Changing climate conditions may lead to the northward spread of vector-borne infectious diseases and potentially enhanced transmission dynamics as a result of warmer ambient temperatures.
Vector-borne diseases (primarily carried by arthropod or small mammal "vectors") and waterborne diarrheal diseases represent a large proportion of infectious diseases, which are the world's leading cause of fatalities. Projected changes in climate almost certainly would make conditions less suitable for the transmission of several vector-borne diseases (e.g., plague and some forms of encephalomyelitis) in much of their current North American range. Other diseases (e.g., Saint Louis encephalitis and western equine encephalomyelitis) might extend their range northward or exhibit more frequent outbreaks. The crucial factor is the availability of appropriate habitats for vectors and (in the case of zoonotic diseases) vertebrate "maintenance" hosts. Although projected changes in climate might provide opportunities for diseases to extend their range, the North American health infrastructure may prevent a large increase in disease cases; providing this protection, however, could increase the demands on and costs of the current public health system.
The transmission of many infectious diseases is affected by climatic factors. Infective agents and their vector organisms are sensitive to factors such as temperature, surface water, humidity, wind, soil moisture, and changes in forest distribution (IPCC 1996, WG II, Chapter 18).
Malaria: Climatic factors, which increase the inoculation rate of Plasmodium pathogens and the breeding activity of Anopheles mosquitoes, are considered the most important factors contributing to epidemic outbreaks of malaria in nonendemic areas. A temperature relationship for sporadic autochthonous malaria transmission in the temperate United States has been observed in New York and New Jersey during the 1990s (Layton et al., 1995; Zucker, 1996). Common to these two outbreaks was exceptionally hot and humid weather, which reduced the development time of malaria sporozoites enough to render these northern anopheline mosquitoes infectious. Such temperature sensitivity of parasite development also has been observed in the laboratory (Noden et al., 1995).
Martens et al. (1995) estimated that an increase in global mean temperature of several degrees by the year 2100 would increase the vectorial capacity of mosquito populations 100-fold in temperate countries. In these countries, however, continued and increased application of control measures-such as disease surveillance and prompt treatment of cases-probably would counteract any increase in vectorial capacity. Similarly, Duncan (1996) showed that projected increases in mean daily temperatures may allow for the development of malaria in Toronto. It was not suggested, however, that climate alone would permit the spread of malaria because many other factors must be considered.
Malaria once prevailed throughout the American colonies and southern Canada (Russell, 1968; Bruce-Chwatt, 1988). By the middle of the 19th century, malaria extended as far north as 50°N latitude. In Canada, malaria disappeared at the end of the 19th century (Bruce-Chwatt, 1988; Haworth, 1988). Serious malaria control measures were first undertaken in the southern United States in 1912 (Bruce-Chwatt, 1988). By 1930, malaria had disappeared from the northern and western United States and generally caused fewer than 25 deaths per 100,000 people in the South (Meade et al., 1988). In 1970, the World Health Organization Expert Advisory Panel on Malaria recommended that the United States be included in the WHO official register of areas where malaria had been eradicated. The history of malaria in North America reinforces the suggestion that although increased temperatures may lead to conditions suitable for the reintroduction of malaria to North America, socioeconomic factors such as public health facilities will play a large role in determining the existence or extent of such infections.
Arboviruses: Dengue fever and dengue hemorrhagic fever (DHF) periodically have occurred in Texas, following outbreaks in Mexico, during the past two decades (Gubler and Trent, 1994; PAHO, 1994). Because of the sensitivity of dengue to climate, especially ambient temperature, it has been suggested that this disease may increase in the United States if a sustained warming trend occurs. However, due to high living standards, this disease is not likely to increase in incidence or geographic distribution in the United States, even if there is a sustained warming trend. Dengue viruses occur predominantly in the tropics, between 30°N and 20°S latitude (Trent et al., 1983); freezing kills the eggs, larvae, and adults of Aedis aegypte, the most important vector (Chandler, 1945; Shope, 1991). It should be noted, however, that the eggs of Ac. Alhopictus, also a vector, are not killed by freezing.
Jetten and Focks (1997) analyzed the impact of a 2°C and a 4°C temperature
rise on the epidemic potential for dengue, including the impacts for cities
in temperate areas (Figure 8-11). Their analysis shows
that areas adjacent in latitude or elevation to current endemic zones may become
more receptive to viral introductions and enhanced transmission. Furthermore,
their study shows that the proportion of the year when transmission can occur
in North America could significantly increase under warming scenarios.
|Figure 8-11: Weeks of potential dengue transmission under current temperature and 2°C and 4°C warming (adapted from Focks et al., 1995; Jetten and Focks, 1997). Presence of dengue virus, mosquito vector, and exposed human populations are required for disease transmission.|
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