example, was first described after it devastated Merino sheep from Europe that were introduced to South Africa in the late 18th century (FAO, 2007b; Verwoerd and Erasmus, 1994). It is therefore not surprising that the initial human occupation of remote ecosystems has resulted in the emergence of vector-borne diseases, given the potential for such circumstances to introduce vector-borne pathogens to immunologically naïve hosts and vectors. Moreover, this is a two-way street: as vector-borne diseases emerge from formerly isolated locations, vector-borne pathogens enter new territories along with their human and animal hosts (Murphy, 1998).

Deforestation and Risk for Malaria

As presenter Jonathan Patz, of the University of Wisconsin, Madison, has observed, land use changes such as deforestation, road construction, and dam building can trigger a cascade of secondary factors known to exacerbate infectious disease emergence, such as forest fragmentation, pathogen introduction, pollution, and human migration (see Patz and Olson in Chapter 1) (Patz et al., 2004).13 In a study they conducted along a road under construction in the Peruvian Amazon, Patz and colleagues determined that as forest density increased, the mosquito biting rate declined, regardless of human population density (Vittor et al., 2006). The change in biting rate, in turn was linked to the species distribution of mosquitoes, such that in deforested sites, the biting rate of Anopheles darlingi, the primary local vector of the malarial parasite Plasmodium falciparum, was more than 278 times higher than for forested areas. Thus, Patz concluded, the change in land use in this area appeared to reduce mosquito biodiversity, increasing the numbers of the malaria vectors and thereby raising the risk of infection. His group is currently investigating possible ecological explanations for the shift in mosquito biodiversity they observed.

Reforestation and Tick-Borne Disease

According to Durland Fish of Yale University, the reversal of deforestation led to the emergence of Lyme disease in the northeastern United States (Barbour and Fish, 1993). Black-legged deer ticks (Ixodes scapularis) carry the bacterial pathogen Borrelia burgdorferi that causes Lyme disease. The adults of this tick species feed exclusively on white-tailed deer; only the nymphs feed on and transmit disease to humans. As long as the deer population of the eastern United States was limited by farming and hunting to a few small, isolated bands, Lyme disease—though probably present—was unrecognized. The decline of agriculture

13

Human-induced land use changes are the primary drivers of a range of infectious disease outbreaks and emergence events and also modifiers of the transmission of endemic infections (Patz et al., 2000).



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