ข้อมูลศึกษาของอาฟริกาเรื่องClimate of diseases

เป็นเรื่องที่น่าสนใจในการจัดการศึกษาเรื่องนี้ ข้าพเจ้าได้นำเสนอในหัวข้อสัมนา ในการสัมนาวิทยาศาสตร์เชิงคำนวณ ประจำภาคการศึกษา

จึงนำมาเสนอไว้เผื่อท่านใดสนใจ พรีเซนเตชั่น ที่ทำไว้ หรืออื่นๆ สามารถขอได้ครับ ขอบพระคุณเจ้าของลิขสิทธิ์ทุกท่าน

Impact of Climate Variability on Infectious Disease in West Africa

Madeleine C. Thomson,1 Stephen J. Connor,1 Neil Ward,1 and David Molyneux2

1International Research Institute for Climate Prediction (IRI), The Earth Institute at

Columbia University, Lamont Campus,

Monell Building, 61 Route 9W, Palisades, NY 10964-8000

2Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, UK

Abstract: The importance of infectious disease as a determinant (as well as an outcome)

 of poverty has recently become a prominent argument for international and national

investment in the control of infectious disease, as can be seen in the recently articulated

 United Nations (UN) Millennium Development Goals (MDGs). Climate variability and

land use change have an enormous impact on health in West Africa, and may

yet undermine the potential for achieving the MDGs, in certain economic-ecological

zones. However, theirunderlying role in determining the burden of disease in the

region on a yearly or decadal basis has never been

systematically studied. In order to improve our understanding of the future

impacts of climate change, it may be more effective to start by investigating the impact

 of inter-annual climate variability, and short-term shifts inclimate (e.g., decadal),

on disease transmission dynamics. This information may inform both current and

future policy decisions with regard to prediction, prevention, and management of

adverse climate-related health outcomes. This article reviews current knowledge of

changes in the epidemiology of infectious diseases associated with climate variability

in West Africa over the last 40 years. Selected examples are considered from

bacterial (meningococcal meningitis), protozoan (malaria), and filarial (onchocerciasis

 and lymphatic filariasis) infections where spatial and temporal disease patterns have

been directly influenced by seasonal, interannual, or decadal changes in climate.

INTRODUCTION

                While infectious and parasitic diseases are no longer the leading causes

of mortality in the developed world, theyremain the main cause of mortality in

middle and lower income countries (Connor,2002a), accounting for morethan

half of all deaths in Africa (Murray et al., 2001). The importance of infectious

disease as a determinant (as well as an outcome) of poverty has recently become

a prominent argument for international and national investment in the control of

infectious disease (WHO, 2001a) as can be seen in the recently articulated United

Nations (UN) Millennium Development Goals (MDGs) (Sachs, 2002). Climate variability

and land use change have an enormous impact on health in West Africa, and may yet

 undermine the potential for achieving the MDGs, in cer economic-ecological zones.

However their underlying role in determining the burden of disease in the region on

a yearly or decadal basis has never been systematically studied. Clearly appropriate

evidence-based economic, environmental, and public health policy instruments are

needed to reduce the potentially adverse impacts. Developing suchevidence in a

systematic way is a prerequisite for its effective use in guiding policy development.

In the countries belonging to the World Health Organization’s sub-region D

(all West African countries), respiratory infection, malaria, and human

immunodeficiency virus/acquired immunodeficiencysyndrome (HIV/AIDS) are

leading causes of mortality and Disability Adjusted Life Years, and their effective

control is perceived as an essential requirement for sustainable livelihood development

 in the region. This article reviews current knowledge of changes in the epidemiology

of infectious diseases associated with climate variability in West Africa over the last

40 years. Selected examples are considered from bacterial, protozoan, and filarial

infections where spatial and temporal disease patterns have been directly influenced

 by seasonal, inter-annual, or decadal changes in climate. Climate of West Africa

The climate of West Africa ranges from the Saharan desert (bounded by 100 mm

mean annual rainfall isohyet) through the Sahelian zone whose southern border is

demarcated by the 400 mm isohyet. South of this line is the savanna zone, where

the Malian horse empire was founded in the 13th century. Further south, the 1000 m

isohyet marks the start of the woodland savanna zone and the northern limits of the

tsetse fly (Glossina morsitans) belt—a formidable barrier to all those dependant on

cattle and horses. Cattle, horses, donkeys, and camels are all susceptible to the often

fatal parasitic disease trypanosomiasis (sleeping sickness) transmitted by this fly.

                The timing of the rainy season in West Africa is highly predictable and is

governed by the movement of the Inter- Tropical Convergence Zone (ITCZ), a

discontinuous belt of thunderstorms marking the convergence of the northern and

southern hemisphere’s surface trade winds which moves northwards and southwards

with seasonal changes in the overhead sun. The ITCZ reaches its maximum northward

 extent around July–August and its southern extent around January–March (during this dry

period, the harmattan winds blow Saharan/Sahelian dust southwestwards across the

 region).

The movement of the ITCZ’s northern edge is not regular and short-term oscillations

of as much as 500–1000 km may occur over a few days. This can cause erratic starts

to the onset of the rainy season. Rainfall in West Africa is the most significant

determinant of the landscape in terms of natural vegetation and land use activity.

The amount of annual rainfall at any geographic location depends heavily on the

duration of the rainy season, that is, the mean time in total during which the ITCZ is

positioned well north of the particular location thus allowing rainfall to occur. Drought

 in West Africa West Africa provides one of the most dramatic examples

worldwide of climate variability that has been directly and quantitatively measured

(Hulme, 2001). Averaged over 30- year intervals, annual rainfall across the Sahel

/Guinea-savannah fell by between 20 and 30% between the pre-independence period

 (1930s to 1950s) and the decades since (post-1960s) (Fig. 1). Thus, the famines that

struck the region in the 1970s and 1980s were embedded in a drought that has lasted

 from the 1960s to the present day (with some amelioration in recent years), which

not only affected the Sahel but extended southward to the Guinea coast. It has been

observed that most of the rainfall deficit of

the dry period 1971–1990 in the Sahel is correlated with a general decrease in the

number of rain events rather than a shortening of the length of the rainy season (Le

Barbe and Lebel, 1997; Le Barbe et al., 2002). This, along with the fact that the

rainfall deficit was equal to 180–200 mm all over West Africa, indicates that there

was no crucial difference in nature between the Sahelian drought and the drought

of the Guinean savanna regions. The explanation of such features resides in either

an abnormal southward position of the ITCZ and/or a less productive ITCZ during

the dry spell. It has been proposed that sulphurous emissions from factories in

industrialized countries may have created the conditions for, or exacerbated the

extent of, this recent drought by weakening the northwards movement of the ITCZ

(Rotstayn and Lohmann, 2002). As a result, the intense thunderstorms which produce

the rains needed for the Sahel croplands fail to materialize. However, the most favored

current hypothesis for the extended drought continues to be large-scale changes in

near-global patterns of sea surface temperature (SST) anomalies (Folland et al., 1986)

 with a possible role of landsurface changes in reinforcing the drought. Anomalies in

SST in the world’s oceans, including those related to the El Nin˜o Southern Oscillation

(ENSO), contribute to rainfall variability in the Sahel (Lamb, 1978; Folland, et al., 1986;

Ward, 1998). Lamb et al. (1978) observe that droughts in West Africa correlate with

warm SST in the tropical South Atlantic. Examining oceanographic and meteorological

data from the period 1901–1985, Folland et al., (1986) found that persistent wet and

dry periods in the Sahel wererelated to contrasting patterns of SST anomalies on a

nearglobalscale. When SST in the northern hemisphere’soceans were cold, rainfall in

the Sahel was low. A recentattempt to quantify the importance of the oceanic

influence

on the variability of the northern African summer monsoonclimate

(including the Sahel) on interannual to interdecadaltimescales has been made

(Giannini et al., 2003).



              Using an ensemble of simulations of the global atmospheric circulation forced

 with the observed record of SST, but no year-to-year changes in vegetation cover or

greenhouse gas concentrations, these authors demonstrate that a warming of the

tropical Pacific and Indian Oceans, of the order of 0.2_C or larger, is associated with

a negative rainfall anomaly in the Sahel PRINCIPAL component (0.73 mm/ day in

observations). Furthermore, a negative rainfall anomaly is consistent with local land

surface temperature anomalies up to 0.6_C warmer than average. From this work,

the authors conclude that SST variability is instrumental in determining the sign of

rainfall anomalies in the Sahel, while coherent land-atmosphere interaction acts to

amplify them. The theory that SST anomalies are the driving force behind the

drought contrasts dramatically with earlier perceptions of the causes of the Sahelian

drought. The responsibility for this regional disaster had been put upon the shoulders

of local peasant farmers, whose overuse of the land was deemed to have resulted in

reduced vegetation cover, the advance of the desert, and localized impacts on the

regional climate regime (McCann, 1999). Major river systems endow some, but not

all, of the countries of West Africa with substantial water resources. One of the first

consequences of the decrease in precipitation has been a significant reduction in

 discharge within these river systems. The fall in precipitation observed before and

after the discontinuity ranges between 15 and 30%; for river discharge, the

corresponding fall varies between 30 and 60% (Gustard and Cole, 2002). This

 translates into a decline in surface water resources across the sub-region in

recent years, which has serious consequences for the management of these resources.

 A further consequence is the increasingly rapid degradation of vegetation and

associated desertification, mainly in the Sahelian zones, which

renders the Sahelian ecosystem even more fragile in the face of predominant

socio-economic activities including agriculture and animal husbandry. Whatever

their cause, droughts in the Sahel are not new. In fact, what characterizes rainfall

in the Sahel is its variability in space and time from year to year, decade to

decade (Nicholson et al., 1990). Human society has adapted to this variability

in rainfall supply, for example, through pastoralism, diversification of income,

and mobility (Batterbury and Warren, 2001). However such adaptability has not

freed the region from vulnerability to periodic famine, the most tragic of which are

recorded as having occurred in 1680, 1750, 1820, 1920, 1968, 1973, and 1984

(Egg and Gabas, 1997). In response to the most recent famines, the international

community has focused efforts on the development of famine early warning systems

to reduce the region’s vulnerability to such disasters. These warning systems use

information on current household vulnerability to shocks and climate/environmental

data in order to assess a region’s vulnerability to famine. The potential value of

seasonal climate forecasts to agriculture (Hansen, 2002), disaster management

(Dilley and Heyman, 1995; Thomson et al., 2003), and health (WHO, 2001b) has

been highlighted and new initiatives are currently underway to assess the value of

such information to regional and local decision makers. However, because of

limitations in the predictability of SST, in particular equatorial and South Atlantic

SST (key determinants of the West African climate), during the spring period, the

potential to predict the seasonal rainfall over West Africa prior to May or June is poor

(Goddard and Mason, 2002). Forecasts are therefore limited in this region in the lead

time they can offer decision makers. While most research to date has focused on the

 predictability of the rainy season, new efforts to predict the length and character

of the dry season are being prompted by health sector interest in view of the potential

 relationship of low humidity and dust torespiratory infections (Molesworth et al.,

2002a).



            Geographical and Socio-economic Characteristics of West Africa There are 15

countries in mainland West Africa (Fig. 2), 3 of which are landlocked. The sub-region

stretches from the southern fringes of the Sahara Desert to the Atlantic Ocean along

 the Gulf of Guinea, from hot dry desert, through

semi-arid savannah, to moist humid forests. Generally the West African countries can

be currently characterized by: high population growth, young populations, high and

weakly controlled urbanization, and weak human development. Periodic war and civil

unrest, often driven by the desire to control minerals (Veeken, 1994; Davies, 2000)

plague countries which are well endowed with natural resources resulting in large

refugee/displaced populations in some countries. While substantial gains have been

made in reducing infant mortality in the last 40 years, deaths rates remain high, life

expectancy is approximately 50 years, and national Gross Domestic Product (GDP)

 figures indicate that these countries include some of the poorest populations in the

world. Two important factors currently driving both economic and political change are

the 10-fold increase of the population over the last century, and the recent opening

up of the region to the world economy, as globalization advances (Cour, 2001).

Political and economic activity in West Africa has always been heavily influenced by

climate, and historians have delimitated key West African historical zones by rainfall

isohyet boundaries which shift periodically. These boundaries impose specific

constraints on human activity (McCann, 1999). For instance, while in all West

African countries there is a predominance of the agricultural sector in the national

economies, the type of agriculture practiced and its potential for economic growth

and development is largely governed by the climatically determined ecological

zones of the region. Recent work for the World Bank by the Food and

Agriculture Organization (FAO, 2001) identified a number of ecological zones,

determined largely by rainfall resources, which characterize particular farming

systems (Fig. 3). These include: Pastoral Systems with a high incidence of severe

poverty and low potential for poverty reduction; Agro-Pastoral Millet-Sorghum

Systems with a high incidence of severe poverty but with a high potential for poverty

 reduction; and Cereal-Root Crop Mixed Systems with a lower incidence of poverty

and a high potential for agricultural growth. The climate and environment in each

of the farming system zones described correspond closely to different zones of

epidemic disease risk (Fig. 4). Therefore, the development policies and practices

 that are followed have important implications not only for agricultural development

 and food security, but also disease transmission risk and vulnerability to severe-

disease outcome. Clearly this has profound implications for sustainable livelihoods

development in the region over the medium–long term.

Climate and Health Interactions in the Arid and Semi-arid Farming Zones of West

Africa Climate impacts on health through a number of distinct, direct and indirect,

mechanisms. For example, directly through heat stress, UV induced melanomas,

etc., or perhaps more significantly, indirectly, through:

                a) its role in determining agricultural output and consequently food

security, which directly effects nutritional status;

                b) its role in the economy via agricultural exports, hydrological power,

etc., which affects the ability of individuals and communities to maintain nutritional

status, prevent infection (e.g., through the purchase of mosquito nets) and, when

 necessary, obtain health care. The impact of climate on the economy will also

affect the ability of governments to provide health services;

                c) its role in determining seasonal and inter-annual demographic

 processes (e.g., seasonal labor migration and environmental refugees),

increasing the likelihood of individuals contracting certain infectious diseases,

such as HIV;

                d) its impact on the spatial and temporal distribution of climate-related

infectious diseases (e.g., malaria, Rift Valley fever, meningococcal meningitis).

Many of these indirect mechanisms interact, and when climatic anomalies coincide

with societal vulnerability, disastrous drought and famine can occur (e.g., in West

Africa in the early 1970s and early 1980s). These situations result in further social

disruption and population displacement, with the major cause of mortality during

 such disasters being epidemics of infectious diseases (Prothero, 1994). Furthermore

, some of the linkages are bidirectional. For example, while agricultural production

 and food security impact on health, infectious disease episodes impact on

agricultural production, affecting the availability of labor at critical times in the

annual agricultural calendar.

Predicting Infectious Disease Transmission

                There is currently extensive interest in predicting the possible impacts

of climate change on the major infectious diseases (Colwell et al., 1998; Kovats

et al., 2001; Patz and Reisen, 2001). However, considerable uncertainty is associated

with such predictions. While there is widespread acceptance that the climate is

changing (Houghton et al., 2001), the future impact of such change on the actual

weather patterns in particular geographic regions remains largely speculative

(McCarthy et al., 2001). Furthermore, while many infectious disease vectors and

agents are known to vary in their spatial and temporal distribution, as a function of

 climate processes (Thomson and Connor, 2000), the detailed

parameterization of key variables, such as vector survivorship, are made difficult by

the paucity of relevant empirical data which relate to disease transmission under

field conditions in Africa. This, combined with the many nonlinearities in the

transmission dynamics of infectious diseases and the importance of nonclimatic

 factors in determining the outcome of transmission, make predicting the impact

of climate change on specific health outcomes a contentious one (Martens et al.,

1997; Rogers and Randolph, 2000). In order to improve our understanding of the

future impact of climate change, it may be more effective to start by investigating

 the impact of inter-annual climate variability, and short-term shifts in climate

(e.g., decadal), on disease transmission dynamics, since this information may

 inform both current and future policy decisions with regard to prediction,

prevention, and management of adverse climate- related health outcomes.

Consequently, research aimed at the use of seasonal climate forecasts by health

 services in Africa needs to emphasize the importance of understanding the

probabilistic nature of future climate/ disease predictions (Thomson et al., 2000b)

 [Palmer et al., manuscript in preparation]. Many of the infectious diseases that

contribute to the very high child mortality rates in West Africa occur within a

‘‘climate envelope’’ (for example: malaria, Rift Valley fever, yellow fever; relapsing

fever, lymphatic filariasis, onchocerciasis) and are further restricted in their

distribution by the microclimate/environment which supports their transmission

 (Thomson and Connor, 2000). However, transmission of such diseases in the

region may vary in intensity between years as a result of inter-annual variation

in climate and human vulnerability (Table 1). Many of these diseases are vector-borne

, but climate may also be a major determinant of the variability of nonvector-borne

diseases such as diarrheal disease, airborne infections such as measles (Brewster

and Greenwood, 1993), and meningococcal meningitis (Cheesbrough et al.,

1995;Molesworth et al., 2002b). Furthermore, climate ‘‘shocks’’ may change an

individual’s exposure to infectious disease through migration (Prothero, 2001)

and changes in behavior (Findley, 1994). A number of examples are presented

 below.

Malaria

                The most obvious result of rainfall decline on infectious disease

transmission in West Africa is the associated change in malaria prevalence and

incidence across the region. A greater than 80% decline in malaria prevalence

 rates have been observed in the semi-arid areas of northern Senegal and Niger

from the early 1960s to the mid 1990s. This reduction is presumably associated

 with a loss of vector breeding sites, a shortening or reduction in intensity of the

malaria transmission season as a result of lower vector survivorship and a loss

of specific vector species. Previous studies have indicated that the different

 cytotypes  of Anopheles gambiae s.s. are highly climate sensitive (Toure et al.,

1994;  Thomson et al., 1997). Observations of malaria prevalence rates before and

after the droughts which occurred since the 1970s, indicate that in the Sahel

(Niayes region, Senegal), endemic malaria decreased drastically after the

 disappearance of the principal mosquito vector, Anopheles funestus, due to

the destruction of its larval sites by cultivation (Mouchet et al., 1996). No

reestablishment of this species was observed in the Niayes region after the very

 wet year of 1995, however, the presence of this vector has been noted more

recently in the irrigated areas of northern Senegal (Konate et al., 2001). It has

 recently been proposed that climate and geography are more significant than

poverty per se in determining levels of malaria burden, and that a reduction in

malaria is likely to proceed a reduction in poverty (Sachs and Malaney, 2002).

The major reductions in malaria transmission in the Sahel that have occurred

have not, however, always resulted in a reduction in poverty, as many countries

 in the region currently rank at the bottom of the development indices. As well

 as climatic variability, other factors such as poor governance, conflict, international

debt, poor terms of trade, and restricted access to the international markets have

 undermined the region’s development agenda. Changes in malaria endemicity

as a result of the Sahelian drought have affected large areas of semi-arid West

 Africa as regions have moved from endemic to epidemic. This change presents

a major obstacle to the development of regional malaria risk maps (Kleinschmidt et al., 2001) as prevalence rates may change dramatically over a few years. The increasing epidemic nature of malaria transmission has, however, been acknowledged in terms of developing control strategies for the region, with new efforts to develop malaria early warning systems based on routinely available meteorological station- or satellite-based information on excess rainfall (WHO, 2001b). Evidence supporting the use of rainfall-driven malaria early warning systems in West Africa is slowly building (Ndiaye et al., 2001). However, rainfall alone may not be a sufficient indicator of malaria transmission in those parts of the Sahel where its impact on the local ecology is governed by hydrological process associated with topography and soil type. In such a situation, an early warning system based on the water levels in rivers or dams may be more valuable—as has been proposed for the Senegal River basin (Faye et al., 1998). Support for the use of hydrological monitoring comes also from Sudan where river flooding is a key trigger for local increases in malaria cases (El Sayed et al., 2000). The flooding of rivers will depend on the rainfall distribution and timing in the entire river catchment rather than on local rainfall. It has been noted that the extent to which flooding is associated with increases in malaria cases is dependent on the timing of the floods in relation to other factors such as local rainfall and humidity (Najera, 1999). Changes in the presence of surface water as a result of the development of irrigation schemes along the major river

systems in West Africa has important implications for malaria transmission. Expectations that the consequent increases in available breeding sites for malaria vectors will result in an inevitable increase in malaria transmission in the vicinity of such schemes has not, however, always been borne out (Ijumba and Lindsay, 2001). The explanation for this finding is still unresolved but, in some cases at least,

can be attributed to displacement of the most endophilic and anthropophilic malaria vector Anopheles funestus by An. arabiensis with lower vectorial capacity.

Lymphatic Filariasis

The ecological determinants of lymphatic filariasis infection are not well understood, although a generalized ecological distribution was proposed by Brengues in the 1970s (Brengues, 1975); see Figure 5. The parameters used to create Brengues’ filariasis map were elevation, vegetation cover, and annual rainfall. The importance of understanding the geographical determinants of this disease have reemerged with the development of efforts to control lymphatic filariasis in Africa as part of a global program for its elimination as a public health problem (Molyneux and Zagaria, 2002). The results of a recent study of the spatial distribution of prevalence rates of Wucheria Bancrofti-(the causative agent of lymphatic filariasis) has shown that this disease is largely absent from some of the Guinea coastal countries such as Togo and Benin—despite these countries being highly endemic for malaria (Gyapong et al., 2002). The apparent contrast in the frequency of human infection between W. Bancrofti and P. falciparum is all the more striking since both parasites are transmitted by the same mosquito vector (namely members of the Anopheles gambiae species complex). Gyapong and colleagues speculate that differences in the spatial distribution of these two diseases may be related to differences in the cytotypes of the vectors since high prevalence levels of filariasis seem to be coincident with the spatial distribution of Mopti and savannah forms of Anopheles gambiae s.str. Given that malaria infection rates have changed dramatically over the Sahel as a result of reduced rainfall, it would seem unlikely that filariasis transmission by Anopheles gambiae s.l. has not also been affected. There is however no evidence to date which can be used to substantiate such a claim.

Onchocerciasis

Prior to the 1970s, onchocerciasis (known as river blindness) was a neglected disease. Its devastating effects where largely borne by those rural populations of West Africa living near the fast flowing rivers of the Sahel. When the Onchocerciasis Control Programme (OCP) was started in 1974, some of West Africa’s richest river lands were uninhabited. In villages sited in river valleys near to the breeding sites of the blackfly vector of onchocerciasis (Simulium damnosum s.l.), it was not unusual to find 60% of the adults afflicted with the disease and 3–5% blind. Communities were forced to abandon their villages en masse. Today, more than 20 years and US $600 million after the program was first launched, the disease has been controlled through one of the most successful public health campaigns in history (Benton et al., 2002). The governments of 11 African nations and 24 donor agencies combined their resources and energies to spray the rivers where the blackflies breed, and to develop and distribute the anti-helminthic drug ivermectin for treatment or prevention of the disease. Villages once emptied by river blindness have been rapidly resettled—bringing new challenges to the delivery of health care in the region (Richards et al., 2001). Understanding the spatial and temporal distribution of the vectors of onchocerciasis has been key to their successful control (Boakye et al., 1998), given that the S. damnosum species complex comprises many distinct sibling species with varying capacities to transmit the pathogen, Onchocerca volvulus. The distribution of the different members of the Simulium damnosum species complex is generally related to phtyogeographic zones, forest and savannah, but seasonal changes in their distribution occurs on an annual cycle as the monsoon winds and their accompanying rainfall aid dispersal and result in enhanced river flow and the creation of breeding sites. According to Baker et al. (1990), members of the S. damnosum complex move average distances of 15–20 km daily and may migrate over a total distance of 400–500 km. Unusual migrations of savannah species of S. damnosum s.l. (the species most commonly associated with the blinding form of the disease) into the forest zones have been observed (Thomson et al., 1996), and this has led to speculation on the possible role of deforestation and rainfall decline on the distribution of different species of S. damnosum s.l. (Walsh et al., 1993). Rainfall has also had a more direct impact on the control program through its role in determining river flow and, therefore, the amount and type of insecticide required to treat specific areas (Hougard et al., 1993). With the extension of the control of onchocerciasis into countries hitherto not included (through the African Programme for Onchocerciasis Control, APOC), a new obstacle has been placed in the way of effective disease control by the presence of another filarial worm, loa loa, in forested West and Central Africa. This is a consequence of recent reports of severe and fatal encephalopathic reactions to ivermectin in individuals with high loa loa microfilarial counts. Consequently, mapping the spatial distribution of loa loa has become a priority for APOC in order to modify its treatment protocols appropriately (Thomson et al., 2000a).

Meningococcal Meningitis

            Neisseria meningitidis (the meningococcus) is responsible for endemic and epidemic meningococcal disease throughout the world. In Africa, meningococcal meningitis often occurs as extensive epidemics—with many thousands of deaths, particularly in the so-called meningitis belt of sub Saharan Africa. This belt is defined as an area between latitudes 4_ and 16_ north where high incidence and recurring epidemics of cerebrospinal meningococcal meningitis coincide with the 300–1100 mm mean annual rainfall isohyets south of the Sahara (Lapeyssonnie, 1963); see Figure 4. Thus, the belt comprises much of semi-arid sub- Saharan Africa and, in particular, the Sahel epidemic waves occur every 5–10 years in the Sahel region of the sub- Sahara and are mostly due to group A meningococci. Risk factors for invasive disease and epidemic outbreaks are not completely understood, but it is likely that a combination of conditions is necessary for an epidemic to occur; immunity, transmission of a virulent strain of meningococcus, poor living conditions, and population movements and crowding are all considered important. Concurrent infections have also been implicated. In addition, environmental factors may play a key role. Asymptomatic carriage of the meningococcus in the nasopharyngeal mucosa is common in the semi-arid zones of Africa, systemic infection causes invasive disease, and most cases result from infection recently acquired from exposure to carriers (Molesworth et al., 2002b). The geographic predominance of epidemics in the belt region and their seasonal occurrence at dry, dusty times of year, ceasing with the onset of the rains (Molesworth et al., 2001) suggest that environmental conditions are important, although the mechanisms by which they may work are poorly understood. It has been postulated that low humidity and dust may damage the mucosal barrier, and/ or inhibit its immune defense mechanism, thus increasing mucosal invasion and the risk of overt disease; indoor overcrowding in adverse environments (intense dust events) may enhance the spread of infection (Greenwood et al., 1987). In a recent review of the spatial and temporal occurrence of meningococcal meningitis epidemics across Africa, it has been clearly shown that the Sahel bears the greatest epidemic burden of meningococcal meningitis with over two-thirds of documented outbreaks and high attack rates (Molesworth et al., 2002b). A recent analysis of epidemic distribution and environmental data indicates that the absolute humidity profiles and land-cover types can be used to distinguish between areas with high and low risk of epidemics, and also that population density and dust may be implicated in some regions (Molesworth et al., 2003). An increase in epidemic occurrence in the southern extent of the meningitis belt may be related to an increase in dust storms in the region (Molesworth et al., 2002a), which may be a consequence of the recent drought periods.

DISCUSSION 

                Climate has had an enormous impact on health in West Africa. However, its underlying role in determining the burden of disease in the region on a yearly or decadal basis is not mentioned in key health policy documents, such as those that reflect the framework and indicators promoted for monitoring the achievements of Roll Back Malaria (RBM) (Remme et al., 2001). Its absence is all the more remarkable given that the stated aim of RBM is ‘‘to halve the burden of malaria through interventions adapted to local needs and strengthening of the health sector.’’ If observations indicating that climate variation alone may be responsible for changes in prevalence rates of 50% in some areas, then indicators of the effectiveness of RBM interventions (which do not incorporate climate variability as a confounding factor) may be either overly optimistic or pessimistic as a result. This is considerable evidence that many infectious diseases such as malaria occur within a climate ‘‘envelope’’ and vary between years in intensity as a result of interannual variation in climate and vulnerability. There is a clear need for researching and documenting more closely the extent to which morbidity and mortality from infectious disease, and their broad socio-economic impacts, is determined by climate, and whether or not climate/environmental information (such as seasonal forecasts or rainfall or vegetation monitoring) can, in practice, improve decision making for control purposes in the affected countries. Currently, there is much interest in developing adaptation strategies for poor countries to cope with the likely impacts of climate change, and yet we know little about

how shifts in climate have affected health outcomes in the recent past. Whether or not the recent drought in West Africa is part of the natural cycle of climate variability or indicative of anthropogenic climate change, we can learn much from the region’s experience in terms of impacts and adaptation to extreme shifts in the region’s climate over a relatively short period of time.

ACKNOWLEDGMENTS

                The authors thank Alessandra Giannini for providing Figure 1, and Valerie Obsomer for providing the digital data used in Figures 4 and 5, respectively. M.C.T., S.J.C., and N.W. are supported by a grant/cooperative agreement number NA67GP0299 from the National Oceanic and Atmospheric Administration.

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