9.7.9. Rodent-Borne Diseases
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).
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.
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).
9.7.10. Water-Related Infectious Diseases
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 weatherfloods or droughtscan 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
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.