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Health Effects (Temperature - Dengue Fever) -- Summary
During his failed run for the Presidency of the United States at the turn of the last century, Al Gore was interviewed by Physics Today about his views on global warming. Among other things, he said that one of the likely consequences of the increase in global temperature predicted by climate models then in vogue would be "expanded geographic ranges for diseases like malaria and dengue fever." Subsequently, in our Editorial of 11 Oct 2000, we contended that Gore was wrong on this point; and subsequent research -- which we summarize below for the case of dengue fever -- has continued to confirm our contention.

In a major review of mosquito-borne diseases by one of the world's premier authorities on the subject, Reiter (2001) analyzed the history of malaria and dengue fever in an attempt to determine if the incidence and range of influence of these diseases would indeed increase in response to CO2-induced global warming. This review indicated that the natural history of these vector-borne diseases is highly complex, and that the interplay of climate, ecology, vector biology and a number of other factors defies definition by the simplistic analyses utilized in models that generate predictions of future geographical changes in these diseases under various global warming scenarios.

That there has, in fact, been a resurgence of these diseases in some parts of the world is true; but, as Reiter notes, it is "facile to attribute this resurgence to climate change." This he shows to be the case via a number of independent analyses that clearly demonstrate that factors associated with politics, economics and human activity are the principal determinants of the spread of these diseases, which factors he describes as being "much more significant" than climate in promoting disease expansion. Consequently, as Reiter concludes, it is inappropriate to use climate-based models to predict the future prevalence of these diseases.

Two years later, Reiter took up the subject again, this time with 19 other scientists as coauthors (Reiter et al., 2003). They began by noting "it has frequently been stated that dengue, malaria, and other mosquito-borne diseases will become common in the United States as a result of global warming (Watson et al., 1996; Jetten and Focks, 1997; Patz et al., 1998; Watson et al., 1998)," undeterred by the fact that the Intergovernmental Panel on Climate Change had played a key role in promoting this claim (as per the two Watson et al. articles). And why were they undeterred? Because they had acquired solid evidence to prove that the IPCC was simply wrong on this point.

In the summer of 1999, toward the end of a significant dengue outbreak in "los dos Laredos" -- Laredo, Texas, USA (population 200,000) and Nuevo Laredo, Tamaulipas, Mexico (population 290,000) -- the team of scientists conducted a seroepidemiologic survey to examine factors affecting dengue transmission in the two cities, which lie adjacent to each other on opposite sides of the Rio Grande, but which experience, according to the team, "massive cross-border traffic across three multi-lane bridges." Their work revealed that "the incidence of recent cases, indicated by immunoglobulin M antibody serosurvey, was higher in Nuevo Laredo [16.0% vs. 1.3%], although the vector, Aedes aegypti, was more abundant in Laredo [91% vs. 37%]." Reiter et al. additionally determined that "environmental factors that affect contact with mosquitoes, such as air-conditioning and human behavior, appear to account for this paradox."

They found, for example, that "the proportion of dengue infections attributable to lack of air-conditioning in Nuevo Laredo [where only 2% of the homes had central air-conditioning compared to 36% of the homes in Laredo] was 55%," which means that 55% of the cases of dengue in Nuevo Laredo would not have occurred if all households there had had air-conditioning. Reiter et al. correctly concluded, therefore, that "if the current warming trend in world climates continues, air-conditioning may become even more prevalent in the United States, in which case, the probability of dengue transmission [there] will likely decrease." Likewise, if the economy of Mexico continues to grow (which it has the opportunity to do if the government does not unrealistically curtail the use of fossil fuels), the use of air-conditioners will likely gain momentum there as well, which would likely lead to even greater decreases in the occurrence of dengue fever in that country. It should be abundantly evident to all, therefore, that the development of wealth, which currently is heavily dependent on the availability of inexpensive fossil-fuel-derived energy, will lead to much greater decreases in mosquito-borne diseases such as dengue fever than any attempted manipulation of the world's climate ever could.

Another factor that could come into play in this regard is the CO2-induced potential for changes in the concentrations of various "phytochemicals" found in certain plants that provide a degree of protection against diseases such as dengue fever. For example, Idso et al. (2000) grew spider lily plants out-of-doors at Phoenix, Arizona in clear-plastic-wall open-top enclosures that had their atmospheric CO2 concentrations continuously maintained at either 400 or 700 ppm for two consecutive two-year growth cycles. At the ends of each of these two-year periods, the underground bulbs of the plants were harvested and the amounts and concentrations of a number of disease-fighting substances found within them were measured. By this means it was learned that the 75% increase in the air's CO2 concentration resulted in a 56% increase in spider-lily bulb biomass, and that it increased the concentrations of five bulb constituents known to possess antiviral properties.

Mean percentage increases in these concentrations were, in the words of Idso et al., "6% for a two-constituent (1:1) mixture of 7-deoxynarciclasine and 7-deoxy-trans-dihydronarciclasine, 8% for pancratistatin, 8% for trans-dihydronarciclasine, and 28% for narciclasine, for a mean active-ingredient percentage concentration increase of 12%." Combined with the 56% increase in bulb biomass, these percentage concentration increases resulted in a mean active-ingredient increase of 75%. If other plants that produce these particular phytochemicals and still other antiviral agents are likewise stimulated by the ongoing rise in the air's CO2 content, it is possible that people's natural resistance to such diseases as dengue fever may be significantly enhanced in a high-CO2 world of the future, simply by eating more of its more health-promoting foods.

Shortly after this time, in a major review of the general subject of infectious diseases in a warming world, Roland Zell (2004) of the Institute for Virology and Antiviral Therapy at the Fredrich Schiller University in Jena, Germany, reviewed what was known about the putative link that some scientists were postulating -- and a host of climate alarmists were championing -- between global warming and the spread of infectious diseases. Upon concluding this review, and after stating that many people "assume a correlation between increasing disease incidence and global warming," he went on to say that after studying the issue in considerable depth, he had to conclude that "the factors responsible for the emergence/reemergence of vector-borne diseases are complex and mutually influence each other," citing as an example of this complexity the fact that "the incidence and spread of parasites and arboviruses are affected by insecticide and drug resistance, deforestation, irrigation systems and dams, changes in public health policy (decreased resources of surveillance, prevention and vector control), demographic changes (population growth, migration, urbanization), and societal changes (inadequate housing conditions, water deterioration, sewage, waste management)." Therefore, as he continued, "it may be over-simplistic to attribute emergent/re-emergent diseases to climate change and sketch the menace of devastating epidemics in a warmer world." Indeed, he reiterated that "variations in public health practices and lifestyle can easily outweigh changes in disease biology," especially those that might be caused by global warming. And, we might add, these public health and lifestyle changes could be implemented now, if we chose to do so, and at a fraction of the cost that would be needed to make even the smallest of changes in the future course of earth's air temperature.

Jumping ahead a few years, Kyle and Harris (2008) wrote that "dengue is a spectrum of disease caused by four serotypes of the most prevalent arthropod-borne virus affecting humans today," and that "its incidence has increased dramatically in the past 50 years," to where "tens of millions of cases of dengue fever are estimated to occur annually, including up to 500,000 cases of the life-threatening dengue hemorrhagic fever/dengue shock syndrome." Consequently, in an effort to better understand what was happening in this regard, they conducted a thorough review of the pertinent scientific literature, exploring "the human, mosquito, and viral factors that contribute to the global spread and persistence of dengue, as well as the interaction between the three spheres, in the context of ecological and climate change."

With respect to the status of dengue fever within the context of global warming, this review indicated, in the words of the two researchers who conducted it, that "there has been a great deal of debate on the implications of global warming for human health," but that "at the moment, there is no consensus." However, "in the case of dengue," as they went on to report, "it is important to note that even if global warming does not cause the mosquito vectors to expand their geographic range, there could still be a significant impact on transmission in endemic regions," as they noted that "a 2C increase in temperature would simultaneously lengthen the lifespan of the mosquito and shorten the extrinsic incubation period of the dengue virus, resulting in more infected mosquitoes for a longer period of time." Nevertheless, they indicated there are "infrastructure and socioeconomic differences that exist today and already prevent the transmission of vector-borne diseases, including dengue, even in the continued presence of their vectors," citing Reiter (2001). Thus, it would appear that whatever advantages rising temperatures may confer upon the dengue virus vector, they can be more than overcome by proper implementation of modern vector control techniques.

In another review of the scientific literature that was published in the same year, Wilder-Smith and Gubler (2008) wrote that "the past two decades saw an unprecedented geographic expansion of dengue," reporting that "each year an estimated 50 to 100 million dengue infections occur, with several hundred thousand cases of dengue hemorrhagic fever and about twenty thousand deaths." They too noted that "global climate change is commonly blamed for the resurgence of dengue," but they stated that "there are no good scientific data to support this conclusion." Thus, in an effort to find a better explanation for dengue's recent global expansion, i.e., one based on real-world data, Wilder-Smith and Gubler reviewed what was known about the problem and pieced together what would appear to be a logical conclusion.

With respect to the occurrence of dengue infections, the two researchers reported that "climate has rarely been the principal determinant of [their] prevalence or range," and that "human activities and their impact on local ecology have generally been much more significant." In this regard, they cite as contributing factors "urbanization, deforestation, new dams and irrigation systems, poor housing, sewage and waste management systems, and lack of reliable water systems that make it necessary to collect and store water," further noting that "disruption of vector control programs, be it for reasons of political and social unrest or scientific reservations about the safety of DDT, has contributed to the resurgence of dengue around the world." In addition, they state that "large populations in which viruses circulate may also allow more co-infection of mosquitoes and humans with more than one serotype of virus," which would appear to be born out by the fact that "the number of dengue lineages has been increasing roughly in parallel with the size of the human population over the last two centuries." Most important of all, perhaps, is "the impact of international travel," of which they say that "humans, whether troops, migrant workers, tourists, business travelers, refugees, or others, carry the virus into new geographic areas," which movements, in their words, "can lead to epidemic waves." Thus, the two researchers conclude that "population dynamics and viral evolution offer the most parsimonious explanation for the observed epidemic cycles of the disease, far more than climatic factors."

Last of all, in a review of the subject as it pertains to Australia, Russell (2009) -- a Professor in the Department of Medicine of the University of Sydney and founding Director of its Department of Medical Entomology -- reports that "during the past 10 years, there has been increasing concern for health impacts of global warming in Australia, and continuing projections and predictions for increasing mosquito-borne disease as a result of climate change." However, he writes that these claims "are relatively simplistic, and do not take adequate account of the current or historic situations of the vectors and pathogens, and the complex ecologies that might be involved." Hence, he reviews the consequences of these several inadequacies for malaria, dengue fever, the arboviral arthritides (Ross River and Barmah Forest viruses) and the arboviral encephalitides (Murray Valley encephalitis and Kunjin viruses). This he does within the context of predictions that have been made for projected climate changes as proposed and modeled by Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Intergovernmental Panel on Climate Change (IPCC).

The abstract of Russell's paper begins with a question: "Will warming climate increase the risk or prevalence of mosquito-borne disease in Australia, as has been projected in a number of scientific publications and governmental reports?" His conclusion -- after considering all of the pertinent literature on the subject -- provides the answer: "there might be some increases in mosquito-borne disease in Australia with a warming climate, but with which mosquitoes and which pathogens, and where and when, cannot be easily discerned." Hence, the mosquito expert concludes that, "of itself, climate change as currently projected, is not likely to provide great cause for public health concern with mosquito-borne disease in Australia."

In light of these several observations, it would appear that Al Gore (and all those who think like him) are 180 degrees out of phase with reality on the issue of CO2, global warming, and vector borne diseases such as dengue. Their prescription for planetary health -- reducing anthropogenic CO2 emissions -- would directly attack that which actually enhances the amounts and concentrations of a number of health-promoting substances found in the foods we eat (see Enhanced or Impaired? Human Health in a CO2-Enriched Warmer World), while producing the negative health consequences that would inevitably accompany the lowering of the standard of living that would result from the huge reductions in the usage of fossil fuels that they claim are needed to attain their illusory dream of remaking earth's climate in the image of their equally illusory wisdom.

References
Idso, S.B., Kimball, B.A., Pettit III, G.R., Garner, L.C., Pettit, G.R. and Backhaus, R.A. 2000. Effects of atmospheric CO2 enrichment on the growth and development of Hymenocallis littoralis (Amaryllidaceae) and the concentrations of several antineoplastic and antiviral constituents of its bulbs. American Journal of Botany 87: 769-773.

Jetten, T.H. and Focks, D.A. 1997. Potential changes in the distribution of dengue transmission under climate warming. American Journal of Tropical Medicine and Hygiene 57: 285-297.

Kyle, J.L. and Harris, E. 2008. Global spread and persistence of dengue. Annual Review of Microbiology 62: 71-92.

Patz, J.A., Martens, W.J.M., Focks, D.A. and Jetten, T.H. 1998. Dengue fever epidemic potential as projected by general circulation models of global climate change. Environmental Health Perspectives 106: 147-153.

Reiter, P. 2001. Climate change and mosquito-borne disease. Environmental Health Perspectives 109: 141-161.

Reiter, P., Lathrop, S., Bunning, M., Biggerstaff, B., Singer, D., Tiwari, T., Baber, L., Amador, M., Thirion, J., Hayes, J., Seca, C., Mendez, J., Ramirez, B., Robinson, J., Rawlings, J., Vorndam, V., Waterman, S., Gubier, D., Clark, G. and Hayes, E. 2003. Texas lifestyle limits transmission of Dengue virus. Emerging Infectious Diseases 9: 86-89.

Russell, R.C. 2009. Mosquito-borne disease and climate change in Australia: time for a reality check. Australian Journal of Entomology 48: 1-7.

Watson, R.T., Zinyowera, M.C. and Moss, R.H. (Eds.) 1996. Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses. Contribution of Working Group II to the Second Assessment of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, UK.

Watson, R.T., Zinyowera, M.C. and Moss, R.H. (Eds.) 1998. The Regional Impacts of Climate Change: An Assessment of Vulnerability. Special Report of the Intergovernmental Panel on Climate Change (IPCC) Working Group II. Cambridge University Press, Cambridge, UK.

Wilder-Smith, A. and Gubler, D.J. 2008. Geographic expansion of Dengue: The impact of international travel. Medical Clinics of North America 92: 1377-1390.

Zell, R. 2004. Global climate change and the emergence/re-emergence of infectious diseases. International Journal of Medical Microbiology 293, Suppl. 37: 16-26.

Last updated 7 October 2009