Early Warning Signs of Global Warming: Spreading Disease
Climate change affects the occurrence and spread of disease by impacting the population size and range of hosts and pathogens, the length of the transmission season, and the timing and intensity of outbreaks (McMichael, 1996; McMichael et al., 1996; Epstein et al., 1998; Epstein, 1999). In general, warmer temperatures and greater moisture will favor extensions of the geographical range and season for vector organisms such as insects, rodents, and snails. This in turn leads to an expansion of the zone of potential transmission for many vector-borne diseases, among them malaria, dengue fever, yellow fever, and some forms of viral encephalitis. Extreme weather events such as heavy rainfall or droughts often trigger disease outbreaks, especially in poorer regions where treatment and prevention measures may be inadequate.
Mosquitoes in particular are highly sensitive to temperature. The mosquitoes that can carry malaria (Anopheline spp.) generally do not develop or breed below about 16° C, and the variety that transmits dengue fever (Aedes aegypti) is limited by winter temperatures below 10° C. Mosquito survival also drops at their upper temperature threshold, about 40° C. With sufficient moisture, warmer temperatures will generally cause an increase in mosquito abundance, biting rates, and activity level, and will accelerate the incubation of the parasites and viruses within them.
Warmer global temperatures will allow an expansion of the geographic range within which both the mosquito and parasite could survive with sufficient abundance for sustained transmission. Model predictions indicate that a 3° C global temperature rise by 2100 could increase the number of annual malaria cases by 50-80 million (not considering factors such as local control measures or health services) (Martens et al., 1995). The largest changes will occur in areas adjacent to current risk areas, at both higher altitudes and latitudes. In these regions, a temperature increase can convert areas that are malaria-free into areas that experience seasonal epidemics. In many cases, the affected populations will have little or no immunity, so that epidemics could be characterized by high levels of sickness and death.
Recent disease outbreaks are consistent with model projections that warmer, wetter conditions will lead to greater transmission potential at higher altitudes and elevations. Mosquito-borne diseases are now reported at higher elevations than in the past at sites in Asia, Central Africa, and Latin America (Epstein et al., 1998). This is coincident with growing evidence for significant warming at high altitude sites in tropical latitudes, as indicated for example by retreating glaciers (e.g., Fitzharris, 1996) and a 150 meter upward shift in the elevation of the freezing level (0° C isotherm) (Diaz and Graham, 1996). In New York City, an encephalitis outbreak in summer 1999 claimed three lives and prompted widespread pesticide spraying. The Centers for Disease Control have identified the West Nile virus as being responsible for this outbreak, a virus transmitted by mosquitoes that feed on infected birds (CDC, 1999a). The disease, which had not been previously documented in the Western Hemisphere, occurs primarily in the late summer or early fall in temperate regions, but can occur year round in milder climates (CDC, 1999b).
Diseases carried by rodents and other mammals may also be impacted by climate change. A recent study found a 60% rise in human plague cases in New Mexico following wetter than average winter-spring time periods (October-May) (Parmenter et al., 1999). Plague has only been in New Mexico since the 1940s, but a large increase in per capita cases occurred in the 1970s and 1980s associated with the wetter than normal conditions. The increased precipitation apparently enhances food resources for small mammals that serve as hosts for the infected fleas. The moister climate may also promote flea survival and reproduction. The study notes that if future climate conditions become more favorable for reproduction and survival of either wild mammal populations or their flea populations, then the probability of human infection via animal-flea-human contacts will likely increase.
Climate change also has the potential to impact diseases of plants and animals, and could lead to significant population declines or even extinction for some threatened or endangered species. Climate change has been implicated, for example, in the spread and emergence of marine diseases. This happens for a number of reasons because the ranges of hosts and pathogens may change, because warming-induced stress may lower disease resistance, and because microbial and contaminant input from terrestrial sources may increase due to runoff from heavy rainfall events (Harvell et al., 1999). Higher sea surface temperatures, for example, increase the stress on corals (see Coral Bleaching), increasing susceptibility to infection. Similarly, a 25-year trend of warming winter temperatures along the eastern US coast has been implicated in the northward expansion of oyster diseases (Harvell et al., 1999). Forests and agricultural crops are also susceptible to the spread of pathogens, especially following extended droughts and floods. The disproportionate warming at night and during the winter (Easterling et al., 1997) may allow destructive insects and pathogens to invade forests at higher latitudes and elevations from which they are now excluded (Kirschbaum and Fischlin, 1996).
References
CDC. 1999a. West Nile-like Virus in the United States. Centers for Disease Control and Prevention, Updated 5 October 1999. http://www.cdc.gov/od/oc/media/pressrel/r991004.htm
CDC. 1999b. Questions and answers about West-Nile Encephalitis, Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Revised 24 November 1999. http://www.cdc.gov/ncidod/dvbid/arbor/West_Nile_QA.htm
Diaz, H.F. and N.E. Graham. 1996. Recent changes in tropical freezing heights and the role of sea surface temperatures. Nature 383, 152-155.
Easterling, D.R., B. Horton, P.D. Jones, T.C. Peterson, T.R. Karl, D.E. Parker, M.J. Salinger, V. Razuvayev, N. Plummer, P. Jamason, and C.K. Folland, 1997. Maximum and minimum temperature trends for the globe. Science 277, 364-367.
Epstein, Paul R. 1999. Climate and health. Science 285, 347-348.
Epstein, P., H. Diaz, S. Elias, G. Grabherr, N. Graham, W. Martens, E. M. Thompson, and J. Susskind. 1998. Biological and physical signs of climate change: focus on mosquito borne diseases. Bulletin of the American Meteorological Society 79, 409-417.
Fitzharris, B.B. 1996. The cryosphere: Changes and their impacts. In Climate Change 1995 - Impacts, Adaptations, and Mitigation of Climate Change: Scientific-Technical Analyses, 241-265 (Eds RT Watson, MC Zinyowera, RH Moss), Cambridge University Press, Cambridge, UK.
Harvell, C.D., K. Kim, J.M. Burkholder, R.R. Colwell, P.R. Epstein, D.J. Grimes, E.E. Hofmann, E.K. Lipp, A.D.M.E. Osterhaus, R.M. Overstreet, J.W. Porter, G.W. Smith, G.R. Vasta. 1999. Emerging Marine Diseases-Climate Links and Anthropogenic Factors. Science 285, 1505-1510.
Kirschbaum M.U.F. and A. Fischlin, 1996. Climate change impacts on forests. In Climate Change 1995 - Impacts, Adaptations, and Mitigation of Climate Change: Scientific-Technical Analyses, 95-129 (Eds RT Watson, MC Zinyowera, RH Moss), Cambridge University Press, Cambridge, UK.
Martens, W.J.M., T.H. Jetten, J. Rotmans, L.W. Niessen, 1995. Climate change and vector-borne diseases: a global modelling perspective. Global Environmental Change 5 (3), 195-209.
McMichael, A.J.. 1996. Human population health, In Climate Change 1995 - Impacts, Adaptations, and Mitigation of Climate Change: Scientific-Technical Analyses, 561-584 (Eds RT Watson, MC Zinyowera, RH Moss), Cambridge University Press, Cambridge, UK.
McMichael, A.J., A. Haines, and R. Slooff (Eds.), 1996. Climate Change and Human Health. World Health Organization, World Meteorological Organization, United Nations Environmental Program, Geneva, 305 p.
Parmenter, R.R. E.P. Yadav, C.A.. Parmenter, P. Ettestad, and K. L. Gage. 1999. Incidence of plague associated with increased winter-spring precipitation in New Mexico American Journal of Tropical Medicine & Hygiene 61 (5), 814 821.
Additional Resources
Colwell, R., 1996. Global climate and infectious disease: The cholera paradigm. Science 274, 2025-2031.
Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases This site provides fact sheets, images, and world maps showing the distribution of several types of vector-borne diseases. http://www.cdc.gov/ncidod/dvbid/dvbid.htm
Epstein, P.P., 1997. Climate, ecology, and human health. Consequences 3 (2), Global Change Research Information Office. http://www.gcrio.org/CONSEQUENCES/vol3no2/climhealth.html
Epstein, Paul R. 1999. Enhanced: Climate and Health. Science 285, 347-348. This enhanced electronic version provides an extensive list of additional websites and literature cited on the topic. http://www.sciencemag.org/cgi/content/full/285/5426/347
World Health Organization Climate and Health http://www.who.int/peh/climate/climate_and_health.htm
