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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 isolines 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 temperature Tmin=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 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 Number that directly relate to climate change Number that examine how climate variables affect health Climate is likely an important factor but is not explicitly addressed 1,357 7 85 706 NIH Studies Health: Exposure pathways: Infectious diseases, respiratory diseases, asthma, heat stress, exposure to environmental toxins, trauma/injury, and cancers Extreme weather, UV radiation, pollution, waterborne, 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.