Climate Change and Human Health

27 April 2008

We were recently asked to "describe a disease or disorder thought to be caused in part by human induced climate change" and to "discuss what portion of its variation is environmental". We chose infectious diseases as our focus. Dr. Robert Shope classified infectious diseases by the number of factors that characterize their spread. A two-factor disease, such as Measles, requires only the Morbillivirus and a human host. As such, two-factor diseases tend to persist wherever human to human transmission is possible, which is truly anywhere two or more people exist and interact and have contact with anyone else. Thus, two-factor diseases tend not to vary greatly with changing environment (Shope, 1991). Though, one dangerous exception can be found in Cholera, a dangerous water-borne illness caused by Vibrio cholerae infection. Incidence of Cholera epidemics increase when harsh storms strike low lying regions. It acts as a 'faux' three-factor disease through fecal/oral transmission amongst members of a community.

However, a true three-factor disease, such as Dengue, uses an intermediate. Flavivirus requires a mosquito (such as Aedes aegypti) to infect a human host. As the intermediate is sensitive to such conditions as mean temperature, humidity and length of breeding season, Dengue and many other similar three and four-factor diseases can and likely will become more widespread with GCC (Shope, 1991). Indeed, as growth zones shift farther North, disease-transmitting insects persist longer and across more inhabited ranges.

Dr. Shope added, "In the special case of segmented genome viruses, ecological overlap of populations creates an abundant opportunity for reassortment of genes that could increase the virulence of the progeny virus. There is no way to anticipate these events, but their potential argues for maintaining a strong biomedical infrastructure and watching closely for new diseases." (Shope, 1991). Thus, human-induced GCC can also yield new viruses by allowing existing ones to overlap and exchange genes in a newly found environment common to both.

The spread of two-factor diseases, such as Cholera, can be effected by climate change. Our immune response to them is generally qualitative. Malaria and other 'three plus' factor diseases, while also responsive to climate change, have a quantitative effect with respect to the genes of their hosts and the environmental factors that promote or inhibit their progress (Mackinnon et al. 2005). Our best defense is to combine strategies that decrease climate change with efforts to monitor the growing ranges and changes available to such diseases.

Mackinnon MJ, Mwangi TW, Snow RW, Marsh K, Williams TN. 2005. Heritability of malaria in Africa. Public Library of Science: Medicine 2(12):e340.

Shope, R. E. 1991. Global climate change and infectious diseases. Environmental Health Perspectives 96: 171-74.
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