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) • Advanced imaging software • 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 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

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)

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 • Development rate to adulthood • Frequency of blood-feeding • Rate at which parasites are required • Parasite incubation time • Decrease • Adult mosquito survival

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 to adult stage

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, 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.