Transcript [12] Human Impacts: Disease

Impacts: Disease
Dr Mark Cresswell
69EG6517 – Impacts & Models of Climate Change
Topics
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Scale of the problem
Some terminology
Malaria
Respiratory diseases
• Impacts
• Future impacts
• Summary
Scale of the problem
• Many countries are vulnerable to diseases
directly influenced by the weather
• Vector-borne diseases (like malaria)
• Respiratory illnesses (like meningitis)
• Water-borne diseases (like cholera)
• Stress illnesses (heat-stroke or
hypothermia)
• Illnesses caused by “mechanical” effects of
extreme weather events
Some Terminology
• An epidemic occurs when the number of
cases exceeds the background average by
a significant number (>2SD)
• An outbreak is a localised number of cases
• En epidemic wave describes an epidemic
that follows a geographical spread as the
disease is transferred from one person to
the next
• Many epidemic follow a “saw tooth” pattern
over a number of years
Some Terminology
• Mortality refers to deaths
• Morbidity refers to cases of disease who
subsequently recover – but nevertheless
present the symptoms of a disease
• A DALY is a Disability Adjusted Life Year
and can be used to measure the economic
impact of disease on the human population
and for countries as a whole
Vector-Borne Diseases
• Malaria is an example of a well known
tropical vector-borne disease
• Any agent of transmission of a bacterium,
virus or pathogen is a vector
• Diseases transmitted by insects are known
as vector-borne
• Malaria is caused by a parasite of the genus
Plasmodium
• The parasite enters the human host after
biting from a female Anopheline mosquito
Stable transmission
Unstable transmission
Respiratory Diseases
• Just as vector-borne diseases such as
malaria are highly dependent on the
weather, so too are respiratory illnesses
such pneumonia and meningitis
• Dry air may desiccate the upper respiratory
tract (nasopharyngeal tract or NPT)
• Since air entering lungs must have 43 gm-3
of absolute humidity and shortfall must be
supplied by the NPT
Respiratory Diseases
• The drier the ambient air, the more moisture
the NPT must supply to tidal air entering the
lungs
• As a general rule, air drier than 10 gm-3
absolute humidity may desiccate the NPT
• If the NPT dries, bacteria are able to
penetrate deep lung tissue – triggering the
disease
• The spread of a disease is enhanced by dry
and dusty conditions (Harmattan)
Respiratory Diseases
• Many respiratory illnesses such as meningitis and
pneumonia are highly dependent on the weather
• Changes in climate may act as a trigger or provide more
favourable conditions for the disease to flourish
ABOVE: CSM bacteria (stained)
LEFT: From Cheesbrough et al. 1995
Using GIS
Most layers of biologically
relevant environmental
information are combined
within a Geographical
Information System (GIS)
Humidity is currently a
“missing” layer
Malaria Model
simplified schematic of Liverpool model
death
Maturing
larvae
Uninfected
death
death
Infected
Infectious
(Sporogonic
cycle)
Infection
Uninfected
Mosquito
Infection
Infected
Infectious
Human
•Underlying model is similar to that described by Aron and May (1982)
•Model assumes no immunity, no superinfection
Malaria Model
prevalence and ERA rainfall
Model Output: West Africa
interannual variability
Rainfall
Prevalence
Impacts
• The following changes to our climate will
make the prevalence of diseases such as
malaria, meningitis and cholera more acute:
•Enhanced precipitation in wet season
•Warmer temperatures in upland areas as temperatures rise
•Drier air – with very low absolute humidities in dry season
•Changes in vegetation patterns
•Floods in lowland areas
•Migration of refugees as a result of extreme weather
Based on IPCC projections
Impacts
ENSO
• ENSO events are steadily increasing in frequency
El Niño and La Niña Events from 1900 to 1999
2000
1990
1980
Year of Event
1970
1960
1950
1940
1930
El Nino Years
La Nina Years
1920
1910
1900
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Number of events
Cresswell et al, 1999
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ENSO
• ENSO provides us with a means of identifying the
impact of climate change on insect populations
• A study was undertaken in the Wajir district of
Kenya in early 1998. Under normal weather
conditions this region is too dry for the vectors
and very little transmission occurs. There had not
been a malaria epidemic since 1952 and the local
health sector was unprepared for the major
outbreak that followed the heavy rains as a result
of ENSO
Cresswell and Kovats, 1999
Role of Climate?
Future Impacts
• In the 2080s it is estimated that some 290
million additional people worldwide will be
exposed to malaria – due to climate change
(McMichael et al, 2003)
• In the absence of climate change, in the
2080s about 9 billion people (~80% of world
pop) would live in areas potentially able to
support malaria – climate change represents
only a 1 to 3% increase in population at risk
Future Impacts
• Increases in summer time mortality due to
enhanced heat stress…and decreases in
winter time mortality due to milder winters in
urban population have been modelled in
accordance with projections of global
warming
• Ingress of brackish water to coastal zones
as sea-level rises will enhance population
numbers of mosquito vectors (IPCC)