Perhaps the earliest measurable changes in human population health related to climate change are relatively rapid shifting of patterns in bacterial, viral, fungal, and parasitic diseases. As living organisms strive to adapt to changes in temperature and weather over time, pursuing both immediate survival and longer-term access to resources, shifts occur in the geographic range of plants, animals dependent on plants, animals dependent on animals, and associated disease patterns. For example, vector-borne diseases are caused by microorganisms that are transmitted by spending part of their life cycle in an animal that then interacts with humans. Diseases may appear in previously unaffected areas in one of several ways, including migration of the vector to newly-warmer or wetter ecosystems.
In its quest for a meal, the mosquito must penetrate the skin of animals and access blood, in the process providing access to organisms harbored in its mouthparts. Malaria (Plasmodium genus, Anopheles genus mosquito) and dengue fever (dengue virus, Aedes genus mosquito) are two major diseases transmitted in this way. Malaria is endemic to tropical areas, largely because the lifecycle of the Anopheles mosquito depends on warm, humid climate and pools of warm standing water for maturation of eggs; the malarial parasite itself does not reproduce in colder temperatures. Global and regional rises in average temperature are expected to extend the range into higher altitudes and more northern latitudes. In Central and South America, researchers have documented an increase in malaria associated with warmer temperatures that occur during El Niño affected periods.
Challenges in measuring regional and global infectious disease changes as they relate to temperature and climate alterations are compounded by many other (sometimes interrelated) human activities. For example, diseases with rapid human-to-human transmission may quickly jump large geographic obstacles such as oceans and mountain ranges through rapid transport capability and increases in overall mobility (travel) of human sub-populations.
Water-borne (especially diarrheal) infectious diseases could increase due to changes in temperature and rainfall patterns. Increasing coastal water temperatures may give rise to more frequent toxic algal blooms. Food-borne diseases, such as salmonellosis, also increase in incidence in warmer months, and may have a significant impact in many areas.
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