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Hydro-Epidemiology of the Nile Basin: Understanding the Complex Linkages Between Water and Infectious Diseases

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The human population of the Nile basin has been vulnerable to water-associated diseases since the dawn of history. In the modern landscape, water development projects and expanded irrigation are considered vital for increasing agricultural productivity and improving the socioeconomic status of rural communities. However, these projects also have the potential to modify hydrological processes in a way that increases the risk of water-associated diseases. To explore these interactions, we first outlined the major hydrological determinants of three important water-associated diseases within the Nile basin: cholera, a water-borne disease; schistosomiasis, a water-based disease; and malaria, a water-related disease. We then reviewed the scientific literature that has examined the influences of dams, irrigation schemes, and other water-management practices on these diseases within the Nile basin. Our synthesis of the literature emphasizes the importance of integrating public health concerns into the planning of new water development projects in the Nile basin and also highlights the potential for utilizing the underlying hydro-epidemiological relationships to enhance mapping and forecasting of water-associated disease risk under current and future climates. © 2014 Springer International Publishing Switzerland. All rights reserved.
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Chapter 12
Hydro-Epidemiology of the Nile Basin:
Understanding the Complex Linkages
Between Water and Infectious Diseases
Michael C. Wimberly and Alemayehu A. Midekisa
Abstract The human population of the Nile basin has been vulnerable to water-
associated diseases since the dawn of history. In the modern landscape, water
development projects and expanded irrigation are considered vital for increasing
agricultural productivity and improving the socioeconomic status of rural communi-
ties. However, these projects also have the potential to modify hydrological processes
in a way that increases the risk of water-associated diseases. To explore these inter-
actions, we first outlined the major hydrological determinants of three important
water-associated diseases within the Nile basin: cholera, a water-borne disease;
schistosomiasis, a water-based disease; and malaria, a water-related disease. We then
reviewed the scientific literature that has examined the influences of dams, irrigation
schemes, and other water-management practices on these diseases within the Nile
basin. Our synthesis of the literature emphasizes the importance of integrating public
health concerns into the planning of new water development projects in the Nile basin
and also highlights the potential for utilizing the underlying hydro-epidemiological
relationships to enhance mapping and forecasting of water-associated disease risk
under current and future climates.
Keywords Hydro-epidemiology ·Nile River basin ·Infectious diseases ·Cholera ·
12.1 Introduction
The Nile basin encompasses one of the largest and most important river drainages
in the world, covering an area greater than 3,400,000 km2and including portions
of 11 countries. Because of its vast size, this area incorporates a broad range of
environmental conditions from deserts to tropical rainforests. The Nile River has
M. C. Wimberly ()·A. A. Midekisa
Geospatial Sciences Center of Excellence,
South Dakota State University, Brooking, SD 57007, USA
A. M. Melesse et al. (eds.), Nile River Basin, DOI 10.1007/978-3-319-02720-3_12, 219
© Springer International Publishing Switzerland 2014
220 M. C. Wimberly andA. A. Midekisa
enormous historical significance; its flows provided the water and nourished the
soils that supported one of the earliest human civilizations in Egypt. Water-associated
diseases, including schistosomiasis and malaria, have been documented in ancient
Egypt and have impacted human populations in this region throughout history (Contis
and David 1996; Nerlich et al. 2008). Today, some of the poorest and most vulnerable
human populations in the world live within the Nile basin, and this region is a
focal point for drought, famine, and infectious disease outbreaks. These crises are
frequently intertwined and are often closely linked to water issues that are exacerbated
by climatic variability and human land-use activities. The purpose of this chapter
is to explore the complex interrelationships between hydrological processes and
infectious diseases in the Nile basin through a review and synthesis of the scientific
Many diseases are associated with water through a variety of pathways, and they
can be categorized based on the specific mechanisms for these interactions. Three
major groups of water-associated diseases (water-borne diseases, water-based dis-
eases, and water-related diseases) are highly prevalent in the Nile basin and will
be the focus of this synthesis (Yang et al. 2012). Water-borne diseases are caused
by microorganisms or chemicals that are directly transmitted via ingestion of con-
taminated water. Well-known examples include cholera, caused by the bacterium
Vibrio cholerae; dysentery, caused by the bacterium Shigella dysenteriae and other
bacteria in the genera Shigella and Salmonella; and typhoid fever, caused by the
bacterium Salmonella typhi.Water-based diseases are caused by parasitic organisms
that spend at least part of their life cycle in the water. They include schistosomi-
asis (also known as bilharzia) caused by trematode worms that utilize snails as an
intermediate aquatic host and guinea worm disease (dracunculiasis) caused by the
nematode worm Dracunculus medinensis which utilizes copepods as intermediate
aquatic hosts. Water-related diseases are those for which water is needed to support
the breeding of the insect vectors that transmit the disease. The most well known and
widespread of these vectors are mosquitoes, and numerous mosquito-borne diseases
occur within the Nile basin. These include malaria, caused by plasmodium micropar-
asites; yellow fever, caused by a flavivirus; lymphatic filariasis, caused by several
species of nematode worms; and Rift Valley fever, caused by a phlebovirus.
Other types of water-associated diseases, including water-carried, water-washed,
and water-dispersed diseases, will not be considered in this review because their
overall prevalence in the Nile basin is low relative to the three major categories
outlined above (Yang et al. 2012). However, these types of diseases may be lo-
cally prevalent in particular areas within the basin. Hydrological processes also have
important, although more indirect, effects on a variety of other disease agents. Soil-
transmitted helminthic infections are caused by multiple species of parasitic worms
that are transmitted through direct contact with contaminated soil, and the risk of these
infections is at least partially dependent on temperature, precipitation, soils, physiog-
raphy, and other factors that influence the environmental reservoirs of these parasites
(Bethony et al. 2006). Ticks are an important disease vector in the Nile basin as well
as the rest of Africa, transmitting a variety of diseases to both humans and animals
(e.g., Maina et al. 2012). Unlike mosquitoes, ticks do not directly depend on aquatic
12 Hydro-Epidemiology of the Nile Basin 221
habitats to complete their life cycles. However, they spend most of the off-host por-
tion of their life cycle at or near the soil surface and are therefore highly sensitive to
climate and its effects on moisture in the upper soil and litter layers (Olwoch et al.
Because of the growing human population and increasing demand for food pro-
duction and energy in Africa, there is a need to expand the availability of water
resources (Boelee and Madsen 2006). This need is being addressed through a vari-
ety of water development projects, including the construction of irrigation schemes,
dams, and water storage facilities such as tanks, ponds, and reservoirs. Irrigation
and water storage can increase the quantity and stability of water availability for
drinking and agriculture, and hydropower dams can additionally supply electric-
ity and help alleviate energy shortages. These water development projects have
the potential to expand considerably in the future. Whereas the total irrigated area
in the African continent is currently 12.2 million hectares, the irrigation potential
of the continent is 42.5 million ha (Boelee and Madsen 2006). However, there is
also growing evidence that these water resource development projects can facilitate
the transmission of water-associated diseases by providing favorable environments
for vectors, hosts, and pathogens (Boelee and Madsen 2006; Keiser et al. 2005;
Steinmann et al. 2006). As a result, there is a need to consider the health impacts of
these modifications to the hydrological system and develop new strategies that can
help to maximize the economic benefits of these projects while minimizing infectious
disease risk (McCartney et al. 2007). Improving our understanding of the underlying
hydro-epidemiology of water-associated diseases and their potential connections to
water resource management will be necessary to achieve this goal.
In the next section, we provide a more in-depth assessment of three important
diseases in the Nile basin that all have a major hydro-epidemiological component.
To cover the breadth of water–disease interactions, we have selected one disease from
each of the three major categories of water-associated diseases that are prevalent in the
Nile basin: cholera, a water-borne disease; schistosomiasis, a water-based disease;
and malaria, a water-related disease. Our goal in presenting these case studies is
to highlight specific aspects of the life cycles of these pathogens that are linked
to the climatic drivers, hydrological processes, and human actions that control the
distribution of water in the environment. A separate section focuses on the health
impacts of water resource development, followed by synthesis and conclusions.
12.2 Case Studies of Water-Associated Diseases
12.2.1 Cholera
Cholera is a water-borne disease caused by the bacterium V. cholerae, for which
humans are the only known animal host. During cholera epidemics, the disease is
transmitted through ingestion of contaminated food and water, and high pathogen
loads are sustained through fecal contamination of wells, rivers, lakes, and other
222 M. C. Wimberly andA. A. Midekisa
sources of drinking water. In the periods between outbreaks, aquatic ecosystems can
serve as natural reservoirs for V. cholerae. The bacterium can survive and grow in
riverine, estuarine, and coastal environments where it is associated with a variety
of flora and fauna, including phytoplankton, algae, and zooplankton (Hunter 1997).
Favorable conditions for cholera include warm temperatures that exceed 10C for a
period of several weeks and estuarine and marine environments with salinity ranging
from 5–30 parts per thousand. However, field and laboratory studies have suggested
that vibrios can also survive in freshwater with high water temperature and elevated
organic nutrient concentrations (West 1989).
Historically, the major environmental reservoirs for V. cholerae have existed in
South Asia, particularly the Bay of Bengal from which cholera has spread worldwide
in a series of modern pandemics (Mutreja et al. 2011; Faruque et al. 1998). The
seventh global pandemic began in 1961 with widespread outbreaks across Asia. In
1971, cholera reemerged in Europe and Africa for the first time in more than 100
years. By the early 1990s, the cholera pandemic had reached the Americas and also
resurged across the African continent. Although cholera has subsided across most
of the globe, including Asia, Europe, and the Americas, similar declines have not
been observed in Africa (Naidoo and Patric 2002; Gaffga et al. 2007). The large
number of cases and high levels of endemicity across Africa suggest that cholera
is now entrenched across the continent. Published data on cholera case numbers,
incidence, and endemicity all suggest that the countries of the Nile basin occupy one
of the geographic hot spots for cholera in Africa. Between 2000 and 2008, all of
countries encompassed by the Nile basin except Egypt reported cholera cases (Ali
et al. 2012). In particular, the Great Lakes region ofAfrica, including lakes Victoria,
Edward, and Albert in the upper Nile basin, has been highlighted as an important
focus of continuous cholera outbreaks since the late 1970s (Nkoko et al. 2011).
Cholera thrives in dense human populations with high levels of poverty and limited
supplies of safe drinking water. Poor sanitation and resulting fecal contamination
of drinking water and food are well known to be major risk factors for cholera
(Tumwine et al. 2002). As a result, communities with low socioeconomic status
that lack adequate health-care systems are particularly at risk (Olago et al. 2007).
Heavy rainfall and associated flooding are also widely recognized as important risk
factors for cholera outbreaks. Floods cause direct contamination of water supplies
and also create humanitarian crises as a result of population displacements that lead
to non-sanitary conditions with limited access to clean drinking water. However,
only a small percentage of cholera outbreaks from 1995–2005 were associated with
flooding in East Africa as compared toWest and South Africa (Griffith et al. 2006). In
contrast, refugee camps and other internal population displacements were associated
with a higher percentage of cholera outbreaks in East Africa compared to other parts
of the continent. For example, one of the worst cholera outbreaks of the seventh
pandemic occurred in the vicinity of Goma, a city in the Democratic Republic of the
Congo located on the shores of Lake Kivu and just across the border from Rwanda
(Echenberg 2011). A main cause of this outbreak was the Rwandan genocide, which
led to major population displacements and the establishment of crowded refugee
camps with unsanitary conditions that facilitated the rapid spread of the disease.
12 Hydro-Epidemiology of the Nile Basin 223
The mechanisms through which cholera has been sustained in Africa are not
completely understood, but a variety of environmental factors have been hypoth-
esized to play a role. Seasonal patterns of cholera cases are associated with the
seasonality of rainfall, and interannual variability in cholera cases is associated with
temperature and rainfall anomalies (Nkoko et al. 2011;Paz2009). In particular,
increases in cholera outbreaks have been found to occur during the warmer-than-
normal conditions that prevail during El Nino events (Nkoko et al. 2011; Olago
et al. 2007). Multiple studies conducted in the headwaters region of the Nile basin
have found spatial concentrations of cholera cases around lakes, suggesting that
these aquatic environments may serve as temporary reservoirs for cholera between
outbreaks (Bompangue et al. 2008; Nkoko et al. 2011; Shapiro et al. 1999).
12.2.2 Schistosomiasis
Schistosomiasis (also known as bilharzia) encompasses an array of diseases caused
by trematode worms that utilize aquatic snails as obligate intermediate hosts. Africa
is the home of 85 % of the global population at risk of schistosomiasis, and these
populations account for 97 % of all infections worldwide (Steinmann et al. 2006). Of
the five major species of schistosomes, Schistosoma haematobium and S. mansoni
are both distributed widely throughout the Nile basin (Gryseels et al. 2006; Schur
et al. In Press). S. haematobium is transmitted by snails in the genus Bulinus and
causes urinary schistosomiasis, whereas S. mansoni is transmitted by snails in the
genus Biomphalaria and causes intestinal schistosomiasis. Manifestations of schis-
tosomiasis in humans range from acute disease that primarily infects travelers with
no acquired immunity to anemia, stunting, liver disease, increased cancer risk, and
a variety of chronic ailments that afflict population in areas where schistosomiasis is
endemic (King and Dangerfield-Cha 2008; Gryseels et al. 2006). Chronic schistoso-
miasis infections can cause significant disability and impose substantial social and
economic burdens on the affected communities (King 2010).
Schistosomes have a complex life cycle in which eggs are shed by the human
hosts in urine or feces. The eggs hatch in freshwater, releasing miracidia that invade
the intermediate hosts, which are specific species of freshwater snails. The miracidia
then multiply asexually in the snails to form sporocysts that are released into the
water as cercariae that penetrate the skin of human hosts and cause infection. Be-
cause the parasite is dependent on the intermediate host to complete its life cycle, the
occurrence of schistosomiasis is constrained by the availability of suitable aquatic
habitats. The host snails are sensitive to a variety of environmental conditions includ-
ing water chemistry, depth, flow, turbidity, shading, and the characteristics of aquatic
vegetation (Hunter 1997). Although these snails are generally associated with slow-
moving water, specific habitat associations depend on the snail species. For example,
Biomphalaria sudanica is typically found in shallower, vegetated habitats located
in marshes or near lakeshores, whereas other species such as B. choanomphala
224 M. C. Wimberly andA. A. Midekisa
and B. stanleyi are associated with deeper lacustrine habitats (Kazibwe et al. 2006;
Standley et al. 2012).
Transmission of schistosomiasis requires direct human contact with water, and
thus the risk of schistosomiasis depends on human behavior in addition to the aquatic
environment. Infection occurs through direct human contact with pathogen-laden
water sources, which often occurs when gathering water for drinking and cooking.
As a result, piped water, laundry and shower facilities, and other improvements that
reduce human contact with water bodies have been shown to reduce the prevalence
and severity of schistosomiasis infections (Esrey et al. 1991). In addition, poor
sanitation and associated contamination of water bodies with feces and urine lead
to dispersal of eggs into the aquatic environment. Therefore, the development of
latrines and other sanitation projects can also limit the transmission of schistosomes
into water bodies and reduce the risk of human infection (Esrey et al. 1991).
12.2.3 Malaria
Malaria, a mosquito-borne disease caused primarily by the microparasites Plasmod-
ium vivax and Plasmodium falciparum, is one of the most common infectious diseases
in the world and is a major public health problem throughout much of the southern
portion of the Nile basin (Fig. 12.1). With the exception of the headwaters of the
White Nile, much of the region has a relatively low prevalence of malaria infection
and can be characterized as mesoendemic (regular but highly seasonal transmission)
or hypodendemic (intermittent transmission). Large malaria epidemics occur most
frequently in highland and semiarid regions and are often associated with interan-
nual fluctuations in rainfall and temperature. These epidemics can be particularly
devastating because they occur in areas where large portions of the population lack
immunity to malaria (Abeku 2007).
Malaria is transmitted between human hosts by anophelene mosquito vectors
that depend on water for egg laying and larval development. Their specific habitat
requirements vary among species (Sinka et al. 2010). For example, Anopheles gam-
biae breeds in temporary, sunlit pools with relatively low levels of vegetation. In
contrast, Anopheles arabiensis is generally associated with drier areas, breeds in a
wider range of habitats than A. gambiae, and is more likely to bite outdoors and to
bite animals than A. gambiae.Anopheles funestus, another important malaria vector,
is associated with naturally occurring habitats such as wetlands and lakeshores with
emergent vegetation and a mix of sunlit and shaded environments. These species
exhibit different geographic ranges within the Nile basin, and the distinctive bio-
nomics of these species thus have implications for malaria transmission in different
regions (Fig. 12.2). In particular, A. arabiensis and A. funestus are all more broadly
distributed within the Nile basin than A. gambiae, which is limited to the southern
portion of the basin.
12 Hydro-Epidemiology of the Nile Basin 225
Fig. 12.1 a The Nile basin with major rivers, lakes, and country boundaries. bPlasmodium falci-
parum parasite rate for ages 2–10 (PfPR). cPlasmosium vivax parasite rates for ages 2–100 (PvPR).
Malaria maps were obtained from the Malaria Atlas Project (Gething et al. 2012; Gething et al.
The strong seasonality of malaria across much of the Nile basin is tightly linked
with its monsoon climates and their effects on both the mosquito vector and the plas-
modium parasite (Cheung et al. 2008; Nicholson 1996). In particular, the highest
seasonal rates of malaria incidence and the most severe malaria outbreaks have his-
torically occurred following major rainy seasons. In the Amhara region of Ethiopia,
the long rains (Kirmet) extend from June through September and provide the major-
ity of total annual precipitation. In an analysis of historical surveillance data from
2000–2010, the monthly numbers of malaria cases from September to December
were higher than any other months of the year, with case numbers peaking in Oc-
tober (Wimberly et al. 2012). Other regions of East Africa also exhibit distinctive
226 M. C. Wimberly andA. A. Midekisa
Fig. 12.2 Probability of occurrence of three major malaria vector species in the Nile basin. a
Anopheles gambiae. bAnopheles arabiensis. cAnopheles funestus. Mosquito maps were obtained
from the Malaria Atlas Project (Hay et al. 2010)
patterns of seasonality. In the highlands of Kenya and Uganda, the dry season ex-
tends from June to October with a period of short rains occurring from October to
December and the long rains occurring from March to May. In these areas, there is
often a smaller early seasonal peak in malaria cases following the short rains, and
another larger peak of malaria cases is during the main epidemic season following
the long rains (Pascual et al. 2008).
Within the Nile basin, more localized patterns of mosquito abundance and malaria
risk are associated with geomorphic landscape characteristics and their influences
on hydrological processes and the resulting prevalence of mosquito habitats. For ex-
ample, malaria incidence is often highest in valley bottoms or close to wetlands than
in drier portions of the landscape (Cohen et al. 2008; Ernst et al. 2006). Similarly,
12 Hydro-Epidemiology of the Nile Basin 227
temporal patterns of malaria incidence are linked with seasonal and interannual fluc-
tuations in temperature and precipitation (Teklehaimanot et al. 2004; Alonso et al.
2011; Pascual et al. 2008; Midekisa et al. 2012). These relationships reflect the
influences of moisture on breeding habitats of mosquitoes and the influences of tem-
perature on developmental rates for both mosquitoes and the malaria parasite (Mbogo
et al. 2003; Koenraadt et al. 2004). However, the strength of the relationship between
weather and malaria cases, the relative importance of different weather variables, and
the time lag at which outbreaks can be predicted all vary with geographic location
(Zhou et al. 2004; Mbogo et al. 2003). In particular, the relative importance of tem-
perature and precipitation for predicting malaria cases in Ethiopia has been found to
vary in cold versus hot environments and in urban versus rural areas (Teklehaimanot
et al. 2004). In general, it is expected that precipitation is likely to be the major
environmental driver of malaria outbreaks in semiarid regions, whereas the effects
of temperature are greater in cooler highland areas (Abeku 2007).
12.3 Water Resource Development and Health Impacts
Water resource development projects such as dams, irrigation canals, and water-
harvesting schemes in Africa have paved the way for expanded generation of
electricity, helped to control flooding, opened new opportunities for arable land,
fostered expansion of urbanized areas, and generally improved the standard of living
for people in the vicinities of these projects (Fenwick 2006). In particular, there has
been demand for the increased construction of dams in the Nile basin (McCartney and
King 2011). However, these projects also affect hydrology in ways that can expand
habitats of the mosquitoes that transmit vector-borne diseases and the aquatic hosts
of water-borne diseases (Fenwick 2006; Lammie et al. 2006; McCartney and King
2011). These changes may increase the risk of water-associated diseases, including
guinea worm, schistosomiasis, lymphatic filariasis, and malaria (Lammie et al. 2006;
Keiser et al. 2005; Steinmann et al. 2006). Therefore, the potential health impact on
inhabitants living nearby water resource development projects should be taken in to
account when planning these projects.
The interaction of water resources development, economic development, and risk
of mosquito-borne diseases is a complex phenomenon, as emphasized in the “paddies
paradox” highlighted by Ijumba and Lindsay (2001). The increased agricultural pro-
ductivity and associated economic development that result from irrigation projects
can improve the economic status of the community, leading to better health-care
access and increased use of bednets and other preventive measures in the affected
areas. As a result, even though irrigation projects can lead to large increases in vector
abundance, they generally do not pose a risk to communities with high level of immu-
nity in places of stable malaria transmission (Keiser et al. 2005; Ijumba and Lindsay
2001). On the contrary, irrigation and associated increases in mosquito abundance
pose a far greater risk in highland and semiarid areas of unstable malaria transmis-
sion where inhabitants have low immunity to the disease. In the highland Ruizizi
228 M. C. Wimberly andA. A. Midekisa
Valley of Burundi, villages in close proximity to irrigation sites had higher vectorial
capacity and elevated malaria prevalence compared to villages located farther from
irrigation sites (Coosemans 1985). In the semiarid Ziway area of central Ethiopia, an
irrigated village similarly had higher malaria prevalence than a nearby nonirrigated
village (Kibret et al. 2010). Much of the Nile basin is considered to have low malaria
prevalence and unstable mesoendemic or hypoendmic transmission (Fig. 12.1), and
it can be expected that there is a potential for irrigation and other water resource
management to increase malaria risk in these areas.
Several studies have also reported linkages between dams and malaria in the Nile
basin. Although the large impoundments created by dams do not provide a suitable
breeding habitat for the anophelene mosquitoes that transmit malaria, dams can raise
groundwater levels, create puddles at the edge of the impoundment as water levels
are drawn down, and lead to water seepage that creates swampy habitats below the
dam (Lautze et al. 2007). These effects are exacerbated by the fact that artificial
impoundments may lengthen the season over which breeding habitats are available.
Increased dam construction in the Uasin Gishu highlands in Kenya was associated
with greater risk of malaria transmission (Khaemba et al. 1994). An assessment of the
Turkwel Gorge hydroelectric dam of Kenya also reported that there was an increase
in malaria risk for inhabitants living in close proximity to the reservoir following
the construction of the dam (Renshaw et al. 1998). In the Rift Valley region of
Ethiopia, malaria cases for residents who lived within 3km of the Koka Dam were
1.5 times as high as for residents living 3–6 km from the reservoir (Lautze et al. 2007).
Another study in the Tigray region of northern Ethiopia found that the incidence of
malaria was almost seven times higher for villages in close proximity to dams as
compared to villages farther from dam sites (Ghebreyesus et al. 1999). In addition to
dams, other types of water-management activities used to support irrigation can also
impact malaria risk. For example, a study in the central highlands of Ethiopia found
that increasing rainwater harvesting was perceived by local residents to be associated
with a longer malaria transmission season and, consequently, a higher risk of malaria
(Kassahun 2008).
Dams and associated irrigation projects increase the risk of water-associated dis-
ease transmission by providing suitable habitats for the snails that are hosts for
schistosomiasis (Steinmann et al. 2006). Irrigation also increases the potential for
schistosomiasis transmission by providing more opportunities for human–water con-
tact and increasing the potential for contamination of water with urine and feces. The
construction of the Aswan Dam in Egypt led to a year-round irrigation scheme that
increased populations of snails that are the hosts for schistosomiasis, resulting in
higher level of infection in the human population (Watts and El Katsha 1997). The
Gezira Agricultural Scheme in Sudan, one of the oldest water resource develop-
ment projects in the Nile basin, was completed in 1924 and provides electricity and
irrigation south of Khartoum (Boelee and Madsen 2006). Although this irrigation
scheme has greatly enhanced the production of cash crops such as cotton, it has also
significantly increased the prevalence of both urinary and intestinal schistosomiasis
in the human population. In southern and central Tigray, Ethiopia, the prevalence of
S. mansoni in humans was higher in irrigated areas than in nonirrigated areas and
12 Hydro-Epidemiology of the Nile Basin 229
higher in areas with a long history of irrigation than in recently constructed irrigation
schemes (Dejenie and Petros 2009). Similarly, schistosomiasis prevalence increased
from 0 to 70 % following the implementation of the Mwea irrigation scheme in Kenya
(Renshaw et al. 1998).
12.4 Synthesis and Conclusions
There are a number of prevalent water-borne diseases within the Nile basin that sig-
nificantly impact the health of this region’s human population. Each of these diseases
is connected with hydrological processes through a distinctive set of causal pathways.
In particular, vector-borne or zoonotic diseases such as malaria or schistosomiasis
are highly dependent on the influences of hydrology on the specific habitats of their
respective vectors and hosts.As a result, it can be difficult to make a broad general-
ization about water-borne diseases. However, this review has documented a number
of commonalities that can begin to provide a framework for understanding this suite
of diseases and developing strategies to improve prevention, control, and elimination
The three major diseases reviewed (cholera, schistosomiasis, and malaria) are all
linked to environmental variabilityin space and time. Large epidemics of both cholera
and malaria can occur within the Nile basin, and outbreaks of both these diseases have
been linked to seasonal and interannual variability in temperature and precipitation.
Spatial patterns of schistosomiasis prevalence in humans and host snail distributions
and infections are associated with geographic patterns of climate and physiography.
The implications of these strong climatic linkages are twofold. First, they emphasize
the region’s sensitivities to future climate change (Hulme et al. 2001). A better
understanding of the potential burden of water-associated diseases under projected
future climates is needed, although the complexities of climate–disease linkages and
the confounding influences of a variety of other important epidemiological factors
make this task an enormous challenge (Hay et al. 2002). Second, these environmental
associations can be leveraged for mapping and forecasting disease risk, with the aim
of enhancing early-detection and early-warning systems for water-associated disease
outbreaks (Ford et al. 2009; Thomson and Connor 2001).
The linkages between water-associated diseases and the development of water-
management projects such as dams and irrigation schemes present both a challenge
and an opportunity. On the one hand, these types of projects have been clearly shown
to increase the risk of diseases such as malaria and schistosomiasis under many con-
ditions through direct impacts on habitats for disease vectors and hosts. On the other
hand, these projects also increase the standard of living of the population and thus
can also indirectly reduce the burden of disease by supporting improvements such
as clean water supplies, better sanitation, enhanced nutrition, increased access to
bed nets and other preventive measures, and improved health infrastructure. Further-
more, experience has shown that it is possible to design water-management projects
so that disease risk can be reduced through active management of water levels and
230 M. C. Wimberly andA. A. Midekisa
flows (Konradsen et al. 2013). Because water resource projects such as dams, ir-
rigation schemes, and water storages are human-made hydrological features, there
is enormous potential for engineers to coordinate with public health experts at the
planning stage to reduce favorable environments for disease transmission (Boelee
and Madsen 2006). Future water resource development projects should thus be based
on integrated approaches that address potential impacts on multiple water-associated
diseases, consider the range of potential climate change scenarios that may influence
disease risk in the future, and enhance the socioeconomic status of the population in
ways that reduce disease risk.
Acknowledgments This work was supported by grant number R01AI079411from the National
Institute of Allergy and Infectious Diseases and by a NASA Earth and Space Science Graduate
Fellowship (Grant Number 11-Earth11F-0286).
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This chapter shows the tremendous influence of the Nile on human culture reaching until today.
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OBJECTIVE: To estimate the global burden of cholera using population-based incidence data and reports. METHODS: Countries with a recent history of cholera were classified as endemic or non-endemic, depending on whether they had reported cholera cases in at least three of the five most recent years. The percentages of the population in each country that lacked access to improved sanitation were used to compute the populations at risk for cholera, and incidence rates from published studies were applied to groups of countries to estimate the annual number of cholera cases in endemic countries. The estimates of cholera cases in non-endemic countries were based on the average numbers of cases reported from 2000 to 2008. Literature-based estimates of cholera case-fatality rates (CFRs) were used to compute the variance-weighted average cholera CFRs for estimating the number of cholera deaths. FINDINGS: About 1.4 billion people are at risk for cholera in endemic countries. An estimated 2.8 million cholera cases occur annually in such countries (uncertainty range: 1.4-4.3) and an estimated 87 000 cholera cases occur in non-endemic countries. The incidence is estimated to be greatest in children less than 5 years of age. Every year about 91 000 people (uncertainty range: 28 000 to 142 000) die of cholera in endemic countries and 2500 people die of the disease in non-endemic countries. CONCLUSION: The global burden of cholera, as determined through a systematic review with clearly stated assumptions, is high. The findings of this study provide a contemporary basis for planning public health interventions to control cholera.
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A rapid health assessment of the Turkwel Gorge hydroelectric dam and proposed irrigation project in Kenya is described. The project will lead to increased risk of a number of communicable diseases including malaria and schistosomiasis, to the human population in the surrounding area. Mitigation measures are available and should be incorporated in the design and operation of the irrigation project to maximise cost-effectiveness.
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Human-made ecologic transformations have occurred at an unprecedented rate over the past 50 years. Prominent among them are water resource development projects. An estimated 40,000 large dams and 800,000 small dams have been built, and 272 million hectares of land are currently under irrigation worldwide. The establishment and operation of water projects has had a history of facilitating a change in the frequency and transmission dynamics of malaria, but analyses of these environmental risk factors are sparse. Here, we present a comprehensive review of studies that assessed the impact of irrigation and dam building on malaria prevalence or incidence, stratified by the World Health Organization's (WHO) sub-regions of the world, and link these studies with the latest statistics on disability adjusted life years, irrigated agriculture, and large dams. We also present estimates of the population at risk due to proximity to irrigation schemes and large dam reservoirs. In WHO sub-regions 1 and 2, which have 87.9% of the current global malaria burden, only 9.4 million people are estimated to live near large dams and irrigation schemes. In contrast, the remaining sub-regions concentrate an estimated 15.3 million people near large dams and up to 845 million near irrigation sites, while here only 12.1% of the global malaria burden is concentrated. Whether an individual water project triggers an increase in malaria transmission depends on the contextual determinants of malaria, including the epidemiologic setting, socioeconomic factors, vector management, and health seeking behavior. We conclude that in unstable malaria endemic areas, integrated malaria control measures, coupled with sound water management, are mandatory to mitigate the current burden of malaria in locations near irrigation or dam sites.
Cholera caused by toxigenic Vibrio cholerae is a major public health problem confronting developing countries, where outbreaks occur in a regular seasonal pattern and are particularly associated with poverty and poor sanitation. The disease is characterized by a devastating watery diarrhea which leads to rapid dehydration, and death occurs in 50 to 70% of untreated patients. Cholera is a waterborne disease, and the importance of water ecology is suggested by the close association of V. cholerae with surface water and the population interacting with the water: Cholera toxin (CT), which is responsible for the profuse diarrhea, is encoded by a lysogenic bacteriophage designated CTX Phi. Although the mechanism by which CT causes diarrhea is known, it is not clear why V. cholerae should infect and elaborate the lethal toxin in the host, Molecular epidemiological surveillance has revealed clonal diversity among toxigenic V. cholerae strains and a continual emergence of new epidemic clones. In view of lysogenic conversion by CTX Phi as a possible mechanism of origination of new toxigenic clones of V. cholerae, it appears that the continual emergence of new toxigenic strains and their selective enrichment during cholera outbreaks constitute an essential component of the natural ecosystem for the evolution of epidemic V. cholerae strains and genetic elements that mediate the transfer of virulence genes. The ecosystem comprising V. cholerae, CTX Phi the aquatic environment, min the mammalian host offers an understanding of the complex relationship between pathogenesis and the natural selection of a pathogen.
Intestinal schistosomiasis is a global disease of enormous public health importance. Assessing the local risk of transmission of the parasite causing the disease requires an appraisal of the distribution of its intermediate snail hosts, Biomphalaria spp.. In East Africa, the Lake Victorian Basin is a major freshwater ecosystem and highly endemic for intestinal schistosomiasis, although detailed distribution data for Biomphalaria have not been collected. We used on-the-ground malacological surveys and conjoint measurement of environmental determinants to develop models for analysis of variables associated with distribution and predictive snail mapping into data-deficient areas. Four expeditions were undertaken to collect snails along the Lake Victoria shoreline, visiting 223 sites overall. Environmental measurements were recorded at the time of inspection and water samples taken for quantification of anion and cation concentrations. The spatial distributions of Biomphalaria choanomphala and Biomphalaria sudanica were modelled in two Bayesian multivariate models: one non-spatial and one spatial with random effects. The results showed that chloride, nitrate, sulphate and the number of sympatric snail species were significant predictors of B. choanomphala whereas habitat, water depth, pH and sulphate were significant predictors for B. sudanica. The range of spatial autocorrelation was large (572.9 km for B. choanomphala and 175.3 km for B. sudanica). Interpolating snail abundance data by kriging revealed two ‘hot-spots’ of high abundance of Biomphalaria. These areas should be targeted in future expeditions to ground-truth the model’s predictions. Our study is the first to use Bayesian methods for determination of biomedically important snail distributions and sets crucial limnological baseline data for assessment of future changes in snail biodiversity in the Lake Victoria Basin.
Schistosomiasis has long been a serious rural health problem in Egypt. Changing irrigation systems are recognized as affecting the distribution of Schistosoma haematobium and S. Mansoni and its snail vectors. In the Nile delta, although the proportion of full-time farmers is declining, an increasing number of women and men are part-time farmers and may be exposed to schistosomiasis. Epidemiological data from two villages indicate that full-time and part-time farmers, both men and women, have relatively high rates of infection with S. mansoni. Water contact studies identify the exposure patterns associated directly with irrigation, and with washing after farm work and cleaning vegetables in the canals. Farmers are aware of the risk of exposure during farming yet feel that they have no alternative but to come into contact with canal water during irrigation. Data on the distribution of the snail vector, and water quality, show that infection in canals flowing through fields in the study villages is possible. Interventions to protect farmers from schistosomiasis include community supported cleaning of canals, health education targeting farmers and improving treatment strategies. In one study village the modification of the water flow of canals may be feasible. In all settings intersectoral collaboration is essential for effective disease control.
Background: Irrigation practices have impact on the distribution of schistosomiasis; so, identifying newly established schistosomiasis transmission foci allows for designing prevention and control strategy. Objective: To establish schistosomiasis prevalence and intensity in relation to development of water bodies for irrigation. Methods: A survey of stool specimens of 2000 school children using Kato thick smear method was carried out in Central and Southern Tigray between October 2001 and January 2002. Results: Among the 1012 males and 998 females examined, 29% males and 27.5% females were found positive for one or more parasite. The prevalence of S. mansoni was 27% in longstanding irrigated, 10.8% in recently constructed irrigation schemes and 1.8% in the non-irrigated rural localities. In the urban setting, its prevalence was 15.5% in areas with water body nearby and 0.5% in areas with no water body nearby (P