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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 thelack of the frigid temperatures a few months back. While it s nota given, the mild winter may lead to an increase in cases ofdiseases spread to people by insects this summer, experts concede. West Nile virus , Lyme disease , and even heartworm in pets all may flourish as a consequenceof the mild winter. More ticks some of which carry the bacterium that causes Lymedisease may have survived the winter than would normally be thecase. And an earlier start to the cycle of mosquito breeding andbiting could potentially lead to more West Nile cases in people andheartworm in pets this summer, they say. They stress the conditional, however. With these diseases, acomplex interplay of factors establishes the risk of infection andthat changes from year to year. The warm winter might contribute to changed risk from thesediseases, says Nick Ogden, a research scientist with the PublicHealth Agency of Canada who specializes in this area. It s too early to be certain, because it s not just whathappens over winter that causes the risk, but also what happensthis summer for West Nile virus and also over subsequent years forLyme disease. In veterinary circles, there has been some concern about heartworm,a disease dogs and cats can be infected with if they are bitten bymosquitoes that carry the parasite. When we had that warm spell back in March, we had mosquitoes allover the place. And that raised a lot of questions with somemosquito-borne diseases, says Dr. Scott Weese, an infectiousdiseases specialist at the Ontario Veterinary College in Guelph,Ont. Weese says the cool temperatures in April and early May could meanthere won t be an explosion of cases this year, despite the earlystart to the mosquito season. But some veterinarians aren t takingchances, having started pets early on the anti-parasitic drugregimen they use every spring to protect against heartworm. Lyme, West Nile, heartworm these are what is known asvectorborne diseases, meaning they are passed to their human oranimal victims by another organism. This is an indirect form ofdisease transmission, in contrast to something like measles wherethe measles virus infects humans directly. In the case of Lyme disease, the causative agent is Borreliaburgdorferi, a bacteria carried by some tick species. In Canada,the main tick species involved in spreading Lyme disease are Ixodesscapularis often called the deer tick and, on the West Coast, Ixodes pacificus. With West Nile virus, infected birds and mosquitoes pass the virusback and forth, creating an ever widening pool of infectedmosquitoes in the spring and early summer. That process amplifiesthe amount of virus in the environment. At some point when there are enough infected mosquitoes, the virusspills over into people when they are bitten by infected bugs. It typically takes until late July or August before human West Nileinfections start to be reported. A short winter and a corresponding early start to the cycle ofmosquito breeding and biting could potentially increase theamplification of West Nile virus in the mosquito species thatspread it. It comes down to the critical mass thing, right? It just allowsfor more generations of transmission and amplification, Weesesays. That said, one can t estimate the severity of a West Nile seasonby the timing of the first crops of mosquitoes in the spring alone. They need to have water and heat in the right amounts at theright time, explains Dr. Bonnie Henry, medical director for theemerging and vectorborne diseases program at the B.C. Centre forDisease Control in Vancouver. If you have a very hot spring and summer, then your risk of WestNile virus goes up and then your risk of increasing number of ticksgo up as well. For West Nile, what is key is how many days there are in the springwhen the average daytime temperature is over 13C, Henry says. Ogden says a warmer winter will typically allow more mosquitoes tosurvive, which means the pool of breeding mosquitoes is larger fromthe start. But whether that translates into a more rapid amplification of WestNile and the corresponding risk that more human infections couldtake occur will depend not only on the weather we had overwinter, but also the weather that s to come, he says. West Nile seems to operate in boom and bust cycles, Ogden says, with thelast large epidemic occurring in 2007. And that was driven by awarm summer followed by a warm winter followed by a warm spring andsummer without a cold snap in spring that would kill off theover-wintering and first breeding mosquitoes. Are parts of Canada on track for a repeat in 2012? It s too soonto say, Ogden insists. As for Lyme disease, it s known that the ticks that carry Borreliaburgdorferi are now found in many more parts of Canada than theywere even a few years ago. In the early years of the last decade,for instance, the only places where infected ticks were found werein the southern most parts of Ontario. Experts are predicting with climate change, the range of theseticks will expand further, putting more Canadians at risk ofcontracting a disease that 10 or 20 years ago they could only havecontracted if they visited Long Point on Lake Erie, Ont., or partsof the Eastern U.S., like Nantucket Island off the coast ofMassachusetts. But whether a single mild winter will have an impact ontransmission risk remains to be seen. One warm winter might not actually produce a great change in Lymedisease risk, Ogden says. But it might contribute to the morelong-term change. 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