Conclusions

Not very long ago, a list of health challenges for the twenty-first century included sanitation and hygiene, vaccination, antibiotics and other antimicrobials, technological advances in detecting and monitoring diseases, serologic testing, viral isolation and tissue culture, and molecular techniques that can help diagnose and track the transmission of new threats (CDC, 1999). The paper went on to discuss the value of molecular genetics and how the US public health system must prepare to address diverse challenges including emergence of new infectious diseases, the re-emergence of old diseases, large food-borne outbreaks, and acts of bio-terrorism. Not until the last sentence was the need for research into environmental factors that facilitate disease emergence mentioned. A mere seven years later, with a growing understanding of the threats that climate change and habitat destruction pose to ecosystems and public health, it is unlikely that such an oversight would be made. The fact is that we are facing the emergence and re-emergence of diseases caused by environmental changes of anthropogenic origin.

As we have seen, the proliferation of suburban development and fragmented landscapes in the northeastern and midwestern US has generated an ideal habitat for people emigrating from the cities, for commensal animal species such as deer that proliferate in the presence of humans, and for the parasites those animals host. This mixture of large numbers of vectors and people brings with it an increased potential for diseases such as Lyme disease, babesiosis, and human granulocytic anaplasmosis. Cases of Lyme disease are on the rise throughout the nation, notably from regions that have experienced similar alterations of landscape, have rapidly growing deer herds, and are becoming increasingly subur-banized (Spielman et al., 1993; Wormser et al., 2007).

The number of tools available to analyze ecological parameters that affect vector distribution has grown considerably in the last two decades - for example, satellite imagery and weather records from around the world support robust analyses of population data that could not be considered in the past. Much of this progress comes as a result of the interest in global warming, a critically important phenomenon that cannot be ignored any longer. Climate constrains the range of many infectious diseases, and weather affects the timing and intensity of outbreaks (Epstein, 1999). A strong argument can be made that diseases transmitted by mosquitoes -malaria, dengue fever, yellow fever, West Nile Virus infection, and several types of encephalitis, for instance - will be the ones that gain most from global warming (Epstein, 2001). This is mainly a function of vector mosquitoes benefiting from larger geographic ranges and faster development times. The future is less clear for ticks in a warmer world. Tick-borne diseases are also sensitive to climatic conditions, but favor cooler temperatures. In Africa, Rogers and Randolph (2000) found mean monthly maximum temperature to be the strongest predictor of tick occurrence at the margins of endemic zones. Only 2°C determined the difference between areas where ticks were present or absent in southeastern Africa. In the southern US, Rocky Mountain Spotted Fever may decline due to the vector tick's (Dermacentor variabilis intolerance of high temperatures and diminished humidity (Haile, 1989; Patz et al., 1996). Brownstein et al. (2003) concluded that I. scapularis distribution is limited to those areas with a mean minimum temperature of -7°C in winter opening the possibility that global warming can increase the range of this tick in the future. Coupled with the ongoing creation of suitable habitat for ticks and their hosts resulting from suburbanization, the likelihood is high that tick-borne diseases will remain a significant threat in developed countries.

The means of combating ticks and reducing risk are limited in number. Host-targeted approaches are laborious, but environmentally more friendly than the classical use of chemical insecticides. To use them to their full advantage, however, requires knowledge of the ecology of a disease system that is frequently difficult to obtain and which takes time to understand; vector-host-pathogen lifecycles are complex and not easily unraveled. Furthermore, the public at risk needs to understand the value of such efforts. This is a formidable task for a sub-urbanized human population that is generally poorly informed about nature and wildlife dynamics (Foster et al., 2002) that drive vector-borne diseases. Most important will be acknowledging the role that humans have in dictating which wildlife species have a place in suburbia, and the types of habitats we permit them to share with us. We need to recognize the direct and indirect cultural control we have over the modern landscape (Foster et al., 2002).

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