The Weakest Link, Goodbye

by Howard Fienberg
April 8, 2002

It is often said that a warming world will lead to greater epidemics of infectious diseases like West Nile and dengue. But the 10th International Congress on Infectious Diseases (March 11-14) did not talk much about climate change. In fact, when discussing common diseases like malaria, most attendees expressed concerns about the pricing of anti-malarial drugs and the growth of drug-resistance.


So why do infectious disease researchers appear so disinterested in global warming? Because their research does not indicate any simple link between global climate changes and the spread of infectious disease. For instance, a new article in the journal Nature (Feb. 21) examining malaria could find no association whatsoever. Oxford zoologist Simon I. Hay and his coauthors looked at temperature, rainfall, vapor pressures and the number of months suitable for the transmission of malaria at four sites in the highlands of East Africa. They found no change in these factors “during the past century or during the period of reported malaria resurgence.”


What else could have caused the uptick in malaria? At Kericho, the researchers point to the growth of anti-malarial drug resistance (those drugs having been the public health response to the large epidemics sixty-some years ago). Similarly, drug resistance, rather than climate changes resulting from local deforestation, seems a more likely culprit in the Usambara mountains of Tanzania. In Southern Uganda, “epidemiological changes have been attributed to the shorter-term climate phenomenon of El Nino, which is suggested to cause changes in vector abundance.” In the fourth site, Muhanga, changes in land use may be at least as important as increases in temperature in causing an increase in malaria.


Across other highland African areas, “increases in malaria have been attributed to population migration and the breakdown in both health service provision and vector control operations.” The researchers conclude that, given the climate variability in East Africa, “associations between local malaria resurgences and regional changes in climate are overly simplistic.”


Changes in the weather can have dramatic impacts on diseases and the pests that spread them. However, as the Nature study hints, and a report from the National Research Council pointed out last year (“Under the Weather”), the relationship traditionally drawn between climate and disease can be misleading.


Other influences, such as ecological, biological and societal changes, can have an even greater impact. Malaria and dengue outbreaks might be caused by anything from deforestation to population increases. And thanks to increased globalization, diseases can be transported worldwide in a matter of hours.


This does not mean that the climate has no impact. The life cycles of many disease pathogens and vectors are directly or indirectly influenced by changes in temperature, precipitation and humidity, affecting “the timing and intensity” of outbreaks. Unfortunately, most of the links made between climate and disease result from computer models.


Computers can calculate anything, but effectively including all relevant factors in a climate model is no easy feat. Inevitably, computer climate models only capture part of the story of infectious diseases. The NRC cautioned that such models are good for some kinds of analyses, but “are not necessarily intended to serve as predictive tools,” since they cannot “fully account for the complex web of causation that underlies disease dynamics.”


University of Edinburgh biologist Mark Woolhouse has highlighted the challenges in this kind of modeling. Commenting on the Nature study on the website BioMedNet (Mar.8), he mentioned three problems: disease surveillance data “are often of poor quality”; outbreaks “undoubtedly” spring from more than one cause; and the trouble in determining the right climate variables to study. So how can researchers be expected “to detect any signal amongst the noise?”


The NRC report stressed that there are a lot more possible influences than climate, including “sanitation and public health services, population density and demographics, land use changes, and travel patterns.” So strong public health measures “such as vector control efforts, water treatment systems, and vaccination programs” remain the most effective weapons in the battle against infectious disease.


Centers for Disease Control entomologist Paul Reiter thinks that North Americans and Europeans have forgotten that malaria and dengue are not just tropical diseases (New Scientist, Sep. 23, 2000). In the 1880's, malaria existed in most of North America. In the 1920's, “epidemics killed hundreds of thousands in the Soviet Union, right up to the Arctic Circle.” In 1922, Texas was estimated to have 500,000 cases of dengue. While public health schemes and socioeconomic improvement moved us away from the mosquitoes, much of the rest of the Earths peoples have not been so fortunate.


It would be a shame if our misperceptions prevented us from properly dealing with infectious diseases, which kill millions of people worldwide every year. Global warming may or may not be a disaster waiting to happen, but lumping it together with more tangible public health problems probably wont help solve either one.

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