Climate change futures: health, ecological and economic dimensions

CLIMATE CHANGE
AND EMERGING
INFECTIOUS
DISEASES
Figure 2.1
BEFORE 1970
Cold temperatures
caused freezing at high
elevations and limited
mosquitoes, mosquitoborne
diseases and
many plants to low
altitudes
MONTANE REGIONS
TODAY
Increased warmth has
caused mountain glaciers
to shrink in the tropics and
temperate zones
Microbes and other living organisms tend to increase
their numbers exponentially; their population levels
reflecting environmental conditions and resource constraints.
Historically, periods of social and environmental
transition have been accompanied by waves of
epidemics spanning multiple continents (Epstein 1992).
Changes in the timing of seasons and greater weather
variability can destabilize natural biological controls
over opportunistic organisms.
Ecological instabilities play key roles in the emergence
of diseases and the resurgence of old scourges (IOM
1992). Predators are needed to control prey and prevent
them from becoming pests (Epstein et al. 1997),
while competitors that are poor carriers of pathogens
may “dilute” them and decrease transmission
(LoGiudice et al. 2003). Species extinctions may play
an insidious role because of the loss of functions they
perform in controlling the proliferation of opportunistic
organisms. Background ecological changes today are
profound: we are using Earth’s resources and generating
wastes at a rapid pace (Wackernagel et al.
2002) and the Millennium Ecosystem Assessment
(2005) found that 60% of ecosystem resources and
services are being used unsustainably (Mooney et al.
2005). Today 12% of birds, 23% of mammals, 25%
of conifers and 32% of amphibians are threatened
with extinction and the world’s fish stocks have been
reduced by 90% since the 1850s. Following mass
extinctions, opportunistic species can flourish until new
communities of organisms are established.
The resurgence of old diseases, such as malaria and
cholera, the redistribution of still others (like West Nile
virus) and the emergence of new diseases are of considerable
concern. Since the late 1990s, the pace of
new and resurgent infections appearing in humans,
animals and plants has continued unabated (Epstein et
al. 2003). Persistent poverty and deteriorating public
health programs underlie the rebound of most diseases
transmitted “person-to-person” (for example, diphtheria
and tuberculosis).
DENGUE FEVER
OR MALARIA
MOSQUITOES
Some mosquitoes,
mosquito-borne
diseases and plants
have migrated upward
PLANTS
In highland regions in Africa, Central and South America, and Asia
plant communities are migrating upward, glaciers are retreating and
mosquitoes are being found at high elevations. Underlying these
changes are higher temperatures (an upward shift of the freezing
isotherm) and thawing of permafrost (permanently frozen ground).
Image: Bryan Christie/Scientific American August 2000
From 1976 to 1996, the World Health Organization
(1996) reports the emergence of over 30 diseases
“new” to medicine, including HIV/AIDS, Ebola, Lyme
disease, Legionnaires’, toxic E. coli and a new hantavirus;
along with a rash of rapidly evolving antibioticresistant
organisms. But the resurgence and redistribution
of infections involving animal vectors, hosts and
reservoirs — mosquitoes, ticks, deer, birds and rodents
— reflect changing ecological balances and an
altered climate (Epstein 2005).
Range Changes: Focus
on Highland Regions
The geographic distribution and activity of insects are
exquisitely sensitive to temperature changes. Today,
insects and insect-borne diseases are being reported
at high elevations in East and Central Africa, Latin
America and Asia. Malaria is circulating in highland
urban centers, such as Nairobi, and rural highland
regions, like those of Papua New Guinea. Aedes
aegypti, the mosquito carrier of dengue and yellow
fever, has been limited by temperature to about
1,000m (3,300 ft) in elevation. In the past three
decades it has been found at 1,700m (5,610 ft)
elevation in Mexico and 2,200m (7,260 ft) in the
Colombian Andes (Epstein et al. 1998).
33 | INFECTIOUS AND RESPIRATORY DISEASES
CASE STUDIES