Will a Warmer World Make Us Sicker?
Scientists are piecing together how climate impacts disease, strange patterns are emerging: mosquito outbreaks can follow drought, shorter migrations can make butterflies sick, and more birds (not fewer) can ward off West Nile virus.
In the late 1990s, a set of alarming maps created a stir in the
scientific community. Based on predictions by a team of Dutch and
Australian researchers and initially published in the journal
Environmental Health Perspectives, the maps charted how global warming
could increase the risk of malaria in seemingly unlikely locales:
northern countries such as Poland, the Netherlands, and Russia.
the next several years, versions of the maps continued to appear in
journals and at scientific meetings as researchers raised the
disquieting possibility that climate change
could trigger an expansion of disease. An article in Scientific
American reprinted one iteration of the maps and declared that "by the
end of the 21st century, ongoing warming will have enlarged the zone of
potential malaria transmission from an area containing 45 percent of
the world's population to an area containing about 60 percent."
Statements like these added to the popular perception that a warmer
world will automatically be a sicker one.
But what if this isn't
true, or is only partially true? Kevin Lafferty, an ecologist at the
U.S. Geological Survey, is among a handful of scientists now raising
these questions and rethinking conventional wisdom. Lafferty recently
published a controversial article in the journal Ecology suggesting
that, while climate change may shift the ranges of certain diseases, it
won't necessarily increase the total amount of territory they affect.
(1) And Sarah Randolph, a parasite ecologist at the University of
Oxford, has reviewed recent disease outbreaks-some of which have been
attributed to global warming-and concluded that human actions and other
factors may have played a larger role than climate.
and Randolph's opinions have stirred intense debate. While there are
credible arguments on both sides, the overriding point is that some
scientists are beginning to see the ecology of disease as far too
complicated to support simple declarations about the impact of global
warming. It turns out that disease ecology is made up of a multitude of
moving parts, ranging from precipitation patterns to animal migrations,
that constantly shift and adjust in relation to each other. And when
climate changes, the end result may be an increase in disease-or not.
Nature vs. Nurture
When tick-borne encephalitis spread throughout the Baltics, was the culprit climate change or the fall of the Soviet Union?
Randolph has spent more than 30 years studying vector-borne diseases
(diseases transmitted to hosts by insects and other animals). She's
also a maverick who has devoted the latest chapter in her career to
digging beneath what she calls "seductive mindsets." As she wrote in a
response to Lafferty's Ecology article, one of these mindsets is that
recent disease outbreaks are caused by climate change, "adding fuel to
the fire of predicted impending doom." (2)
encephalitis (TBE), a nasty viral disease that can cause inflammation
of the brain. Beginning in the 1980s and 1990s, a rise in temperature
appeared to correspond with a TBE surge in several European countries,
where thousands of people were stricken with headaches, fever, and
other unpleasant symptoms. In Sweden, some scientists suggested that
warming had triggered the rise in TBE cases and that future climate
change would exacerbate the scourge.
Randolph decided to conduct
her own investigation. In a 2007 study, her team examined county-level
TBE trends in the Baltic countries of Estonia, Latvia, and Lithuania;
they found patterns that couldn't be explained solely by climate. While
temperatures rose in 1989 across the Baltics, TBE cases in individual
counties began spiking anytime between 1990 and 1998.
investigate further, Randolph's team studied the region's social and
economic history. After the Baltics broke from Soviet rule in the early
1990s, unemployment rates-and poverty-surged. Poorer people were less
likely to be vaccinated, the researchers found, and more likely to
forage for food in tick-filled forests. This suggested to Randolph
that, contrary to popular assumptions, the disease surge probably had
far more to do with human actions than planetary changes.
case isn't unique. "In the last two decades," Randolph argues, "there's
practically no examples where a vector-borne disease can be pinned on
Of course, Randolph is only one player in a
contentious debate, and other scientists say they have found links
between climate and diseases such as malaria, dengue, and cholera. Just
because disease is influenced by a myriad of factors doesn't mean we
should ignore climate, warns Richard Ostfeld of the Cary Institute of
Ecosystem Studies. "To me, that's kind of like saying because we know
that obesity is also a risk factor for heart disease, we don't need to
worry about smoking," he says.
Dry and High
Why did mosquito populations surge after drought dried up their habitat?
researchers piece together the disease puzzle, some of the most
complicated variables revolve around the mosquito. Notorious for
transmitting malaria, West Nile virus, and other pathogens, mosquitos
are expected to develop faster at higher temperatures, raising concerns
that global warming could spur disease outbreaks. But as researchers
like Jonathan Chase unravel how mosquitos respond to key climate
conditions, they're reaching surprising new conclusions.
ecologist at Washington University in St. Louis, Chase didn't set out
to discover anything about mosquitos. He wanted to know how droughts
affected biodiversity, so his team built artificial wetlands by filling
outdoor tanks with dirt and water. To simulate different wetland drying
patterns, they left some tanks full year-round, while others were
drained annually. A third type of tank was drained only once in three
years, mimicking wetlands that generally retain water but go dry during
You might think periodic droughts would diminish
mosquito populations by drying up habitat. But the number of mosquitos
in the third group of tanks skyrocketed to more than 20 times the
amount in the other tanks.
Chase thought back to figure out what
might have happened. He knew that, since those tanks usually held
water, they housed mosquito predators and competitors that were poorly
suited to dry conditions. When the "drought" killed many of them off,
mosquitos likely seized the opportunity to multiply. With their fast
breeding times, Chase reasoned, mosquitos could quickly recolonize the
area before their predators rebounded, resulting in the population boom.
see whether the idea held up in nature, Chase turned to data from a
survey of Pennsylvania wetlands. Sure enough, ponds that dried out only
during a drought showed a spike in mosquito larvae the following year.
The results confirmed what Chase had suspected-the drought had opened a
"giant habitat for a small window of time," he says, allowing mosquitos
Chase's team took the research a step further to see
how this might affect the spread of disease. They analyzed data on
human West Nile virus cases in the United States between 2002 and 2004,
comparing the trends to changes in precipitation. In the western United
States, the number of cases increased after dry years, as expected. But
in the eastern part of the country, the pattern was the opposite:
outbreaks happened after rainy years.
The variation might arise
from a difference in mosquito species, says Chase. Mosquitos that
spread the virus in the western United States tend to dwell in wetlands
and thus would benefit if a drought wiped out fellow inhabitants. But
mosquito species on the other side of the country prefer to breed in
puddles and water-filled containers, so they could take advantage of
Chase's mosquito findings illustrate how much
scientists still have to learn before they can accurately forecast the
effects of climate change on disease. "You have these simple notions
that one factor will work in one way," says Andrew Read, an
infectious-disease researcher at Pennsylvania State University who was
not involved in Chase's work. "But in the context of community ecology
and food chains, anything can happen."
Do long migrations keep butterflies healthy?
has long been feared that climate change will enable disease to run
rampant through animal populations. A warming world could alter the
borders of suitable habitats, leading migratory species to new
territory and exposing them to diseases they haven't encountered
before. But scientists are only beginning to get their arms around the
mechanisms that might allow disease to weaken some populations while
others emerge unscathed.
Karen Oberhauser, an ecologist at the
University of Minnesota, has been pursuing answers to questions about
how climate change might affect the monarch butterfly. Today, eastern
North American monarchs can log up to 5,200 kilometers on their annual
trips to forests in Mexico. But in a 2003 study, Oberhauser found
that global warming might make these forests too wet for monarchs.
Instead of flying to Mexico, she says, the butterflies might take a
shorter migration route to the Gulf Coast of the United States.
the face of it, shorter migration flights might not seem alarming. But
Sonia Altizer, a former student of Oberhauser's who is now at the
University of Georgia, has been finding surprisingly strong links
between the length of monarch migrations and the prevalence of disease. Altizer
examined nearly 15,000 monarchs to determine which were infected with
the parasite Ophryocystis elektroscirrha, which can cause wing
deformities and shorten life spans. Among monarchs that travel long
routes to Mexico, less than 8 percent of the butterflies were heavily
infected. But for western North American monarchs that take shorter
flights to California, the numbers went up to about one-third. And in a
Florida population that didn't migrate at all, more than 70 percent
Altizer's team speculated that migration might
weed out infected butterflies, which wouldn't survive strenuous trips.
To investigate, they attached monarchs to the end of a butterfly
"treadmill"-a horizontal rod that could spin around a pivot-and let
them fly in circles. The researchers found that infected butterflies
stopped an average of 14 percent sooner and traveled 10 percent slower
than uninfected butterflies. If monarchs start wintering in Texas
instead of Mexico, the population might accumulate more diseased
butterflies, says Altizer.
Shorter or stalled migrations might
pose a threat to other migratory species as well. For instance,
reindeer and fall armyworm moths may also shake off parasites through
seasonal migrations, either by ridding themselves of sick individuals
or leaving contaminated sites. Climate change could even create
year-round habitat that encourages migratory species to stay put,
Altizer says, strengthening the foothold of infectious diseases.
Parasites Lost and Found
Why do warmer Arctic summers give musk oxen nosebleeds?
climate and animal movements are changing, so are the organisms that
play a key role in disease ecology: parasites. Often carried by insects
or other animals to their hosts, parasites are the infectious agents
behind many human and wildlife diseases. And as climate change begins
to alter the life cycles and biodiversity of these organisms,
scientists say, it could have a powerful impact on disease patterns.
Kutz, a wildlife parasitologist at the University of Calgary, began
studying one Arctic parasitic worm in 1994. The worm penetrates the
feet of slugs, using them as a growth chamber until the slugs are eaten
by musk oxen. The worms then take up residence inside the musk oxen's
lungs, causing nosebleeds, weakening the animals, and making them
vulnerable to predators such as grizzly bears.
To investigate how
climate change might affect the parasite's life cycle, Kutz spent two
summers on the Arctic tundra, tracking the worm's growth. She
calculated that a couple of decades ago, the tundra would have been too
cold for the worm to develop in one summer. But around 1990, rising
temperatures probably allowed the parasite to grow faster, shortening
its development time from two years to one.
This finding suggests
that small changes in temperature can trigger large jumps in parasite
life cycles, says Kutz. Since the faster growth rate would have allowed
more worms to survive to maturity and infect the musk oxen, she
speculates, it might explain why musk oxen numbers declined in a 1994
survey-although there aren't enough data to say for sure.
parasite equation is complicated by the fact that, in addition to
allowing some parasites to develop faster, climate change could drive
others to extinction. Those that can't handle warmer conditions might
try to find new hosts to the north or let existing hosts carry them to
cooler regions, Kutz suggests. Once they reach new habitat, they will
face competition from other parasite species. If they can't win the
struggle for animal hosts, she says, they may run out of places to go.
could also be in trouble if their hosts become endangered or extinct,
the USGS's Kevin Lafferty says. Biodiversity is expected to decline
with climate change, and the disappearance of one animal species could
threaten multiple parasite species. "Listen, you don't want to be a
parasite of a polar bear or a penguin," he says.
As outlined in a
recent article in Proceedings of the Royal Society B, one possibility
is that the disappearance of certain parasites could simply allow
remaining parasites to spread. And parasites that lose mammal hosts to
extinction might just switch to a different host species-possibly
But Lafferty cautions that it's far too early to leap
into crisis mode. Instead of adding to the slew of doom-and-gloom
climate predictions, he believes it's first necessary to withhold
judgment and construct a more-complete portrait of disease ecology.
It's a daunting task, but it's also within reach. Ecologists already
have the tools to study intricate systems, Lafferty suggests, and that
could allow them to disentangle the contributions of various factors,
including climate, to disease. Until this happens, predictions of
climate-driven disease spread are likely to be insufficient and
incomplete. "The outcome is important enough," Lafferty says, "that we
should get it right."
Strength in Numbers
Can biodiversity thwart the spread of disease?
scientists are starting to believe biodiversity could act as a powerful
repellent to infectious disease. "Biodiversity gives insects a choice
of what to bite," says Andy Dobson, an infectious disease ecologist at
Princeton University. In other words: If there's a large number of
species to choose from, a disease-carrying bug could miss its target
and bite a species that isn't susceptible.
Some recent studies
have affirmed this. A team led by Brian Allan at Washington University
in St. Louis found that West Nile virus infection was more common in
areas with low bird diversity, areas which also tend to harbor the
species most likely to transmit the virus. In another study,
researchers removed small mammals from plots in Panama and observed
higher hantavirus infection rates among remaining host species. But
the disease buffer could vary depending on which species are lost or
gained. In some low-diversity communities, animals that transmit a
particular disease may have already dropped out, says Peter Hudson, an
ecologist at Pennsylvania State University. Alternatively, the presence
of certain species could help spread the disease.
If some species
are a particularly good disease buffer, it could be tempting to try to
add more of them to ecosystems. But that's probably infeasible, says
Richard Ostfeld of the Cary Institute of Ecosystem Studies. Opossums
are known to reduce Lyme disease risk, he says, "but are we going to go
out and air-drop opossums into suburban neighborhoods? I don't think
1. Lafferty, K.D. 2009. The ecology of climate change and infectious diseases. Ecology 90(4):888-900.
2. Randolph, S.E. 2009. Perspectives on climate change impacts on infectious diseases. Ecology 90(4):927-931.
Dunn, R.R. et al. 2009. The sixth mass coextinction: Are most
endangered species parasites and mutualists? Proceedings of the Royal
Society B DOI:10.1098/rspb.2009.0413.
Further Reading: Pascual, M. and M.J. Bouma. 2009. Do rising temperatures matter? Ecology 90(4):906-912.
Ostfeld, R.S. 2009. Climate change and the distribution and intensity of infectious diseases. Ecology 90(4):903-905.