and cats can be infected with if they are bitten by mosquitoes that carry the parasite.

"When we had that warm spell back in March, we had mosquitoes all over the place. And that raised a lot of questions with some mosquito-borne diseases," says Dr. Scott Weese, an infectious diseases specialist at the Ontario Veterinary College in Guelph, Ont.

Weese says the cool temperatures in April and early May could mean there won't be an explosion of cases this year, despite the early start to the mosquito season. But some veterinarians aren't taking chances, having started pets early on the anti-parasitic drug regimen they use every spring to protect against heartworm.

Lyme, West Nile, heartworm — these are what is known as vectorborne diseases, meaning they are passed to their human or animal victims by another organism. This is an indirect form of disease transmission, in contrast to something like measles where the measles virus infects humans directly.

In the case of Lyme disease, the causative agent is Borrelia burgdorferi, a bacteria carried by some tick species. In Canada, the main tick species involved in spreading Lyme disease are Ixodes scapularis — often called the deer tick — and, on the West Coast, Ixodes pacificus.

With West Nile virus, infected birds and mosquitoes pass the virus back and forth, creating an ever widening pool of infected mosquitoes in the spring and early summer. That process amplifies the amount of virus in the environment.

At some point when there are enough infected mosquitoes, the virus spills over into people when they are bitten by infected bugs.

It typically takes until late July or August before human West Nile infections start to be reported.

A short winter and a corresponding early start to the cycle of mosquito breeding and biting could potentially increase the amplification of West Nile virus in the mosquito species that spread it.

"It comes down to the critical mass thing, right? It just allows for more generations of transmission and amplification," Weese says.

That said, one can't estimate the severity of a West Nile season by the timing of the first crops of mosquitoes in the spring alone.

"They need to have water and heat in the right amounts at the right time," explains Dr. Bonnie Henry, medical director for the emerging and vectorborne diseases program at the B.C. Centre for Disease Control in Vancouver.

"If you have a very hot spring and summer, then your risk of West Nile virus goes up and then your risk of increasing number of ticks go up as well."

For West Nile, what is key is how many days there are in the spring when the average daytime temperature is over 13C, Henry says.

Ogden says a warmer winter will typically allow more mosquitoes to survive, which means the pool of breeding mosquitoes is larger from the start.

But whether that translates into a more rapid amplification of West Nile and the corresponding risk that more human infections could take occur "will depend not only on the weather we had over winter, but also the weather that's to come," he says.

West Nile seems to operate in boom and bust cycles, Ogden says, with the last large epidemic occurring in 2007. "And that was driven by a warm summer followed by a warm winter followed by a warm spring and summer without a cold snap in spring that would kill off the over-wintering and first breeding mosquitoes."

Are parts of Canada on track for a repeat in 2012? It's too soon to say, Ogden insists.

As for Lyme disease, it's known that the ticks that carry Borrelia burgdorferi are now found in many more parts of Canada than they were even a few years ago. In the early years of the last decade, for instance, the only places where infected ticks were found were in the southern most parts of Ontario.

Experts are predicting with climate change, the range of these ticks will expand further, putting more Canadians at risk of contracting a disease that 10 or 20 years ago they could only have contracted if they visited Long Point on Lake Erie, Ont., or parts of the Eastern U.S., like Nantucket Island off the coast of Massachusetts.

But whether a single mild winter will have an impact on transmission risk remains to be seen.

"One warm winter might not actually produce a great change in Lyme disease risk," Ogden says. "But it might contribute to the more long-term change."