Defining climate and weather and their basic relationship to infectious disease transmission

The term weather refers to the state of the atmosphere in a particular place and time, with respect to wind, temperature, cloud, moisture, etc. Weather data are transformed into climate data when averaged over time. That is, climate refers to the weather that prevails in an area over a long period. Climate is what you expect; weather is what you get!

It is helpful to think of climatic conditions as setting the basic spatial and seasonal limits of infectious disease transmission - via the survival and replication of pathogens, the season of activity (pathogen and vector), the geographic range of vectors and hosts, and the behaviors of susceptible human populations. Within these climatic constraints, shorter-term weather can affect the timing and intensity of outbreaks (as with outbreaks of cholera and other diarrheal diseases in the wakes of Hurricane Mitch in 1998, and Hurricane Katrina in 2005).

The distribution of malaria illustrates the importance of climate in determining the geographic regions where the disease has the potential to become established. That is, climatic conditions set the geographic limits of the disease. Malaria is caused by four species of a protozoan parasite (plasmodium), transmitted between humans by the bite of infective female Anopheles mosquitoes. Of the four protozoa that infect humans, two of them predominate overwhelmingly: Plasmodium vivax and P falciparum. Falciparum (which causes far greater mortality than vivax) requires warmer conditions, and predominates in most tropical and sub-tropical regions. Vivax malaria, with its capacity for overwintering dormancy, predominates in cooler, temperate zones. The many environmental factors that affect malaria incidence include altitude (Bodker et al., 2003), topography (Balls et al., 2004), land use, irrigation, and other environmental disturbance (Carlson et al., 2004).

Meanwhile, it is important to stress that various other complex and interacting factors influence the occurrence and prevalence of infectious diseases within climatically suitable envelopes. The most upstream of these factors may well be poverty (Winch, 1998), as the availability of population wealth enables development and maintenance of the public health infrastructure necessary for detecting, tracking, and protecting against infectious diseases, as well as the treatments for managing them. Dengue infection provides a good illustration of how proximal geographic regions that share the same climate and weather, but have widely disparate economic resources, can exhibit significantly different rates of this disease (Reiter et al., 2003; see also Chapter 4).

Historically, vivax malaria occurred widely within temperate zones (e.g. Europe and Scandinavia, North America, Australia), but has been effectively eradicated from those regions for the past half-century. Other social and demographic factors that affect malaria transmission include human population density and immunity;

housing location and condition (screens, air conditioning, piped water); and the use of bed nets, mosquito control programs, and medical treatments.

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