Vector-Borne Diseases and Global Change

In introducing their study about what is known -- and unknown with respect to vector-borne diseases in humans, Gage et al. (2008) note that "temperature, precipitation, humidity, and other climatic factors are known to affect the reproduction, development, behavior, and population dynamics of the arthropod vectors of these diseases," which suggests that the subject is extremely complex. And truly it is.

Based on their study, the four researchers -- all of whom hail from the U.S. Centers for Disease Control's National Center for Zoonotic, Vector-Borne, and Enteric Diseases -- conclude that "the precise impacts" of the various climatic changes that are typically claimed to occur in response to rising atmospheric CO₂ concentrations "are difficult to predict." Indeed, they say that "in some areas, climate change could increase outbreaks and the spread of some vector-borne diseases while having quite the opposite effect [italics added] on other vector-borne diseases."

In further discussing this situation, they write that "the mere establishment of suitable vectors for a particular agent does not necessarily mean that spread to humans will commonly occur, as indicated by the limited transmission of dengue and malaria in the southern U.S.," because, as they continue, "in these instances, competent vectors are present and infected individuals or vectors occasionally enter this region, but local transmission has been limited by factors unrelated to the climatic suitability of the areas for the relevant vector species [our italics]." And they add that "in instances where a vector-borne disease is also zoonotic, the situation is even more complex, because not only must the vector and pathogen be present but a competent vertebrate reservoir host other than humans must also be present."

So what are some of the non-climatic factors that impact the spread of vector-borne diseases of humans? In this category, Gage et al. list "many other global changes concurrently transforming the world, including increased economic globalization, the high speed of international travel and transport of commercial goods, increased population growth, urbanization, civil unrest, displaced refugee populations, water availability and management, and deforestation and other land-use changes," as well as could be added the many different ways in which these phenomena are dealt with by different societies.

In light of the many complex phenomena that concurrently impact the spread of vector-borne diseases, it is clearly unjustified to claim that any future warming of the globe will necessarily lead to a net increase in the global incidence of vector-borne human diseases, for just the opposite could well be true, depending on the type and degree of a number of current and potential societal impacts on the world of nature, as well as the diverse natures of the evolving states of the planet's multiple human societies.