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9.7.1. Malaria
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).
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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., Uganda—Lindblade
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
breeding sites.
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.
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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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