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## An Overview of the Effects of Climate on Malaria Transmission

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### An Overview of the Effects of Climate on Malaria Transmission Barbara Wendelberger 27 April 2010 – PowerPoint PPT presentation

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Title: An Overview of the Effects of Climate on Malaria Transmission

1
An Overview of the Effects of Climate on Malaria
Transmission
Barbara Wendelberger 27 April 2010
2
Some Simplifications to MARA
• Anopheles gambiae s.l.
• Plasmodium falciparum.
• Independent analyses of rainfall and temperature

3
Why Climate Mappings Fail
• Lack of data
• Use of crude geographic and climate iso-lines
• No clear, reproducible numerical definitions
• Prevents ability to compare data

4
Improvements
• Large global data sets
• Up to 1.6 billion observations daily
• Climate data
• Population data
• Satellite imagery and topography
• Geographical Information Systems (GIS)
• Overlaying of varying levels of understanding
• Ex. Rainfall and temperature

5
Finding Stability Distributions
• MARA
• Finding the limits of the distribution of stable
malaria areas
• Based on temperature and rainfall data
• R0 (vectorial capacity)
• Main component strongly determined by climate
• Reproduction rate of malaria parasite and
mosquito vector

6
Modeling Problems
• Malaria is not definable
• in space because the edge of the distribution is
indistinct
• in time because both intensity and distribution
wax and wane with natural periodicity of events

7
Logic
• Boolean Logic
• Climate has only two states
• Suitable for transmission (1)
• Unsuitable for transmission (0)
• Fuzzy Logic
• An extension of Boolean logic
• Allows fractions
• Suitable (1)
• Semi-suitable (between 0 and 1)
• Unsuitable (0)

8
Transmission Areas
• Perennial always able to sustain transmission
• Seasonal suitable for a short season each year
• Epidemic long-term variation in climate renders
suitable conditions irregularly
• Malaria-free always unsuitable
• Long term monthly means exclude rare epidemic
zones

9
A fuzzy model that demonstrates the different
suitability zones
10
Temperature Effects
• Sporogonic duration (n)
• n DD _
• T Tmin
• DDdegree days for parasite development (111)
• Tmean temperature
• Tmintemperature at which parasite development
ceases (16 C)
• Mosquito survival (p)
• p e (-1/(-4.41.31T-0.03T2)
• Defined by Martens
• Assumes constant humidity

11
Temperature, p, and n
• pn percentage of vector cohort that survives
the required temperature time period
• ld larval density
• 1 ___
• (0.00554T 0.06737)

12
Temperature, p, and n
13
Rainfall
• Best studied when temperature is not limiting
• No direct, predictable relationship between
rainfall and Anopheles gambiae s.l.
• Anopheles gambiae s.l. breed more prolifically in
temporary, turbid water bodies, such as those
formed by rain
• Impacts
• Humidity
• Saturation deficit
• Temporary and permanent bodies of water

14
Sustainability
Temperature cut-off point between epidemic and
no-malaria zone 18ºC 22ºC allows stable
transmission The rainfall requirement for stable
transmission is 80mm/month for at least 5
months
15
Climate/Transmission Relevance
More limiting variable used.
16
Climate Change and Health Research (NIH Portfolio
Analysis-funded activities in 2008)
Number of studies in some way related to climate change 1,357
Number that directly relate to climate change 7
Number that examine how climate variables affect health 85
Climate is likely an important factor but is not explicitly addressed 706
17
NIH Studies
• Health
• Infectious diseases, respiratory diseases,
asthma, heat stress, exposure to environmental
toxins, trauma/injury, and cancers
• Exposure pathways
• Extreme weather, UV radiation, pollution,
water-borne, vector-borne, and zoonotic diseases
• Study Types
• Laboratory experiments, population studies, field
ecology, and mathematical modeling

18
Deaths
• The WHO
• 160,000 deaths due to climate change in 2000
• From malaria, malnutrition, diarrhea, flooding,
and heat waves
• BUT
• How does this compare to climate-related deaths
in other years?
• What is the error? Could this number be within
the range of the normal number expected?

19
NIH Initiatives
• The NIH is interested in studies that directly
examine climate impacts on human health.
• Research needs to bridge the gap between global
scale and micro studies.

20
Could Global Warming Increase Malaria Prevalence?
• Optimum constant temperatures for adults and
larvae
• 23ºC to 24ºC
• Development rates
• Increased development for both parasite and
vector with increased temperature
• Could increase it to the point of weakening the
progeny
• Density
• At 30ºC, when density increases, survival
increases
• At 27ºC, when density increases, survival
decreases

21
Current Predictions Based On
• Continuing change in global temperature
• The present distribution of malaria parasites and
their mosquito vectors

22
Warming Effects
• High Temperature
• Increase
• Frequency of blood-feeding
• Rate at which parasites are required
• Parasite incubation time
• Decrease

23
Thermodynamics
24
Negative Correlation Coefficients?
• Data
• Dar es Salaam (Tanzania)
• Dodowa (Ghana)
• -0.7 (mean max monthly temp/number of cases)

25
Could the Malaria Endemicity Center Move?
• Multiple factors suggest yes
• Intrinsic optimum temperature model
• Exhibits the effects on enzyme inactivation in
relation to development
• Co-evolution of vector and parasite (23ºC to
24ºC)
• Temperature and the sexual events of the malaria
parasite in the mosquito gut
• Relative transcription levels of rRNA involved
in sporogony
• The success of mosquito development from aquatic

26
The Bottom Line
• Climate is a complex variable
• Study individual components
• Understand how they interact and affect each
other
• If temperatures continue to increase, then the
center of malaria endemicity will likely move to
avoid temperatures that are too hot to encourage
stable development
• Tropics are not equivalent to hot environments

27
Research Sources
• Ahumada, J.A.,D. Lapointe, and M.D. Samuel. 2004.
Modeling the Population Dynamics of Culex
quinquefasciatus (Diptera Culicidae), along an
Elevational Gradient in Hawaii. J. Med. Entomol.
41 (6)1157-1170.
• Armstrong J.A., and W.R. Bransby-Williams. 1961.
The Maintenance of a Colony of Anopheles gambiae
With Observations on the Effects of Changes in
Temperature. Bull. WHO 24, 427-435.
• Craig, M.H., R.W. Snow, and D. le Seuer. A
Climate-Based Distribution Model of Malaria
Transmission in Sub-Saharan Africa. Parasitology
Today, vol. 15, no. 3, 1999.
• Hay, S.I., Snow, R.W. and Rogers, D.J. (1998)
Prediction of malaria seasons in Kenya using
multi-temporal meteorological satellite sensor
data. Trans. R. Soc. Trop. Med. Hyg. 92, 1220
• Ikemoto, T. 2008. Tropical Malaria Does Not Mean
Hot Environments. J. Med. Entomol. 45(6) 963Ð969
• Lindsay, S.W. and Martens, W.J.M. (1998) Malaria
in the African highlands past, present and
future. Bull. WHO 76, 3345.
• Lyimo, E.O., W. Takken, and J. C. Koella. 1992.
Effect of rearing temperature and larval density
on larval survival, age at pupation and adult
size of Anopheles gambiae. Entomol. exp. appl.
63 265-271.
• Taylor, D. Trans-NIH group assesses response to
climate change.
• Special thanks to Derrick Parker for the variety
of literature that he made available for my
research.