Malaria is one of the world's most serious and complex public health problems. The disease is caused by four distinct species of plasmodium parasite, transmitted between individuals by Anopheline mosquitoes. Each year, it causes an estimated 400-500 million cases and more than 1 million deaths, mostly in children (WHO, 1998a). Malaria is undergoing a global resurgence because of a variety of factors, including complacency and policy changes that led to reduced funding for malaria control programs in the 1970s and 1980s, the emergence of insecticide and drug resistance, human population growth and movement, land-use change, and deteriorating public health infrastructure (Lindsay and Birley, 1996). Variation in malaria transmission also is associated with changes in temperature, rainfall, and humidity as well as the level of immunity (Lindsay and Birley, 1996). All of these factors can interact to affect adult mosquito densities and the development of the parasite within the mosquito (see Table 9-2).
Very high temperatures are lethal to the mosquito and the parasite. In areas where mean annual temperature is close to the physiological tolerance limit of the parasite, a small temperature increase would be lethal to the parasite, and malaria transmission would therefore decrease. However, at low temperatures, a small increase in temperature can greatly increase the risk of malaria transmission (Bradley, 1993; Lindsay and Birley, 1996).
Micro- and macroenvironmental changes can affect malaria transmission. For example, deforestation may elevate local temperatures (Hamilton, 1989). Changes in types of housing may change indoor temperatures where some vectors spend most of the time resting (Garnham, 1945). In Africa, deforestation, vegetation clearance, and irrigation can all provide the open sunlit pools that are preferred by important malaria vectors and thus increase transmission (Chandler and Highton, 1975; Walsh et al., 1993; Githeko et al., 1996; Lindsay and Birley, 1996).
Malaria currently is present in 101 countries and territories (WHO, 1998a). An estimated 40% (i.e., 2.4 billion people) of the total world population currently lives in areas with malaria. In many malaria-free countries with a developed public health infrastructure, the risk of sustained malaria transmission after reintroduction is low in the near term. Other areas may become at risk as a result of climate change if, for example, malaria control programs have broken down or if transmission currently is limited mainly by temperature. Environmental conditions already are so favorable for malaria transmission in tropical
African countries that climate change is unlikely to affect overall mortality and morbidity rates in endemic lowland regions (MARA, 1998). Furthermore, reductions in rainfall around the Sahel may decrease transmission in this region of Africa (Mouchet et al., 1996; Martens et al., 1999). Future climate change may increase transmission in some highland regions, such as in East Africa (Lindsay and Martens, 1998, Mouchet et al., 1998; Cox et al., 1999; see Box 9-2). Studies that map malaria in Africa indicate that, at the broad scale, distribution of the disease is determined by climate, except at the southern limit (MARA, 1998). Malaria transmission currently is well within the climatic limits of its distribution in mid- to high-latitude developed countries because of effective control measures and other environmental changes. However, in South America the southern limits of malaria distribution may be affected by climate change. The southern geographical distribution limit of a major malaria vector in South America (An. darlingi) coincides with the April mean isotherm of 20°C. If temperature and rainfall increase in Argentina, An. darlingi may extend its distribution in southern Argentina, whereas if rainfall decreases, conditions may become unfavorable for An. darlingi (Carcavallo and Curto de Casas, 1996).
Box 9-2. Have Recent Increases in Highland Malaria been Caused by Climate Warming?
"Highland malaria" usually is defined as malaria that occurs around its altitudinal limit, exhibiting an unstable fluctuating pattern. There has been considerable debate about the causes of the resurgence of malaria in the African highlands. Early in the 20th century, malaria epidemics occurred at elevations of 1,500-2,500 m in Africa, South America, and New Guinea (Mouchet et al., 1998; Reiter, 1998a). Highland malaria in Africa was effectively controlled in the 1950s and 1960s, mainly through the use of DDT and improved medical care. Important changes that have contributed to the subsequent resurgence include changes in land use, decreasing resources for malaria control and treatment, and population growth and movement (Lindsay and Martens, 1998; Malakooti et al., 1998; Mouchet et al., 1998; Reiter, 1998a). There are insufficient historical data on malaria distribution and activity to determine the role of warming, if any, in the recent resurgence of malaria in the highlands of Kenya, Uganda, Tanzania, and Ethiopia (Cox et al., 1999).
That malaria is sensitive to temperature in some highland
regions is illustrated by the effect of El Niño. Increases in
malaria have been attributed to observed El Niño-associated warming
in highland regions in Rwanda (Loevinsohn, 1994) and Pakistan (Bouma
et al., 1996). However, increases in rainfall (sometimes associated
with El Niño) also trigger highland epidemics (e.g., UgandaLindblade
et al., 1999). Lindsay et al. (2000) found a reduction
in malaria infection in Tanzania associated with El Niño when
heavy rainfall may have flushed out Anopheline mosquitoes from their
Most increases in malaria transmission entail single epidemics or a sequence of epidemics that occur over a 1- to 2-year period. Although many epidemics are triggered by transient increases in temperature and/or rainfall, the short time scale of events and the difficulty of linking different epidemics in different parts of the world make it difficult to say if long-term climate change is a factor. Furthermore, there has been little work that identifies where malaria transmission currently is limited by temperature and therefore where highland populations are at risk of malaria as a result of climate change. To determine the role of climate in the increase in highland malaria, a comprehensive research effort is required, together with implementation of a sustainable disease surveillance system that combines trend analyses across multiple sites to account for substantial local factors.
Malaria was successfully eradicated from Australia, Europe, and the United States in the 1950s and 1960s, but the vectors were not eliminated (Bruce-Chwatt and de Zulueta, 1980; Zucker, 1996). In regions where the vectors persist in sufficient abundance, there is a risk of locally transmitted malaria. This small risk of very localized outbreaks may increase under climate change. Conditions currently exist for malaria transmission in those countries during the summer months, but few nonimported cases have been reported (Holvoet et al., 1983; Zucker, 1996; Baldari et al., 1998; Walker, 1998). Malaria could become established again under the prolonged pressures of climatic and other environmental-demographic changes if a strong public health infrastructure is not maintained. A particular concern is the reintroduction of malaria in countries of the former Soviet Union with economies in transition, where public health infrastructure has diminished (e.g., Azerbaijan, Russia).
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