TORONTO - It's not just our day-to-day weather and other aspects of the environment that will be altered by global climate change: disruptions in seasonal temperatures and precipitation patterns will also likely affect human health because of a rise in infectious diseases, experts suggest.
In an article in Tuesday's issue of the Canadian Medical Association Journal, infectious disease specialists predict that global warming will increase the risk of infectious disease by expanding the geographic ranges of species known to carry diseases that jump from animals to humans.
As mean temperatures rise in Canada and the rest of North America, mosquitoes and other insects known to carry diseases that can be spread to humans would likely expand their breeding areas - and also the length of time they could survive in a season, said co-author Dr. David Fisman, a scientist at the Hospital for Sick Children's research institute in Toronto.
With West Nile virus, for instance, the number of mosquitoes carrying the disease could rise dramatically and a longer summer could mean transmission to humans well into October or November, he said Monday. West Nile season now usually ends by September in parts of Canada, depending on the weather.
Ticks that carry Lyme disease and typically hitch a ride on deer species could also survive longer in more northerly regions of Canada.
Fisman said it is difficult to specify a time frame for such a blooming of infectious pathogens, but he suggested such changes could occur in the next several decades.
"We're not trying to predict the future," he said. "What we're saying is that there's a real possibility."
"Human beings are just one group of organisms that live in a really complicated ecosystem. Ecosystems include both physical and biological components and we seem to be in a situation where the physical component of the ecosystem -- that is weather and climate -- is changing at a rate that is faster and faster and faster."
Co-author Amy Greer, a research fellow at Sick Kids Hospital, said it is not just warmer temperatures that could play havoc with the spread of infectious diseases.
"There are also a lot of predictions of ... large precipitation events - so much more rain occurring in a short period of time, which can lead to flooding," said Greer, noting that winters could be marked by more rainfall than snowfall.
"And those precipitation events or flood events have been linked to outbreaks of things like gastroenteritis (intestinal disorders). When you have precipitation events, that allows for agricultural run-off where we have pathogens that then get into water systems, for instance."
A perfect example, she said, is the flooding that contaminated the water supply in Walkerton, Ont., with E. coli bacteria, leading to the deaths of some residents and long-term health effects in others.
Dr. Andrew Simor, head of microbiology and infectious diseases at Toronto's Sunnybrook Health Sciences Centre, called the paper "an excellent article" and agreed there's no question that climate change will significantly influence the extent and kinds of infectious diseases that will confront humans.
In fact, it's already happening, said Simor, who was not involved in writing the article.
For example, the fungal disease Cryptococcus gattii - previously seen only in Australia and other subtropical regions - has taken root in Vancouver Island and is spreading locally in mainland B.C and the U.S. northwest. The fungus can cause serious infection in the lungs and brains of both humans and animals.
"And there is no question it's climatic changes that have allowed this fungus to appear and (infect) humans," Simor said.
Fisman said warmer temperatures and more rainfall could lead to increased vegetation growth over a longer period in Canada's North, which could in turn create an explosion in rodent populations. Among the diseases carried by certain rodents are hantaviruses, which can be fatal to humans.
"We would expect in northern areas of Canada, where the degree of warming is supposed to be greatest, longer summers and more vegetation available for rodents more of the year," he said. An explosion in rodents could mean rising numbers of predator animals, which can carry such diseases as rabies.
To meet the growing infectious disease threat, Fisman and his colleagues say federal and provincial public health bodies need to strengthen the testing and surveillance infrastructure, so new cases of existing diseases and the emergence of new ones can be properly diagnosed and tracked.
"So unless we are able to identify and count the number of cases of disease that we have and look for year-on-year trends, we're never even going to know whether things are getting worse at the population level until things get so bad it will be sort of a slap in the face," Fisman said.
It's not just human cases that need to be monitored, said Greer, but also infectious diseases in livestock and pets.
"It's becoming more apparent ... many of these pathogens do have animal or insect components to them or are pathogens that are able to be harboured by both animals and humans," she said.
"I think we like to think of ourselves as separate from our environment or animals, but I think it also calls for us to strengthen the infrastructure between the human health community and the veterinary health community."
If veterinarians see an increase in the number of ticks on pets, for instance, that information is important for the human health community to know, she said.
Besides beefing up public health infrastructure, Simor said practising physicians and medical students need to be better educated about these diseases, some of which have up to now been considered rare, subtropical infections, so they can recognize and properly treat them.
"Clearly, as if there weren't already enough reasons, this is yet another important reason to ensure that we don't contribute further to global warming," he said.
"I do think this highlights how humans so very much interact with the environment, change the environment, and we're not always aware how those changes can affect us in the long run. This is an important wake-up call."
