15.2.4.2. Potential Indirect Health Impacts of Climate Change

15.2.4.2.1. Vector-borne diseases

15.2.4.2.1.1. Encephalitis

In the midwestern United States, outbreaks of St. Louis encephalitis (SLE) appear to be associated with a sequence of warm, wet winters; cold springs; and hot, dry summers (Monath, 1980). In the western United States, a 3-5°C increase in mean temperature may cause a northern shift in the distribution of western equine encephalitis (WEE) and SLE outbreaks and a decrease in the range of WEE in southern California (Reeves et al., 1994). In Canada, the ranges of eastern equine encephalitis (EEE), snowshoe hare virus (SHV), and WEE probably would expand with global warming. All three already have been reported in Canada or adjacent U.S. states, albeit sporadically (McLean et al., 1985; Artsob, 1986; Tourangeau et al., 1986; Keane and Little, 1987; Heath et al., 1989; Carman et al., 1995; Duncan et al., 1998).

15.2.4.2.1.2. Malaria

In the United States, sporadic autochthonous malaria transmission was observed in New York and New Jersey during the 1990s (Layton et al., 1995; Zucker, 1996). Malaria is imported into Canada, however.

According to studies by Martens et al. (1995), Martin and Lefebvre (1995), and Duncan (1996), with global warming, malaria may extend northward into temperate countries. These studies note, however, that many of these countries had regular epidemics of malaria in the 19th century and the first half of the 20th century and that continued and increased application of control measures—such as water management, disease surveillance and prompt treatment of cases—probably would counteract any increase in vectorial capacity.

Malaria once prevailed throughout the United States and southern Canada (Bruce-Chwatt, 1988). As recently as 1890, the census recorded more than 7,000 malaria deaths per 100,000 people across the American South and more than 1,000 malaria deaths per 100,000 people in states such as Michigan and Illinois. It is important to note that diagnoses and reporting did not meet today's standards. By 1930, malaria had been controlled in the northern and western United States and generally caused fewer than 25 deaths per 100,000 people in the South. In 1970, the World Health Organization (WHO) Expert Advisory Panel on Malaria recommended that the United States be included in the WHO official register of areas where malaria had been eradicated.

In Canada, vivax malaria became widespread at the end of the 18th century, when refugees from the southern United States settled in large numbers as far north as "the Huron" in the aftermath of the American War of Independence. Malaria was further spread with the building of the Rideau Canal (1826-1832) (Duncan, 1996). By the middle of the 19th century, malaria extended as far north as 50°N. In 1873, the great malarious district of western Ontario was only a fraction of a large endemic area, extending between Ontario and the state of Michigan. In Canada, the disease disappeared at the end of the 19th century (Bruce-Chwatt, 1988; Haworth, 1988; Duncan, 1996).

The history of malaria in North America underscores the fact that increased temperatures may lead to conditions that are suitable for the reintroduction of malaria to North America. Socioeconomic factors such as public health measures will continue to play a large role in determining the existence or extent of such infections (Shriner and Street, 1998).

15.2.4.2.1.3. Dengue and Yellow Fever

Although the Aedes aegypti mosquito already is found in the southern United States, socioeconomic factors play a large role in determining the actual risk of climate-sensitive diseases (see Chapter 9). For example, socioeconomic differences between Texas and bordering Mexico determine disease incidence: A total of 43 cases of dengue were recorded in Texas during 1980-1996, compared to 50,333 in the three contiguous border states in Mexico (Reiter, 1999).

15.2.4.2.1.4. Lyme Disease and Rocky Mountain Spotted Fever

Lyme disease—the most common vector-borne disease in the United States—currently circulates among white-footed mice in woodland areas of the Mid-Atlantic, Northeast, upper Midwest, and West Coast of the United States (Gubler, 1998). In 1994, more than 10,000 cases of the disease were reported (Shriner and Street, 1998). Although possible tick vectors have been reported in various parts of Canada, self-reproducing populations of infected ticks are believed to occur only in Long Point, Ontario (Barker et al., 1992; Duncan et al., 1998). Lyme disease has been predicted to spread within Canada with increased temperatures (Grant, 1991; Canadian Global Change Program, 1995; Environment Canada, 1995; Guidotti, 1996; Hancock and Davies, 1997). However, in assessing climate-induced risks for Lyme disease, the ecology of two mammalian species along with projections for land use make predictions very difficult (see Chapter 9). Finally, Grant (1991) has suggested that, with warming, Rocky Mountain Spotted Fever might increase in some localities in Canada.

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