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Climate Change:
Changing patterns of Infectious
Diseases in Humans?
Dr Dilys Morgan
Health Protection Agency Centre for
Infections
Climate Change does not
just mean global warming
Stern review
“Climate change threatens basic elements of life for
people around the world – access to water, food
production, health, and use of land and the
environment”
Impacts of climate change are not evenly distributed –
the poorest countries will suffer earliest and most
Climate change may initially have small positive effects
for a few developed countries but….”
Health risks
• Hazards are diverse global and probably irreversible
over human time scales
• Health impacts are potentially huge
• Impact will be inequitable
• Impacts may be avoidable
Water
0ºC
1ºC
2ºC
3ºC
4ºC
5ºC
Significant changes in water availability/
one study projects more than one billion
people suffer water shortages in the
2080s, many in Africa, while a similar
number gain water
Sea level rise threatens
major world cities,
including London,
Shanghai, New York,
Tokyo and Hong Kong
Small mountain glaciers
disappear worldwide –
potential threat to water
supplies in several years
Greater than 30% decrease in
runoff in Mediterranean and
Southern Africa
Food
0ºC
1ºC
2ºC
4ºC
3ºC
5ºC
Falling crop yields in many developing regions
Severe impacts in
marginal Sahel
region
Rising number of people at risk
from hunger (25 – 60% increase in
the 2080s in one study with weak
carbon fertilisation), with half of
the increase in Africa and West
Asia
Rising crop yields in high-latitude
developed countries if strong
carbon fertilisation
Entire regions
experience major
declines in crop yields
(e.g. up to one third in
Africa
Yields in many developed
regions decline even if
strong carbon fertilisation
Extreme Weather Events
0ºC
1ºC
2ºC
3ºC
4ºC
5ºC
Rising intensity of storms, forest fires, droughts, flooding and heat
waves
Small increases in hurricane
intensity lead to a doubling
of damage costs in the US
Floods
• Contamination of water supplies with sewage
• Standing water => mosquito breeding areas
• Failure of water treatment facilities
• Extreme heat 2003 >44,000 deaths in Europe
• Some short term mortality displacement deaths
• Milder winters could => reduced deaths
ECOSYSTEMS
0ºC
1ºC
Coral reef ecosystems
extensively and
eventually irreversibly
damaged
2ºC
3ºC
4ºC
Possible onset of
collapse of part or all of
Amazonian rainforest
Large fraction of ecosystems unable to maintain current form
Many species face
extinction (20 – 50% in one
study)
5ºC
Risk of rapid climate change and major irreversible
impacts
0ºC
1ºC
2ºC
3ºC
4ºC
5ºC
Risk of weakening of natural carbon absorption and possible
increasing natural methane releases and weakening of the Atlantic
THC*
Onset of irreversible melting of
the Greenland ice sheet
Inreasing risk of abrupt, large-scale
shifts in the climate system (e.g.
collapse of the Atlantic THC and the
West Antartic Ice Sheet
*Thermohaline circulation
Indication of uncertainty of what may happen…..
Predicting what may happen…
Global Ocean circulation system
North Atlantic Conveyor
Climate change and health
WHO
Huge world-wide consequences for human health –
hazards are diverse global and
But effects on human infections?
Problems of estimating effect of climate
change on Infectious diseases
Transmission of infectious disease determined by many
factors:
Extrinsic - social, climatic, ecological
Intrinsic - human immunity
Route of transmission etc etc
Temperature, precipitation, humidity -
Each affects pathogens and vectors directly and indirectly
through ecological change and can affect timing and
intensity of outbreaks
Estimating effects of climate change on
infectious diseases
But incidence disease also affected by factors such as
sanitation, public health services, population density and
demographics, land use changes and travel patterns
Models developed to stimulate the effects of climate change
on malaria, dengue and cholera but not necessarily
prediction tools because of physical and biological feedback
and human adaptation
Theoretical vs actual….. behavioural adaptations could
mitigate many adverse impacts eg vaccines, drugs, public
health programmes
Prediction not an exact science and uncertainty could
underestimate as much as overestimate
Difficult to estimate overall risk in specific country or Region
Direct vs indirect effect of climate change e.g. migration
The evidence…
Long list of infections reported to be associated
(or have potential to) with climate change
Vector-borne diseases
Malaria, yellow fever
Dengue, filariasis, Ross River fever
Lyme Disease, Tick borne encephalitis
Leishmaniasis
Chagas Disease
Onchocerciasis
Trachoma
Schistosomiasis
Rodent – borne infections
• Plague
• Hantavirus
• Leptospirosis
Food borne/waterborne infections
• Diarrhoeal diseases
• Salmonella
• Campylobacter
• Cholera
Interpreting reports
Long-term trends difficult to interpret
Inconsistency of datasets
World is changing with lots of other developments
Human behaviour – time and culture dependent
Trend to report emerging infections as related to climate
change but…sars; ebola/marburg; chikungunya??
Confounders
Ecological changes:
Economic development and land use; Agriculture; dams,
changes in water ecosystems; deforestation & reforestation;
flood/drought; famine; climate change
Human demographics and behaviour:
Societal change; population growth and migration (rural to
urban); war/conflict; urban decay; sexual behaviour; IV drug
use; overcrowding.
Confounders 2
Technology and industry:
Globalisation of food supplies; changes in food processing
and packaging; organ/tissue transplantation; immunosupp
drugs; widespread use of antibiotics.
Microbial adaptation and change:
Microbial evolution; response to selection in environment
Breakdown in public health measures:
Curtailment or reduction in preventative or
vector control programmes; inadequate sanitation
Confounders 3
International travel and commerce:
Worldwide movement of goods and people, rapid air travel.
Role of climate change among other drivers of global
change?
Many infections do have seasonal variation e.g.
meningitis in sub-Saharan Africa hot dry season
Evidence [publications] about relationship between
natural variation in meteorological variables (especially
temperature) extensive
Estimation via modelling and extrapolation less
extensive and also encompasses several controversies,
including that of adaptation
WHO tried to address this
“Comparative Quantification of Health Risks:
Global and Regional Burden of Disease Attribution to
Selected Major Risk Factors”
Looks at mortality and burden of disease (DALYs) attributable to CC in
2000 and modeling up to 2030 based on alternative climate scenarios
Vector-borne Diseases
Vector
Major diseases
Mosquito
malaria, filariasis, dengue fever, yellow fever
Ticks
Lyme disease, tick-borne encephalitis
Sandflies
Leishmanisis
Triotomids
Chagas disease [Trypanosomiasis cruzii]
Tsetse African trypanosomisis
VBD
Complex
Dependent on many features
Complex life cycles of vectors and parasites and contact
with humans
Balance between vectors surviving long enough for a
parasite to complete life cycle
Human factors
• Need exposure - No. of bites
• Behaviour
• Immunity of population (genetic, acquired, drugs, vaccination)
• Herd immunity -> endemic stability
• Population density
Mosquito-borne infections
Predicted changes of warm climatic zones, less extreme low
temps and more frequent extreme high temps
Milder winters-early springs may lengthen survival, and
higher temperatures may speed up parasite development
Faster vector and pathogen development, with more
generations per year +
Reduced mortality of vector at low temperatures
Range expansion of warm-climate vectors and pathogens in
previously temperate areas
Shorter life span of vector at high temperatures
Climate change and VBD
Rainfall patterns altered patterns of mosquito breeding;
more flushing of breeding sites with flooding or standing
water for breeding sites
Would make temperate environments more receptive
to tropical VBD while having less negative effects on
tropical environments
Malaria
Studies have reported a relationship between malaria
transmission and climate change in highland areas of Kenya
No link when examining longer term trends
Fraught with confounders and contribution of other factors drug resistance, immune status, population movement, land
use etc
Malaria
Large changes of risk in countries at the edge of current
endemic areas. However, most of disease burden will still
be associated with small proportionate change in highly
endemic areas (particularly Africa)
Dengue fever
Often urban events with man-made waste acting as water
containers for breeding of Stegomyia (Aedes) aegypti
Annual average vapour pressure was most important
individual predictor of DF distribution, and geographic limits
of DG transmission are strongly determined by climate
Seems to have smaller climatic influence on disease than
for other arboviruses (estimated 1/3 of world’s population at
risk) but higher temps accelerate transmission of dengue
even in absence of rain.
But also need source of infection, vectors and susceptible
population for epidemic to occur
Modelling indicates climate change will increase
latitudinal range of dengue and extend the duration
of the transmission season in temperature countries
Other VBD
Leishmaniasis: visceral and cutaneous
Transmitted by phlebotomus, modelling indicates climate
change could extend range
Chagas Disease [Trypanosomiasis cruzi]:
Transmitted by triotomine bug in rural areas South America,
modelling indicates climate change would reduce risk
Tickborne encephalitis in Europe
TBE – studies describing link between TBE and climate
change 1960-1998, increase in cases due to milder winter
and early spring, but confounders ++
Indicators that fragile transmission cycle of TBE may
actually be disrupted by climate change
Lyme borreliosis: England and Wales, annual
totals, 1997 to 2006
Increase in UK due to:
Increased awareness of the disease
Greater access to diagnostic facilities
More sensitive diagnostic methods
Introduction of the enhanced surveillance scheme in late 1996, and
more complete reporting of cases since 2000
Increased deer population
Increasing popularity of activity holidays such as walking, trekking, and
mountain biking
The Countryside and Rights of Way Act 2000
Changes in population sizes and geographical ranges of I. ricinus both
in the UK and throughout Europe as a result of successive mild winters,
warmer summers and early springs
Why is so difficult to estimate risks from VBD?
Knowledge re vector distribution and abundance lacking even
in industrialised countries such as UK. Comprehensive
studies are expensive and logistically difficult.
Climate-driven change difficult to assess:
• Few long-term studies
• Geographic limitation
• Standardisation of collection methods lacking
• Need to be frequent surveys to attribute variations to longer
term effects of CC and not just inter-annual climate variability
• Vector mapping usually where disease is most prevalent
• Reporting bias
So is it “absence of evidence of effects of climate
change on infectious diseases” or “evidence of
absence of effects of climate change” …..??
Food/waterborne infections
In industrialised countries positive association between
salmonella notifications and mean monthly temperature
(more influential earlier in production process rather than at
food preparation stage or consumption)
Campylobacter and listeria commonest in summer months
but temperature association less clear cut
Seasonal variation of diarrhoeal disease in less developed
countries - but low rainfall vs high rainfall?
Studies showing relationship between short-term
phenomenon such as El niňo and cholera outbreaks in
Bangladesh
Food/Water-borne infections UK
Food: Warmer summers are likely to be associated with an
increase in food-borne diseases
Water:
 increased rainfall (over short periods) leading to
increased numbers of bacteria in surface water,
 increased water temperature leading in an increase in
algal blooms in reservoirs and a
 decrease in the efficiency of chemical coagulation: a
major method of removal of microbes from drinking water.
Food/waterborne infections
Relative importance depends of modes of transmission
Heavily influenced by level of sanitation
Effects of climate change on diarrhoeal diseases in Less
Developed Counties harder to predict, but small increase in
RR has huge impact (WHO RR 1.08-1.09 in 2030)
Problems with extrapolating from El
niňo or other short-term events
Longer term risks population learns to adapt – Public Health
policy, interventions, education, behavioural change,
vaccines, drugs
Vulnerability = impact X (1-adaptive capacity)
Impact exposure= Exposure X sensitivity
Adaptive capacity = adaptation products and practices
X resources
Less Developed Countries
Disportionate impact of effects of climate change
More endemic diseases + less ability to adapt and respond
Small increase in risk large impact
Less Developed Countries
Reduction in water supplies
Already have 1.1 billion without access to adequate supplies
of safe water
And 2.4 billion do not have access to sanitation
Improved water supply and sanitation has been shown to
reduce morbidity of diarrhoea; ascaris infections; guinea
worm; trachoma; diarrhoea-specific mortality, and general
child mortality
Climate Change 2007 – Impacts,
Adaptation and Vulnerability (IPPC)
Climate change will
• continue to change the range of some infectious disease
vectors (high confidence)
• have mixed effects on malaria; in some places the
geographical range will contract, elsewhere the
geographical range will expand and the transmission
season may be changed (very high confidence)
• increase the burden of diarrhoeal diseases (medium
confidence)
WHO Comparative Quantification of Health Risks
“Climate change will bring some health benefits such as
lower cold-related mortality and greater crop yields in
temperate zones, but these will be greatly outweighed by
increased rates of other diseases particularly ID and
malnutrition in developing regions”
Increased RR of diarrhoea in developing countries
Richer countries no or little additional risk of diarrhoea
Large changes in RR of P falciparum malaria in countries at
the edge of the current distribution
Emphasise effects of CC are predicted to be heavily
concentrated in poorer populations in LDC and will have
most impact on under 5 year olds
Health Effects of Climate Change in the
UK (Department of Health)
Vector-borne diseases:
outbreaks of malaria in the UK are likely to remain rare
possibility of outbreaks of malaria in other European countries
possibility that more effective vectors may arrive in the UK.
Tick-borne disease are likely to become more common in the
UK, but more likely to be due to changes in land use and leisure
activities than to climate change
The likelihood that Tick-borne encephalitis will become
established in the UK is very low
Warmer summers are likely to be associated with an increase in
foodborne diseases
To manage potential risk from ID as a result
from climate change
Need early warning systems
Improve preparedness
Surveillance and response complemented by prediction
and prevention
Improve forecasting capability and establish faster
analytical, observational, and computer developments
 Multidisciplinary approach– meteorologists, ecologists,
entomologist, epidemiologists, public health, social
scientists, communications, GIS etc
Conclusions
Climate change will have huge impact on the world
Specific effects of climate change on infectious diseases are
difficult to predict
CC is just one component of global change which can affect
a range of infectious diseases
Any contribution of CC to change in ID in industrialised
countries is likely to be modest
But would have significant and major impact on less
developed countries