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Vector-borne and Zoonotic Diseases

Impacts on Risk

VBZD ecology is complex, and weather and climate are among several factors that influence transmission cycles and human disease incidence. Impacts in certain ecosystems are better understood; however, for others such as marine ecosystems, their role in VBZD has not been well characterized. Changes in temperature and precipitation patterns affect VBZD directly through pathogen host-vector interactions, and indirectly through ecosystem changes (humidity, soil moisture, water temperature, salinity, acidity) and species composition.

Social and cultural behaviors also affect disease transmission. Many VBZD exhibit some degree of climate sensitivity, and ecological shifts associated with climate variability and long-term climate change are expected to impact the distribution and incidence of many of these diseases. For instance, the range of Lyme disease is expected to expand northward as the range of the deer tick that transmits it expands. In another example, the frequency of hantavirus pulmonary syndrome outbreaks, caused by human exposure to the virus in deer mice urine or feces, may change with increasingly variable rainfall in the desert Southwest, which affects the populations of deer mice and other rodents through changes in production of the pine nuts on which they feed. Similarly, certain VBZD may decrease in particular regions as habitats become less suitable for host or vector populations and for sustained disease transmission. Coastal and marine ecosystems will be particularly impacted by increasing temperatures, changes in precipitation patterns, sea-level rise, altered salinity, ocean acidification, and more frequent and intense extreme weather events. These changes will directly and indirectly affect ocean and coastal ecosystems by influencing community structure, biodiversity, and the growth, survival, persistence, distribution, transmission, and severity of disease-causing organisms, vectors, and reservoirs. Also of concern for both terrestrial and aquatic/marine ecosystems is the loss of biodiversity (which underlies ecosystem services) that further exacerbates the impacts of climate change on vectors or animal reservoir populations. Such alterations in ecosystem functions may alter the emergence of VBZD in populations within the United States. With the loss of predators, insect vectors may increase, making necessary either chemical or mechanical controls.

Projecting VBZD incidence is difficult given the complexity of VBZD transmission cycles, the variability of regional and local impacts of climate change, and the limited information currently available regarding the ecology of many VBZD. For instance, while malaria transmission increases with temperature and humidity, the decrease in disease incidence seen with prolonged drought may negate these effects. Human rural and urban development efforts, such as the creation of clean water sources for animal husbandry or swamp clearance to increase availability of land for human settlement, also have significant impacts on transmission dynamics that can offset climate impacts. The incidence of VBZD in the United States will likely increase under anticipated climate change scenarios, for several reasons. The distribution of vectors currently restricted to warmer climates will expand into the United States. For example, the habitats of two potent mosquito vectors of malaria, Anopheles albimanus and Anopheles pseudopunctipennis, currently range as far north as northern Mexico, and would presumably expand northwards across the U.S.–Mexico border. The extrinsic incubation period of pathogens in invertebrate vectors is highly dependent on ambient temperature. Since the lifespan of vector species is relatively constant, changes in the incubation period due to precipitation and temperature significantly alter the likelihood of transmission. Also, large disruption and subsequent movement of human populations create conditions for wider distribution of pathogens and greater exposure to vector species. And, climate change is already affecting the biodiversity of marine and terrestrial ecosystems, which in turn will alter the dynamics of predator–prey relationships, as well as vector and reservoir pathogen populations. This may alter the types and quality of subsistence animal foods, and present dependent communities with new pathogen risks. The time scale of this threat will be continuous unless mitigating measures are taken. Economic and regulatory restrictions continue to slow the development and use of new modes of action against vectors.

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