15.2.4.2.2. Rodent-borne diseases
15.2.4.2.2.1. Hantavirus
In 1993, Sin Nombre virus, which causes hantavirus pulmonary syndrome (HPS),
emerged in the Four Corners region of the southwestern United States. Unusually
prolonged rainfall associated with the 1991-1992 El Niño was implicated
as a causal factor in the outbreak (Engelthaler et al., 1999; Glass et
al., 2000). As of 1999, 231 cases had been confirmed in the United States,
with a mortality rate of 42% (Patz et al., 2000). A total of 16 cases
of HPS have been identified in Canada. These cases occurred in British Columbia,
Alberta, and Saskatchewan (Stephen et al., 1994; Werker et al.,
1995; Duncan et al., 1998).
HPS could undergo changes in occurrence related to increased contacts between
rodents and people. Because the virus is present in rodents in the United States
and Canada and changes in climate and ecology are known to affect rodent behavior,
it is assumed that changes in the incidence of HPS would result with global
warming (Duncan et al., 1998), but they will be difficult to predict
because of local rainfall variability.
Adaptive measures to reduce the risks of contracting vector- and rodent-borne
diseases include providing information, vaccination, and drug prophylaxis for
travellers, as well as use of repellants, surveillance, and monitoring (Patz
et al., 2000).
15.2.4.2.2.2. Diseases associated with water
More than 200 million people in the United States have direct access to treated
public water supply systems. Nevertheless, 9 million cases of water-borne diseases
are estimated to occur each year (Bennett et al., 1987). Although most
of the water-borne disease involves mild gastrointestinal illness, some disease
causes severe outcomes, such as myocarditis (Patz et al., 2000).
Giardia occurs in American and Canadian watersheds, resulting in widespread
human exposure (Schantz, 1991; Chow, 1993; Moore et al., 1993; Wallis
et al., 1996; Olson et al., 1997; Duncan et al., 1998).
A study of 1,760 water samples from 72 municipalities across Canada showed that
giardia cysts were found in 73% of the raw sewage samples, 21% of the raw water
samples, and 18.2% of the treated water samples (Wallis et al., 1996).
Cryptosporidium—considered to be one of the most common enteric pathogens
worldwide (Meinhardt et al., 1996)—is less common in Canada than
giardia cysts, however. Cryptosporidium was found in only 6.1% of raw sewage
samples, 4.5% of raw water samples, and 3.5% of treated water samples (Wallis
et al., 1996).
Increases in ambient temperatures, a prolonged summer season, increased heavy
rainfall and/or runoff events, and many watersheds with mixes of intensive agriculture
and urbanization led to recent large outbreaks of cryptosporidium in the United
States (MacKenzie et al., 1994; Goldstein et al., 1996; Osewe
et al., 1996) and the UK (Bridgeman et al., 1995) and may be indicative
of the future. In 1993, more than 400,000 cases (including 54 deaths) resulted
from a cryptosporidium outbreak in the Milwaukee, Wisconsin, water supply (MacKenzie
et al., 1994). A positive correlation between rainfall, cyrptosporidium
oocyst and giardia cyst concentrations in river water, and human disease outbreaks
has been noted (Weniger et al., 1983).
The largest ever reported outbreak of toxoplasmosis was traced to the municipal
water supply of the greater Victoria area of British Columbia (British Columbia
Toxoplasmosis Team, 1995; Den Hollander and Noteboom, 1996; Mullens, 1996; Bowie
et al., 1997; Duncan et al., 1998). There already is evidence
that exposure to the causative parasite in Canada is widespread (Tizard et
al., 1977, 1978). Given the increasing number of feral cats in Canada and
the persistence of sporulated oocysts in a variety of environments (Dubey and
Beattie, 1988), areas in Canada that are hospitable to oocyst survival are likely
to expand as a result of climate change.
Warm marine water may favor growth of toxic organisms such as red tides, which
cause three varieties of shellfish poisoning: paralytic, diarrheic, and amnesic.
Domoic acid—a toxin produced by the Nitzchia pungens diatom that causes
amnesic shellfish poisoning—appeared on Prince Edward Island for the first
time in 1987. A total of 107 patients were identified, of whom 19 were hospitalized.
Of those requiring hospitalization, 12 people required intensive care because
of coma, profuse respiratory secretions, or unstable blood pressure. A total
of four people died as a result of eating contaminated mussels (Perl et al.,
1990). The outbreak coincided with an El Niño year, when warm eddies
of the Gulf Stream neared the shore and heavy rains increased nutrient-rich
runoff (Glavin et al., 1990; Perl et al., 1990; Teitelbaum et
al., 1990; Hatfield et al., 1994; Shriner and Street, 1998). In the
United States, marine-related illness increased during El Niño events
over the past 25 years. During the strong El Niño event of 1997-1998,
precipitation and runoff greatly increased counts of fecal bacteria and viruses
in local coastal waters in Florida (Harvell et al., 1999). Contamination
of water bodies by animal and human wastes can stimulate harmful algae such
as Pfiesteria that have been demonstrated to cause illness in humans and death
in some species of fish (Burkholder et al., 2000).
Potential adaptive measures to reduce water-borne disease include improved
water safety criteria, monitoring, treatment of surface water, and sewage/sanitation
systems (Patz et al., 2000). Land-use management should include consideration
of water supply and quality.
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