Evolution of Malaria in Africa for the Past 40 Years: Impact of Climatic and Human Factors
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Jaumal of the American Mosquito Control Association, 14(2):121-13O, l99B Copyright O 1998 by the American Mosquito Control Association, Inc. EVOLUTION OF MALARIA IN AFRICA FOR THE PAST 40 YEARS: IMPACT OF CLIMATIC AND HUMAN FACTORS JEAN MOUCHET! SYLVIE MANGUIN,, JA6QUES sIRcoULoN,' STEPHANE LAVENTURE,3 OUSMANF FAYE,4 AMBROSE W. ONAPA,s PIERRE CARNEVALE,6 JEAN JULVEZ? AND DIDIER FONTENILLE8 ABSTRACT. Different malarial situations in Africa within the past 40 years are discussed in order to evaluate the impact of climatic and human factors on the disease. North of the equator. more droughts and lower rainfall have been recorded since 19721 and in eastem and southern Africa, there have bgen aliemating dry and wet periods in relation to El Nifio. Since 1955, the increase in human population from 125 to 450 million has resulted in both expansion of land cultivation and urbanization. In stable malaria areas of West and Central Africa and on the Madagascar coasts, the endpmic situation has not changed since 1955. However, in unstable malaria areas such as the highlands and Sahel significant changes have occurred. In Madagascar, cessation of malaria control programs resulted in the deadly epidemic of 1987-88. The same situation was observed in Swaziland in 1984- 85. In Uganda, malaria incidence has increased more than 30 times in the highlands (1,500-1,800 m), but irs altitudinal limit has not overcome that of the beginning of the century. Cultivation of valley bottoms and extension of settlements are in large part responsible for this increase, along with abnormally heavy rainfall that favored the severe epidemic of 1994. A similar increase in malaria was observed in neighboring highlands of Rwanda and Burundi, and epidemics have been recorded in Ethiopia since 1958. In contrast, in the Sahel (Niayes region, Senegal), stricken by droughts since 1972, endemic malaria decreased drastically after the disappearance of the main vector, Anopheles funestus, due to the destruction of its larval sites by cultivation. Even during the very wet year of 1995, An. funestus did not reinvade the region and malaria did not increase. The same situatron was observed in thd Sahelian zone of Niger. Therefore, the temperature increase of 0.5'C during the last 2 decades cannot be incriminated as a major cause for these malaria changes, which are mainly due to the combination of climatic. human. and operational factors. KEY WORDS Malaria, Afnca, Anopheles gambiae, Anopheles funeslas, climatic factors, human factors MALARIA IN AFRICA dent on accumulation of rain water in pools. Vec- torial competence of An. gambiae s.s. and Ar.fu- Since the l5th century, travelers described the nestus is remarkable, and their sporozoitic indices unhealthy nature of African coasts where Europe- (SIs) are greater than lVo, if not 3Vo. Anopheles ans were decimated by fevers, and where, inland, arabiensis is not as good a vector with an SI often life expectancy did not exceed 2-3 (Carlson months lower than 17o. 1984). After 1950, global malaria eradication be- Even though malaria occurs throughout Africa, came a worldwide goal for public health, resulting there are significant differences between regions. in a number of pilot projects in Africa with re- Malaria stability and equilibrium between host, markable studies on vectors and parasite preva- vector, and parasite are conditioned by the compe- lence. However, studies on morbidity and mortality tence and longevity of vectors (Macdonald 1957). were not conducted. In stable malaria regions, transmission occurs every Africa is a continuous malaria focus from the year at high levels and can be either perennial or southern border of the Sahara (20'N) to South Af- seasonal. The human population acquires a strong rica and Namibia (30"S). Howeveq endemicity dis- immunity during the lst few years of life at the appears above 1,800-2,000 m in the mountainous price of a high infantile-juvenile mortality. In un- regions of Central and East Africa. The 3 major stable malaria regions, transmission is lower and vectors involved are Anopheles gambiae Glles irregular from year to year, with the human popu- 1902, Anopheles arabiensls Patton 1905, and lation acquiring less immunity and epidemics oc- Anopheles funestus Giles 190O. The lst 2 species curring in all age classes (Macdonald 1957). are mainly linked to rainfall, and the 3rd is depen- Stable malaria covers most of West and Central Africa, the eastern coast of Africa, Madagascar coasts, and I Comoro Islands, with An. gambiae s.s. ORSTOM, 213 rue Lafayette,75010, Paris, France. 'LIN-ORSTOM, and An. funestus as the main vectors. Unstable ma- 911 Avenue Agropolis, B.p 5045, laria, witlr An. arabiensls as the major vector, in- 34032, Montpellier, France. 3 cludes the southern border of the Sahara, the Pasteur Institute, BP 1274, Antananarivo, Madagascar. moun- a tainous areas of University of Dakar, Department of Zoology, Dakar, Central Africa, countries south of Senegal. the Zartbezi River, and the highlands of Madagas- 5 Vector Control Unit, Ministry of Health, Kampala, car. However, local presence of An. funestus and, Uganda. An. gambiae s.s. can sustain local stable malaria 6 ORSTOM, 0l BP 1434, Bouake 01, C6te d,Ivoire. foci. Between these 2 situations of stable and un- 7 Ministry of Health, Paris, France. stable malaria, all intermediates occur, in particular 8 Pasteur Institute, BP 22O, Dakar, Senegal. in Sahel and East Africa (Mouchet et al. 1993). t2l t22 JounNar-or rne AN{enrcnNMosqurro CoNrnor_AssocrnrroN VoL. 14,No.2 Sahel 0,5 -0,5 -l -2 1920t'92s 1930 1935 1940 1945 1950 i955 19601965 Lg70 lg75 19801985 1990 Year Fig. l. Annual rainfall anomaly indices in the Sahel. Anomaly indices were calculated with respect to mean l93l- 90 rainfall (from Hulme 1992). MAJOR CLIMATIC EVENTS (Nicholson et al. 1988): 1) West and Central Africa (including the Congolese The principal climatic factors influencing malaria basin), fed by a humid Atlantic flow; 2) East Africa, transmission are temperature and rainfall. The for- fed not only by this flow but mainly by the Indian mer determines the length of the larval mosquito Ocean's maritime flow; and 3) Southern Africa and cycle and the sporogonic cycle of the parasite in Madagascar, sig- nificantly affected by the El Niflo/Southern the mosquito; the latter determines the number and Oscil- lation (ENSO) phenomenon. productivity of breeding sites and consequently the vector density. These 3 main geographic groups have their own specificities. The Sahelian zone, from Mauritania to Numerous studies on the Quaternary period western Ethiopia, has undergone showed that the African continent went through tre- an extended rain deficit since 1965 (Hulme mendous climatic variations during the last inter- 1992), resulting in the shift of mean isohyets 100-200 glacial era. During the humid Holocene (9,000 be- km south, as well as a 2O-3OVa deficit of pluviometric means fore present [BP]), the Great Rift lakes were at their during 1961-90 compared to 1931-60 (Fig. highest level, and rainfall was relatively constant 1). Drought paroxysms were recorded in 1972--73 and all year round, most likely 3OVo higher than the 1983-84. During these periods the flow of the Senegal, average recorded in 1931-60. Temperatures were Niger, and Chari rivers dropped significantly 2oC below current levels. During the past 100 years and Lake Chad was reduced by 22. for which quantified records are available, climatic variations have been noticed. In East Africa, exceptional rainfall occurred dur- ing the early 1960s resulting in a lo-l1Vo increase of current means, which explains the augmentation Temperature of the flow of the White Nile during the past de- cades. In mountainous Jones and Briffa (1992) used a temperature da- areas, the spatial heteroge- neity of rainfall can be important tabase available since 1895 and covering im area even a few kilo- meters away. For between latitudes 35'N and 40'S and between lon- instance, during the lst semester of t994, rainfall at Kabale (Uganda) gitudes 20'W and 55oE. Based on one century, all was consistent with the normal mean (492.7 tendencies have been detected including an atmo- mm), whereas at Kis- izi (50 km away) rainfall (1,004.6 spheric warming of 0.55"C in 1905 and 1940, a was twice as high mm). distinct cooling between 1940 and 1955, followed In southern Africa, by temperature stability until 1980. In the early alternating dry and humid years were observed past 1990s, another temperature increase of 0.2'C was during the decades. El Nifro/Southern recorded, followed by atmospheric cooling in Oscillation events have a significant impact; all El Nifro years (1972-73, L992-93. However, 1995 was the warmest year re- 1982-83, corded globally. 1986-87) were both hot and drv. Rainfall IMPACT OF HUMAN FACTORS Africa south of the Sahara has a wide range of In 1955, the Second Pan-African Conference on pluviometric regimes, from desert to equatorial Malaria estimated the population living in Africa zones. Three geographic groups can be identified south of the Sahara at 125 million. In 1995. the JuNe1998 MnI-lnIl w Apntcl, Ct-nraertc AND HUMAN F,qcrons r23 paid estimate exceeded 450 million; this population in- plateaux were moved to the western plains and crease has resulted in galloping urbanization and a high price to the disease (Krafsur and Armstrong environmental changes due to expansion of land 1978). The presence of refugee camps can also cultivation and development of new agricultural temporarily increase malaria endemicity in areas methods, in particular irrigation. where it is low. A forest is a closed environment where the sun's rays do not reach the ground and heliophilic vectors EVOLUTION IN STABLE MALARIA ZONES are absent.