Rodent-borne diseases are zoonoses that are transmitted directly to humans by contact with rodent urine, feces, or other body fluids (Mills and Childs, 1998; Peters, 1998). Rodents are principle hosts for arthropod vectors such as fleas (see Section 9.7.7) and ticks (see Section 9.7.8). Environmental factors that affect rodent population dynamics include unusually high rainfall, drought, and successful introduction of exotic plant species. Rodent-borne pathogens are affected indirectly by ecological determinants of food sources that affect rodent population size (Williams et al., 1997; Engelthaler et al., 1999).

9.7.9.1. Leptospirosis

Leptospirosis is an acute febrile disease caused by the bacteria Leptospira. It probably is the most widespread zoonotic disease in the world and is particularly common in the tropics (PAHO, 1998). Infection is caused by exposure to water, damp soil, or vegetation contaminated with the urine of infected wild and domestic animals (e.g., rodents and dogs) (Thiermann, 1980). Outbreaks often occur after heavy rainfall and during floods (Kriz et al., 1998; Trevejo et al., 1998). Therefore, any increase in flooding associated with climate change may affect the incidence of this disease.

9.7.9.2. Hantaviruses

Several hantaviruses are capable of causing severe, often fatal, illness in humans (PAHO, 1998). Each has a specific geographic distribution that is determined by that of the primary rodent host (Schmaljohn and Hjelle, 1997). Humans are infected by aerosol exposure to infectious excreta or occasionally by bites. The better known of these diseases are hemorrhagic fever with renal syndrome, caused by Hantaan virus, in China and Korea and hantavirus pulmonary syndrome in the Americas, caused by several viruses that are specific to their rodent host (Schmaljohn and Hjelle, 1997). Outbreaks of disease may be associated with weather that promotes rapid increases in rodent populations, which may vary greatly between seasons and from year to year (Glass et al., 2000). Many hantavirus infections occur in persons of lower socioeconomic status, where poorer housing and agricultural activities favor closer contact between humans and rodents (Schmaljohn and Hjelle, 1997). Arenaviruses (Lassa, Junin, Machupo, etc.), which are ecologically similar to hantaviruses, may respond similarly (Mills and Childs, 1998).

There are complex relationships between human health and problems of water quality, availability, sanitation, and hygiene. Predicting the potential impacts of climate change on water-related diseases therefore is difficult because access to a clean safe water supply is determined primarily by socioeconomic factors. Extreme weather—floods or droughts—can increase the risk of disease via contamination of water resources, poor hygiene, or other mechanisms. Currently, the World Health Organization (WHO) estimates that more than 1 billion people worldwide are without access to safe drinking water and that every year as many as 4 million die prematurely because they do not have access to safe drinking water and sanitation. Increases in water stress are projected under climate change in certain countries (see Chapter 4), but it is difficult to translate such indicators directly into the attributable risk for water-related diseases. Water scarcity may necessitate use of poorer quality sources of freshwater, such as rivers, which often are contaminated. Decreases in water supplies could reduce the water available for drinking and washing and lower the efficiency of local sewerage systems, leading to increased concentration of pathogenic organisms in raw water supplies.

Excessive precipitation can transport terrestrial microbiological agents into drinking-water sources. For example, some outbreaks of cryptosporidiosis, giardia, and other infections have been triggered by heavy rainfall events in the UK and United States (Lisle and Rose, 1995; Atherholt et al., 1998; Rose et al., 2000; Curriero et al., 2001). Significant correlation between the cumulative monthly distribution of cholera cases and the monthly distribution of precipitation has been observed in Guam (Borroto and Haddock, 1998). In many countries, handling of sewage is not separate from the drainage system for stormwaters. It is important that water resource management can adapt to changes in the frequency of precipitation extremes to minimize the risk of microbiological contamination of the public water supply.

Cholera is a water- and food-borne disease and has a complex mode of transmission. In tropical areas, cases are reported year-round. In temperate areas, cases are reported mainly in the warmest season. The seventh cholera pandemic currently is spreading across Asia, Africa, and South America. A new serogroup (V. cholerae O139) appeared in 1992 and is responsible for large epidemics in Asia. During the 1997-1998 El Niño, excessive flooding caused cholera epidemics in Djibouti, Somalia, Kenya, Tanzania, and Mozambique (WHO, 1998b). Birmingham et al. (1997) found a significant association between bathing and drinking water from Lake Tanganyika and the risk of infection with cholera. Warming in the African Great Lakes may cause conditions that increase the risk of cholera transmission in the surrounding countries (WHO, 1998b). See Section 9.8 for a discussion of cholera in coastal waters.

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