Volume 12, Number 30: 29 July 2009
In a paper recently published in Ecology, Harvell et al. (2009) begin their analysis of this intriguing subject by writing that an early hypothesis about climate change and infectious diseases posited that "increased temperature and rainfall" -- which are predicted to follow the projected increase in the atmosphere's CO2 concentration -- "could facilitate the emergence and persistence of many infectious microorganisms," concluding that "a warmer world would be a sicker world," in support of which statement they cited the earlier study of Harvell et al. (2002).
Well, the idea "clicked" in the mind of many a climate alarmist; and it has now become one of the primary tenets of their worldwide movement. But the science of the subject has not lain dormant; and what seemed self-evident several years ago is currently under intense reanalysis.
"Of all the infectious diseases," as Harvell and his 2009 coauthors continue to write, "it seemed likely that human vector-borne diseases would respond most promptly to localized warming events." More specifically, they state that "in temperate climates, we might expect the range and activity of mosquitoes and the pathogens they vector, such as malaria and dengue, to increase with warmer temperatures." However, "from a later vantage point in 2009," they say that "surprisingly, insect-vectored diseases resoundingly do not show a net expansion in range or increase in prevalence [our italics]."
As for why this is so, the five scientists give three explanations that they attribute to Lafferty (2009): "(1) anthropogenic activities directly influence the distributions of vectors and infectious disease in ways unrelated to climate, (2) vectors and pathogens are limited by thermal maxima, so that temperature changes lead to shifts rather than expansions in distribution, and (3) other factors such as host acquired immunity and vector or parasite life history traits are linked to habitat suitability in addition to climate." And they note the important role that may be played by "evolutionary changes in properties of the host or pathogen."
In concluding their paper, Harvell et al. say that "ecologists need to consider how host biology, including movement behavior and acquired immunity, can mediate the impacts of global change on parasite/pathogen dynamics and disease severity," because, as they continue, "at present, many of these mechanisms are poorly known."
Clearly, although we may now well know that a warmer world is not necessarily a sicker world, there is much we still can learn about this important subject.
Sherwood, Keith and Craig Idso
References
Harvell, D., Altizer, S., Cattadori, I.M., Harrington, L. and Weil, E. 2009. Climate change and wildlife diseases: When does the host matter the most? Ecology 90: 912-920.
Harvell, C.D., Mitchell, C.E., Ward, J.R., Altizer, S., Dobson, A.P., Ostfeld, R.S. and Samuel, M.D. 2002. Climate warming and disease risks for terrestrial and marine biota. Science 296: 2158-2162.
Lafferty, K.D. 2009. The ecology of climate change and infectious diseases. Ecology 90: 888-900.