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An Overview of the Effects of Climate on Malaria Transmission


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

An Overview of the Effects of Climate on Malaria
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

  • 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
  • in time because both intensity and distribution
    wax and wane with natural periodicity of events

  • 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

A fuzzy model that demonstrates the different
suitability zones
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

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
  • 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

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
Climate/Transmission Relevance
More 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

  • 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
  • 23ºC to 24ºC
  • Development rates
  • Increased development for both parasite and
    vector with increased temperature
  • Could increase it to the point of weakening the
  • Density
  • At 30ºC, when density increases, survival
  • At 27ºC, when density increases, survival

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

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
  • 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
  • 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, 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