- The deadly surprises just keep coming -- West Nile virus,
SARS, hantavirus, E. coli. The health-care system is reeling from an onslaught
of new diseases and the resurgence of old foes.
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- In the past three decades, according to Harvard University
biologist Douglas Causey, "previously unknown diseases have surfaced
at a pace without precedence in the annals of medicine."
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- "Since the 1970s, there have been 30-plus new diseases
that have emerged. We also have old diseases that are reappearing where
they've been eliminated or appearing where they've never been before,"
said Kirsty Duncan, a University of Toronto adjunct professor of geography
who has studied the potential re-emergence of malaria in Ontario.
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- These diseases are spreading for various reasons -- notably
global air travel -- but there is an important factor that many political
leaders and public-health officials perhaps do not fully appreciate: climate
change.
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- Scientists warn that rising global temperatures and increasing
weather extremes are playing a significant role in the emergence and spread
of infectious diseases and are accelerating the potential for more nasty
surprises. The wildfire advance of West Nile across North America is, they
say, a harbinger of things to come.
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- While the impact of new diseases will vary depending
on the resilience of health programs in affected countries, severe acute
respiratory syndrome has shown that even in developed countries, human
and financial resources can be pushed to the wall by unanticipated outbreaks.
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- Earth's average temperature has been rising for 150 years,
particularly the past two decades -- something most climate experts attribute
mainly to greenhouse-gas emissions from human activities. Nine of the warmest
years in that period occurred in the 1990s and 2000s, including the past
six.
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- Global temperatures are projected to increase 1.4 to
5.8 degrees Celsius by 2100 and the warming will be even greater at high
latitudes. Canada has already warmed almost two to three times the global
average in the past 150 years.
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- During the summer of 2001, Canada experienced an unprecedented
24 days above 30 C. "From 1961 to 1990, the average was two to four
days," said Prof. Duncan, who was the principal author of a United
Nations report on human-health impacts of climate change. By 2050, it is
projected that 50 per cent of summer days may reach those temperatures.
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- Extreme weather events are also projected to increase,
including the boom-and-bust cycle of severe droughts followed by heavy
rains that has occurred in North America in recent years. "More drought
out West and more drought in Southern Ontario are anticipated," said
Dominique Charron, senior epidemiologist with Health Canada's Centre for
Infectious Disease Prevention and Control in Guelph, Ont. "Rain, when
we get it, will likely be in more intense bursts."
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- This increased climate variability will probably be even
more important than temperature increases in promoting outbreaks of some
diseases, according to Paul Epstein of Harvard Medical School's Center
for Health and the Global Environment.
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- In fact, climate affects all the links in the chain of
disease transmission: disease-causing organisms such as viruses and parasites;
"vectors" that transmit the disease, such as mosquitoes, ticks
and rodents; hosts, such as birds, that provide a reservoir for disease
organisms; and predators that keep disease-carriers under control.
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- Mosquitoes are highly sensitive to rising temperatures,
which accelerate their metabolic rate, biting rates and feeding frequency,
Dr. Charron said.
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- Temperature also affects their reproduction and growth.
Prof. Duncan cites the example of Anopheles mosquitoes, one of three Ontario-native
species that can transmit malaria. At temperatures below 23 C, a female
takes 72 hours to lay eggs; above 23 C, she takes 48 hours. At 20 C, the
eggs hatch in 2.5 days; at 33 C, they hatch in 33 hours. At 20 C, the eggs
take 20 days to reach adulthood; at 31 C, they take just seven days.
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- Pathogens inside mosquitoes are also sensitive to temperature.
At 18 C, one species of the malaria parasite plasmodium takes 30 days to
develop; at 25 C, it takes just 10 days.
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- Since Anopheles mosquitoes live only a few weeks, higher
temperatures increase the odds the parasite will mature and become infective
while the mosquito is alive and biting. And because warming accelerates
the mosquitoes' breeding cycle, there are more generations of vectors.
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- Moreover, warmer nights and warmer winters resulting
from climate change can extend the geographic range of mosquitoes, increasing
their ability to spread disease. One study estimated that an increase of
several degrees in the global temperature by 2100 could multiply the vectoral
capacity of mosquitoes by 100 times in temperate regions.
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- Drought followed by heavy rainfall can also encourage
the outbreak of deadly diseases. Such was the case in the U.S. Southwest
in 1993 with the sudden emergence of hantavirus pulmonary syndrome, caused
by inhaling virus particles in rodent droppings.
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- Scientists believe the outbreak went like this: First,
drought killed off rodent predators, such as owls, hawks and snakes. Heavy
rains followed, increasing the rodents' food supply and, in the absence
of predators, their population exploded. It is believed that this allowed
hantavirus, previously inactive or isolated, to spread widely among the
rodents. When drought returned, the resulting food shortages forced the
rodents into human dwellings, where they passed the disease to humans.
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- Human cases of hantavirus have occurred in Canada's western
provinces and infected mice have been found in eastern Canada, but Dr.
Charron said climatic and ecological conditions here are not the same as
in the United States and hantavirus "hasn't established itself to
the same level. We haven't had any outbreak situations."
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- There are similarities between the hantavirus scenario
and the surprise outbreak of West Nile in New York in 1999. The primary
vector, the Culex pipiens mosquito, lives mainly in cities, laying eggs
in stagnant, polluted water in sewers and elsewhere. "During a drought,
the pools become richer in rotting vegetation that the mosquito needs to
survive," Prof. Duncan said.
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- The mild winter of 1998-99 was followed by the hottest,
driest spring and summer New York City had experienced in a century. According
to Dr. Epstein, it is likely that drought killed off mosquito predators
and also caused birds to congregate around sparse water sources where mosquitoes
bred.
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- The heat wave allowed the virus to mature rapidly inside
mosquitoes, which quickly infected many birds, creating a large reservoir
of the virus. As some birds died and others flew off in search of water,
the mosquitoes increasingly turned to humans for blood. Then came heavy
rains that created new breeding sites, causing the population of infected
mosquitoes to explode.
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- In 2002, the second warmest year on record, conditions
were ripe for further spread. Many parts of North America experienced a
very warm winter and a spring and summer of drought followed by heavy rains.
West Nile made "a furious dash" westward, Dr. Epstein said, spreading
to 44 states and five provinces.
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- In early August, the number of U.S. cases suddenly tripled
from the previous week and was expected to exceed last year's totals. The
virus has spread rapidly to western states not previously affected. It
now infects 230 animal species, including more than 130 species of birds.
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- The winter of 2002-03 was colder than last year, but
dead infected birds still turned up in Ontario more than a month earlier
than last year and the first Canadian cases have also been earlier.
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- "There doesn't seem to have been a delay in onset,"
Dr. Charron said. It has been suggested that this might be because the
mosquitoes were protected by relatively warm urban sewers, but she noted
that West Nile continues to spread through non-urban areas as well.
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- "We're all a little awed by how the virus has spread
and how comfortably established it has become in the bird population. The
birds and the mosquitoes here are completely able to support the transmission
of the virus as though it had evolved here."
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- It may take a few years for the pattern to stabilize,
but she said it's likely West Nile is "here to stay."
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- The pattern of warm winters followed by hot, dry summers
may encourage the spread of other vector-borne diseases, such as encephalitis,
transmitted by mosquitoes, and Lyme disease and Rocky Mountain spotted
fever, spread by ticks. Prof. Duncan said warming temperatures may create
conditions suitable for the re-introduction of malaria in North America
if they allow parasites sufficient time to mature inside mosquitoes.
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- Water-borne diseases also may increase as a result of
heavy rainfall that washes animal wastes into water supplies. Extreme rain
has been implicated in outbreaks of cryptosporidiosis in the United States
in 1993, toxoplasmosis in British Columbia in 1995, and deadly E. coli
in Walkerton, Ont., in 2000.
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- Weather and climate are, of course, not the only factors
that influence the outbreak and spread of diseases; social and economic
factors also are important. Surveillance programs and testing and control
measures -- such as monitoring for dead birds and eliminating mosquito
breeding areas -- can make a difference.
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- A UN report notes that while climate change "might
provide opportunities for diseases to extend their range, the North American
health infrastructure may prevent a large increase in disease cases."
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- However, it warns that providing protection "could
increase the demands on and costs of the current public health system."
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- The truth of this statement, written before SARS took
a billion-dollar bite out of Ontario's health-care system, is now painfully
obvious. SARS has prompted considerable soul-searching about how often
even advanced public-health systems can withstand such challenges.
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- Climate experts say these looming health issues are just
one more reason to get serious about cutting greenhouse-gas emissions.
Unfortunately, even if we do -- and for the most part we're not remotely
close to achieving it -- some warming is inevitable. Because of the enormous
carbon "debt" literally hanging over our heads, temperatures
will keep rising over the next century even if all emissions stopped today.
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- In the near term, then, humans must adapt to whatever
climate change brings. Quentin Chiotti, a senior scientist with Pollution
Probe who studies adaptation issues, said that although Canadians are in
many ways well adapted to a changing climate, West Nile and SARS have shown
that there are gaps in the adaptive capacity of the health-care system.
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- It's necessary to improve surveillance and monitoring
systems to catch new diseases early and to enhance the ability to respond
to sudden outbreaks. Health-care workers also need to be better informed
about the relationship between climate and infectious diseases because
many are "not making the connection," Dr. Chiotti said.
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- Dr. Charron said the fact that Health Canada has her
studying the issue is an indication of increased awareness. The challenge
for health-care workers, she said, is knowing what to look for and how
to react when they see it. "SARS shows us that it's not that easy
to notice something new, especially if what you're seeing looks like the
flu."
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- Because front-line health workers are so busy, the research
community must support them by helping to identify things that "might
be more likely than others," she said. "We don't want to ask
physicians to look for everything all the time."
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- One thing doctors should be thinking about when they
see patients is the weather. If a patient has diarrhea and it's been raining
heavily, a doctor should "think about a water source," Dr. Charron
said.
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- "Certainly, Walkerton has triggered that thought
process in pretty much everyone now. It's impossible to say with any certainty
what the future will bring in terms of changing climate and changing diseases,
but I'd like to take the lessons of SARS, West Nile and Walkerton to heart.
We need to put systems in place so that we can deal with these [diseases],
see them coming, catch them early and react appropriately."
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- - Lydia Dotto is a freelance science writer and the author
of Storm Warning: Gambling with the Climate of Our Planet.
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