Sir--Paul Reiter (March 14, p 839)1 misses the main point: it is not that vector-borne diseases have never before occurred at high elevations during especially warm years. Rather, it is the resurgence of highland malaria, dengue fever, and their vectors in Latin America, central Africa, and Asia during the 1980s and 1990s, coincident with three other changes--namely, (1) the widespread and accelerating retreat of tropical summit glaciers,2 (2) the upward displacement of plants,3 and (3) the main underlying measurement, the upward shift of the freezing level (0°C isotherm) in the mountains (30°N to 30°S latitudes)--150 metres since 1970, equivalent to a 1°C warming.4
Insects, in the fossil record, are excellent indicators of climate change, their distribution shifting rapidly with warming and cooling--particularly in response to changes in night-time and winter temperatures. These temperatures have risen twice as fast as daytime temperatures (1·86°C per 100 years vs 0·88°C per 100 years) since 1950,5 which is best explained by enhanced evaporation from warmer oceans,2 leading to the increased cloudiness that blocks outgoing night-time infrared radiation.
Our understanding of Earth's climate system, and the impact of our rapidly altering of the chemical components of the lower atmosphere, are based on an n of 1 (one earth). Pattern recognition, consistency of data with model projections, and internal consistency among datasets are our primary tools for evaluating trends and risks. It is axiomatic that climate circumscribes the range of vector-borne diseases, whereas weather affects the timing and intensity of outbreaks. Models project that global warming will provide conditions conducive to transmission at higher latitudes and higher elevations, and four independent sets of physical and biological data indicate climatic warming.
The global emergence, resurgence, and redistribution of infectious disease in the latter part of the twentieth century is--as Reiter rightly argues--multifactorial, involving land-use change, local biogeography, population migration, immunological history, control measures, and--most fundamentally--the level of socioeconomic development. We are all concerned about emerging infectious diseases, but many are also worried about climate change from burning fossil fuels. The true challenge in the coming decades will be to develop healthy economies with clean energy sources.
Paul R Epstein
Center for Health and the Global Environment, Harvard Medical School, Boston, MA 02115, USA
1 Reiter P. Global-warming and vector-borne disease in temperate regions and at high altitude. Lancet 1998; 351: 839-40.
2 Intergovernmental Panel on Climate Change (IPCC). In: Boughton JT, Mairo Filho LG, Callander BA, Harris N, Kattenberg A, Maskell K, eds. Climate change '95: the science of climate change. Contribution of working group I to the second assessment report of the IPCC. Cambridge: Cambridge University Press, 1995: 149, 370.
3 Pauli H, Gottfried M, Grabherr G. Effects of climate change on mountain ecosystems--upward shifting of alpine plants. World Resources Rev 1996; 8: 382-90.
4 Dias HE, Graham NE. Recent changes in tropical freezing heights and the role of sea surface temperature. Nature 1996; 383: 152-55.
5 Easterling DR, Horton B, Jones FD, et al. Maximum and minimum temperature trends for the globe. Science 1997; 277: 363-67.
Sir--Paul Reiter1 incorrectly quotes me as making "adamant" claims that global warming has already caused malaria, dengue, and yellow fever to invade higher latitudes in the temperate regions and high altitudes in the tropics. Reiter's statement seems to be based on a misquote from a Lancet news article, in which I state, "there are some early signs of malaria and other vector-borne diseases being experienced at higher altitudes than was previously the case".2 I made no comment about higher latitudes in temperate countries, where in most cases public health infrastructure is likely to be adequate to prevent substantial increases in incidence.
Recent reports, for example on Ethiopia3 and Kenya (Githeko A, personal communication), suggest shifts in altitude in malaria consistent with increases in temperature. Increases in temperature might be associated with increased incidence of malaria in the Usumbara mountains of Tanzania,4 and increases in temperature and rainfall have been associated with a steep rise in malaria cases in Rwanda. Changes in climate may be due to interannual variability, climatic cycles such El Niño, local deforestation, or global climate change; indeed all four may co-exist. Lindsay and Martens5 have shown with mathematical modelling that such changes in incidence are likely to occur with climate change, but clearly other factors may also be at work including those outlined by Reiter. The historical data on high altitude transmission of malaria cited by Reiter do not necessarily mean that climate change will not effect the incidence of malaria at high altitudes.
I have argued consistently for better monitoring of the potential health impacts of climate change to improve understanding of associations between short-term climate variablility, longer term underlying trends in global climate, local environment change, changes in public-health infrastructure on the one hand, and the occurrence of vector-borne diseases and other potential impacts on health of global warming on the other.
Andy Haines
Department of Primary Care and Population Sciences, Royal Free and University College London School of Medicine, London NW3 2PF, UK
1 Reiter P. Global-warming and vector-borne disease in temperate regions and at high altitude. Lancet 1998; 351: 839-40.
2 McCurry J. Physicians add their warnings to Kyoto summit. Lancet 1997; 350: 1825.
3 Tulu A. Determinants of malaria transmission in the highlands of Ethiopia: the impact of global warming on mortality and morbidity ascribed to malaria. London University, PhD Thesis, 1996.
4 Matola YG, White GB, Magayuka SA, et al. The changed pattern of malaria endemicity and transmission at Amani in the eastern Usambara mountains, north-east Tanzania. J Trop Med Hyg 1987; 90: 127-34.
5 Lindsay S, Martens WJM. Malaria in the African highlands: past, present and future. Bull World Health Organ (in press).
Author's reply
Sir--Paul Epstein is absolutely correct: I try, but am quite unable to see his point. I feel like Alice1 in her conversation with the White King:
"I see nobody on the road," said Alice
"I only wish I had such eyes," the King remarked in a fretful tone. "To be able to see Nobody! And at that distance too!"
The global warming notion is far from being universally accepted.2 Moreover, as a medical entomologist, I am disturbed by the tangle of syllogistic delusions in the health aspects of the debate, and the Machiavellian way in which they are presented to the public. For example, at his Kyoto press conference, Epstein stated: "Malaria is now occurring in the . . . highlands of Papua New Guinea. This is exactly in the same area where glaciers are retreating and plants are migrating up mountains". True, but even in the 1940s scientists warned that "with the march of civilization into the highlands there was every chance that the malaria problem could become . . . severe"3. They described large populations of malaria-free so-called stone-age peoples who had been discovered in a unique state of isolation in these highlands. To protect them, labourers entering from the malarious lowlands were held in compulsory quarantine for 2 weeks and given curative malaria therapy. The eventual failure of this measure was exacerbated by anopheline populations that increased rapidly after forest clearance and other human activities.4,5 Malaria transmission was never attributed to climate change, by malariologists.
In the context of global climate change, Andrew Haines' arguments are equally misleading. For example, the article he cites on malaria in Tanzania clearly states: "There was a pronounced warming . . . at Amani during the 1960s when considerable forest clearance occurred on the Amani hills, followed by a cooling trend as re-forestation progressed". The statement is supported by a graph of mean annual temperature that shows a steady decline (total 3°C) from 1970 to 1976. Clearly this was an effect of the local environment on the local meteorology, not a global impact on the regional climate.
The principal reasons for the rise in malaria were carefully described: (1) a sizeable influx of people from malarious lowlands, and (2) major ecological disturbance--deforestation, road construction, proliferation of dams, ditches, pools, &c--which opened up the area to Anopheles gambiae and An funestus, the classic African malaria vectors. Moreover, the altitude (600-1000 m) was well below the 2600 m maximum for transmission in neighbouring Kenya, first observed in the early half of the century. Last, Haines' mention of a mathematical model is a classic example of irrelevant proof, for it has long been common knowledge that vectorial capacity is a function of temperature. The model is based on this relation, so, inevitably, it indicates that incidence might increase with warmer climate.
Like Epstein, I worry about climate change, but my concern is with the dissemination of fallacious logic to journalists who are more likely to focus on crisis than on reason. I stand firmly by my original message: however worthy the cause, the distortion of science to make dramatic predictions of unlikely disasters is not in the public interest.
Paul Reiter
Dengue Branch, Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Dengue Branch, San Juan, PR 00921, USA (e-mail: ipr1@cdc.gov)
1 Carroll L. Alice's adventures in wonderland, and through the looking-glass and what Alice found there. Oxford: Oxford University, 1983.
2 Kerr RA. Greenhouse forecasting still cloudy. Science 1997; 276: 1040-42.
3 Peters W, Christian SH, Jameson JL. Malaria in the highlands of Papua and New Guinea. Med J Australia 1958; 2: 409-16.
4 Peters W, Christian SH. Studies on the epidemiology of malaria in New Guinea. Part IV. Unstable highland malaria: the clinical picture. Part V. Unstable highland malaria: the entomological picture. Trans R Soc Trop Med Hyg 1960; 54: 529-48.
5 Attenborough RD, Burkot TR, Gardner DS. Altitude and the risk of bites from mosquitoes infected with malaria and filariasis among the Mianmin people of Papua New Guinea. Trans R Soc Trop Med Hyg 1997; 91: 8-10.
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