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An Overview of the Effects of Climate on
Malaria Transmission
Barbara Wendelberger
27 April 2010
Some Simplifications to MARA Anopheles gambiae s.l. Plasmodium falciparum. Independent analyses of rainfall and
temperature
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
Improvements Large global data sets
Up to 1.6 billion observations daily Climate data Population data Satellite imagery and topography
Geographical Information Systems (GIS) Advanced imaging software
Overlaying of varying levels of understanding Ex. Rainfall and temperature
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
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
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)
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
A “fuzzy” model that demonstrates the different suitability zones
Temperature Effects Sporogonic duration (n)
n = DD _ T – Tmin
DD=degree days for parasite development (111) T=mean temperatureTmin=temperature at which parasite development ceases
(16 C)
Mosquito survival (p) p = e (-1/(-4.4+1.31T-0.03T^2)
Defined by Martens Assumes constant humidity
Temperature, p, and n
pn = percentage of vector cohort that survives the required temperature time period
ld = larval density
= 1 ___(0.00554T – 0.06737)
Temperature, p, and n
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
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
Climate/Transmission RelevanceMore limiting variable used.
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
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
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?
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.
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
Current Predictions Based On Continuing change in global temperature
The present distribution of malaria parasites and their mosquito vectors
Warming Effects High Temperature
Increase Development rate to adulthood Frequency of blood-feeding Rate at which parasites are required Parasite incubation time
Decrease Adult mosquito survival
Thermodynamics
Negative Correlation Coefficients?Data
Dar es Salaam (Tanzania) Dodowa (Ghana)
-0.7 (mean max monthly temp/number of cases)
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 to adult stage
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”
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, 12–20
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, 33–45.
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.