GLOBAL CLIMATE CHANGE
AND MOSQUITO-BORNE DISEASES
Theodore G. Andreadis
Department of Entomology and Center for Vector
Biology and Zoonotic Diseases
The
ABSTRACT
World climate is in a warming phase that began in the
early decades of 18th century and is occurring faster than any period in last
thousand years. Sea levels have risen
approximately two mm per year, arctic sea ice has declined by 7.4% per decade,
snow cover and glaciers have diminished in both hemispheres, and worldwide
hydrological cycles are accelerating resulting in an increased intensity,
frequency and duration of droughts, heavy precipitation events and flooding. Observed climatic changes in North America
alone have included fewer cold days and nights, more frequent heat waves and
warm spells, more intense precipitation events, total rainfall and hurricanes,
and increases in areas affected by drought.
Predictions from United Nations Intergovernmental Panel on Climate
Change (IPCC) indicate that in next 90 years global temperature will increase
between 1.8 oC and 4.0 oC
and sea levels will rise by as much as two feet.
Mosquitoes, in their role as vectors, are critical
components in the transmission cycle of many disease causing pathogens that
affect hundreds of millions of people world-wide including Malaria, Dengue,
Yellow Fever, Japanese Encephalitis, Chikungunya, Rift Valley Fever,
Climate change may affect the incidence of
mosquito-borne diseases through its effect on four principal characteristics of
vector mosquito populations that relate to pathogen transmission.
1. Geographic
and Temporal Distribution: Range shifts
in vector distribution that brings tropical mosquito vectors into contact with
new susceptible human populations.
Temporal change would include an extension of the transmission season
allowing mosquitoes to transfer pathogens for a longer period of time.
2. Population
Density: Changes in the population
density of the mosquito vector that result in an increased frequency of contact
with humans. This could arise from
increased overwintering survival due to warmer temperatures, a shortening of
larval development times, more frequent feeding by adults, quicker digestion of
blood meals, and increased adult survival at higher latitudes. Increased precipitation would increase the
number and quality of larval breeding sites and epic rainfall events could
synchronize mosquito host seeking and pathogen transmission. An associated increase in humidity would
further serve to increase mosquito survival.
Lower rainfall amounts and
draught conditions in other regions while generally decreasing the number and
quality of larval breeding sites, could equally serve to create larval habitat
by causing rivers to dry into “pools” that serve as production sites (eg. dry season malaria) and decreased rainfall would likely
increase container-breeding mosquitoes by forcing increased water storage.
3. Prevalence
of Infection by Zoonotic Pathogens:
Changes in the prevalence of pathogen infection in the reservoir host or
mosquito vector population that would increase the frequency of human contact
with infected mosquito vectors
4. Pathogen
Load: Changes in pathogen load brought
about by changes in the rates of pathogen reproduction, replication, and
development in the vector mosquito.
Increases in temperature would result in a decrease in the extrinsic
incubation period of pathogen in the mosquito vector (i.e. the period of time
from when a mosquito takes an infectious bloodmeal until it transmits the
pathogen). Pathogens inside the mosquito
mature faster in heat, increasing transmission efficiency and the likelihood of
the disease being spread.
Given this scenario, the greatest effects of climate
change on transmission of mosquito borne diseases are likely to be observed at
the temperature extremes of the range of temperatures at which transmission occurs,
and effects are likely to be expressed in the increased frequency of short-term
epidemics and long-term gradual changes in disease trends. However it is important to recognize that
climate change is only one of many factors affecting the incidence of
mosquito-borne diseases. Socioeconomic
factors and human activities that impact the local ecology are equally
important and may in fact have a greater impact. These include demographic changes (population
growth, migration, urbanization); societal changes (inadequate housing, water
deterioration, migration); changes in public health policy (decreased resources
for surveillance, prevention and vector control); insecticide and drug
resistance; deforestation and irrigation systems and dams.
As noted by Reiter (Environmental Health
Perspectives, 2001), “The natural history of mosquito-borne disease is complex
and the interplay of climate, ecology and vector biology defies simplistic
analysis.” Adaptations to climate change
and variability will largely depend on the level of health infrastructure in
the affected regions. We really don’t
know how projected climate change will affect the complex ecosystems required
to maintain these mosquito-borne diseases.
More research is needed to better understand the influence of weather
and climate on these pathogens in their natural transmission cycles. “Assessments that integrate
global climate scenario-based analyses with local demographic and environmental
factors will be needed to guide comprehensive, long-term preventive health
measures” (Gubler et al. Environmental Health
Perspectives, 2001).