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. Author manuscript; available in PMC: 2011 Oct 31.
Published in final edited form as: Trans R Soc Trop Med Hyg. 2000 Mar-Apr;94(2):113–127. doi: 10.1016/s0035-9203(00)90246-3

Annual Plasmodium falciparum entomological inoculation rates (EIR) across Africa: literature survey, internet access and review

Simon I Hay 1,*, David J Rogers 1, Jonathan F Toomer 1, Robert W Snow 2,3
PMCID: PMC3204456  EMSID: UKMS36405  PMID: 10897348

Abstract

This paper presents the results of an extensive search of the formal and informal literature on annual Plasmodium falciparum entomological inoculation rates (EIR) across Africa from 1980 onwards. It first describes how the annual EIR data were collated, summarized, neo-referenced and staged for public access on the internet. Problems of data standardization, reporting accuracy and the subsequent publishing of information on the internet follow. The review was conducted primarily to investigate the spatial heterogeneity of malaria exposure in Africa and supports the idea of highly heterogeneous risk at the continental, regional and country levels. The implications for malaria control of the significant spatial (and seasonal) variation in exposure to infected mosquito bites are discussed.

Keywords: malaria, Plasmodium fulciparum, entomological inoculation rate (EIR), biting rate, sporozoite index, transmission, disease control, Africa

Introduction

Malaria continues to pose a major public-health threat to many countries in Africa (Snow et al., 1999a). The launch of Roll Back Malaria (RBM) (Nabarro & Tayler, 1998; WHO, 1998) therefore was regarded as timely by many. International public health initiatives such as RBM, aimed at reducing continental burdens of malaria, require an understanding of contemporary malaria distribution, risk and burden (Snow et al., 1996; Le Sueur et al., 1997, Snow et al., 1998a). Several attempts have been made to explore continental malaria distribution (Sutherst, 1993; Lindsay & Martens, 1998; Lindsay et al., 1998; Craig et al., 1999) and disease burden (Snow et al., 1999a, 1999b) using climate models and malariometric data gathered from the literature. There is increasing evidence, however, that the relationship between the frequency of infection and disease outcome is complex and control options should be selected accordingly (Snow et al., 1997, 1998b; Snow & Marsh, 1998: Gupta et al., 1999a, 1999b).

In the absence of a comprehensive reference on malaria exposure in Africa, and owing to its importance in international efforts in malaria control, an overview of the available entomological evidence describing the average annual risk across Africa of receiving a Plasmodium falciparum infected bite from the local vector population [the annual entomological inoculation rate (EIR)] was initiated. This review focuses particularly on the spatial heterogeneity of annual EIR within Africa and its implications for the rationalization of malaria control. A discussion of some of the methodological difficulties involved in comparing annual EIR information is also provided. There is further consideration of the limitations and benefits of data-sharing over the internet. The data described were predicted for non-surveyed areas using remotely sensed imagery from meteorological satellites (D. J. Rogers et al., paper in preparation), to provide the first annual EIR surfaces for the African continent.

Materials and Methods

Annual EIR definition

The activity of the anopheline vector of malaria provides the basis for calculating the EIR, h’, the daily number of infective mosquito bites received per person (MacDonald, 1957). Algebraically h’ = mas; where m is the anopheline density in relation to humans; a is the average number of persons bitten by 1 mosquito in a day, and s is the proportion of mosquitoes with sporozoites in their salivary glands. It is obvious that if the EIR value is to be representative of the year, the estimates of the biting rate and the sporozoite index must be repeated at a monthly (or higher) frequency, for at least a year or complete transmission season. The annual EIR is a favoured measure for assessing malaria endemicity (Burkot & Graves, 1995) and thus the suitability of vector control (Coosemans et al., 1992), as well as the risk of epidemic development (Onori & Grab, 1980). Measuring the annual EIR presents several major difficulties, however, since the entomological methods used in its estimation have not been standardized (Githeko et al., 1996).

Human biting rate

The most direct way to measure the human biting rate (the product of ma) is the human bait catch (WHO, 1975). This involves a team waiting in a given location, usually throughout the night, collecting all the mosquitoes that attempt to feed on exposed individuals. Despite being expensive, technically difficult to replicate and unethical in areas of drug-resistant malaria, this method is unique in that it directly samples human-biting mosquitoes (Le Goff et al., 1997). Other sampling methods such as pyrethrum spray collections and light and exit traps depend on mosquito behaviours that are less directly associated with feeding on humans (Garret-Jones, 1970; Service, 1993). Fortunately, the sampling biases between the most commonly used techniques, human bait and light traps, have been investigated (Lines et al., 1991; Faye et al., 1992; Mbogo et al., 1993a; Davis et al., 1995; Smith, 1995). Correction factors have also been suggested to account for children experiencing lower biting rates than adults in the same location (Port et al., 1980). Details of the protocol used for the standardization of data are presented below.

Sporozoite index

Measurements of the sporozoite index (s) require the number of infective mosquitoes (those with sporozoites in their salivary glands) in the local population to be determined (WHO, 1975). Ideally, but not always, the sporozoite index is derived from the biting rate sample. The traditional method was to dissect all sampled mosquitoes for their salivary glands and subject them to procedures designed to help reveal potential sporozoites under the microscope. More recently, the enzyme-linked immunosorbent assay (ELISA) techniques, which detect Plasmodium-specific circumsporozoite antigens from mosquito head and/or thorax samples, are being increasingly used owing to their greater sensitivity and species specificity (Burkot et al., 1984). No attempt was made to standardize the sporozoite index in this study because sensitivity and specificity of microscopy (Kilian et al., 2000) and ELISA techniques will vary between studies.

Identification of sources of annual EIR data

Online abstracting databases (Biological Abstracts®, BIOSIS, Philadelphia, Pennsylvania, USA, MEDLINE®, US National Library of Medicine, Bethesda, Maryland, USA; CAB HEALTH, CAB International Inc., Wallingford, Oxfordshire, UK; and the Zoological Record Online®, BIOSIS, Philadelphia, Pennsylvania, USA) were searched with the following keywords (entomological inoculation rate, EIR, h’, biting rate, ma, sporozoite index, s, Plasmodium falciparum Anopheles gambiae, Anopheles funestus, Anopheles, vectorial capacity, malaria transmission, malaria control, bednets, ITBN, human bait, pyrethroid spray, light trap, exit trap, bionomics). This search resulted in a large number of references which were scrutinized for annual EIR data. From this subset of publications a ‘key author list’ was compiled and these names were re-entered into the abstracting databases and further relevant manuscripts were retrieved. The bibliographies of all recovered manuscripts were then checked for potential additional references. These references were collected and the searching strategies repeated until no new information was forthcoming. The list of papers found for each author was then compiled and a letter sent to each individual requesting they check that the bibliography was complete and the data were abstracted correctly. Forty-three letters were sent, to which there were 21 replies, mostly with further information from the ‘grey’ literature.

Recording and standardization of the annual EIR values

The values recorded in Table 1 are P. falciparum-infected bites per adult, per night indoors, using human biting rates averaged over a year. In most studies the author(s) had expressed their data in the above format. When adjustment was necessary, this commonly involved converting light-trap catches to their human-bait equivalent by multiplying by 1·5 (Lines et al., 1991) and more rarely converting child EIR values to those of adults by multiplying by 3·57 (Port et al., 1980). In addition to the annual EIR, the average annual biting rate and the sporozoite index are also presented. It should be noted, however, that owing to the approximation inherent in rounding, the product of the biting and sporozoite indices may not exactly match the recorded annual EIR. For each location an index of the length of the transmission season was expressed as the number of months in which 75% of the annual EIR was transmitted. When studies did not provide enough information for this to be calculated the length of the transmission season only was recorded. The percentage of the total annual EIR transmitted by Anopheles gambiae s.l. freshwater species, An. funestus and all other species was also noted. Finally, the land-use in which the study site was located was classified as dominantly rural, irrigated rice or urban.

Table 1.

Geo-referenced locations in Africa for which annual P. falciparum EIR data were published

Country and site Location1
(Long., Lat.)
Land-use2 Study date Sporozoite
index3 (%)
Biting rate4
(ma annual)
EIR5
(annual)
Seasonality6
(months)
Relative EIR
transmission by
species7 (%)
Citation
Benin
 Cotonou, Gbégamé
  quarter
2·45, 6·36a U Jan 1987–Dec 1987 2·80e 1179g 33·0 1 100:0:0 Akogbeto et al., 1992
 Cotonou, Ladji quarter 2·43, 6·38a R Jan 1987–Dec 1987 1·60e 3666g 58·0 4 100:0:0 Akogbeto et al., 1992
 Cotonou, Sainte-Rita
  Norde quarter
2·42, 6·37a R Jan 1987–Dec 1987 1·40e 3363g 47.0 4 100:0:0 Akogbeto et al., 1992
 Ganvié 2·42, 6·47b R Jan 1993–Dec 1993 0·43e 2555g 11·0 6 P:A:P Akogbeto, 1995
 Ganvié, near lake Nokoué 2·42, 6·42a R Jan 1993–Dec 1995 0·37 e 2917g 10·8 6 100:0:0 Akogbeto & Nahum, 1996
Burkina Faso
 Barna −4·42, 11·38b R Jan 1981–Dec 1981 NANA NANA 175·2 3 57:35:8 Carnevale & Robert, 1987
 Baré −4·10, 11·08b R Jan 1981–Dec 1981 NANA NANA 91·3 3 57:35:8 Carnevale & Robert, 1987
 Barkournbilen and
  Barkoundouba
−1·23, 12·67b R Aug 1994–Jun 1995 NAf NAh 140·0 4–5 P:P:A Modiano et al., 1996
 Bobo-Dioulasso −4·30, 11·20b U Nov 1991–Jan 1993 0·10e 2150g 2·2 NA 100:0:0 Lochouarn & Gazin, 1993
 Bobo-Dioulasso, Colrna-
  Nord quarter
−4·30, 11·21a U/R Jan 1985–Dec 1985 0·19e 2421 g 4·6 NA 100:0:0 Robert et al., 1986
 Bobo-Dioulasso,
  Diaradougou quarter
−4·29, 11·18a U Jan 1985–Dec 1985 0·19e 74 g 0·1 NA 100:0:0 Robert et al., 1986
 Bobo-Dioulasso,
  Dioulassoba quarter
−4·30, 11·19a U Jan 1985–Dec 1985 0·19e 289 g 0·6 NA 100:0:0 Robert et al., 1986
 Bobo-Dioulasso, Sarfalao
  quarter
−4·30, 11·20b U Jun 1993–Sep 1993 0·50e 800 g 4·0 NA 100:0:0 Gazin et al., 1996
 Dandé and Tago −4·55, 11·59a R Jan 1983–Dec 1984 3·99 e 1380g 55·0 3 69:31:0 Robert et al., 1985
 Desso −4·28, 11·35b R Jan 1981–Dec 1981 NANA NANA 208·1 3 57:35:8 Carnevale & Robert, 1987
 Gounghin Nord −1·55, 12·37a U Jul 1984–Dec 1984 NAe NAh 0·0 0 0:0:0 Rossi et al., 1986
 Karangasso −4·63, 11·22b R Jan 1985–Feb 1986 4·13 f 6360g 262·8 4 57:41:2* Boudin et al., 1991
 Karangasso, Koko suburb −4·65, 11·22a R Feb 1985–Feb 1986 NAe NAg 116.0 4 P:P:P Robert et al., 1988
 Karangasso, Koko suburb −4·65, 11·22a R May 1985–Apr 1986 2·60 e 4548g 116·1 5 51:49:0 Carnevale et al., 1988
 Karangasso, Koko suburb −4·65, 11·22a R May 1986–Apr 1987 4·56 e 4913g 223·8 4 59:41:0 Carnevale et al., 1988
 Karangasso, Massasso
  suburb
−4·64, 11·21a R Feb 1985–Feb 1986 NAe NAg 370·0 4 P:P:P Robert et al., 1988
 Karangasso, Massasso
  suburb
−4.64, 11.21a R May 1985–Apr 1986 5·75 e 7012g 403·3 4 60:37:3 Carnevale et al., 1988
 Kongodjan −4·45, 11·58a R Jan 1983–Dec 1984 1·78 e 7480g 133·0 4 52:48:0 Robert et al., 1985
 Kongodjan −4·45, 11·58a R Jan 1983–Dec 1983 NAe NAg 133·0 3 P:P:A Gazin et al., 1988
 Koro −4·20, 11·15b R Jan 1981–Dec 1981 NANA NANA 171·6 3 57:35:8 Carnevale & Robert, 1987
 Koubri −1·38, 12·15a R Jul 1984–Dec 1984 NAe NAh 441·6 3 P:P:A Rossi et al., 1986
 Ouagadougou, Kologh
  Naba suburb
−1·54, 12·39a U Jul 1984–Dec 1984 NAe NAh 1·5 3 P:P:A Rossi et al., 1986
 Ouagadougou,
  Nongremassm suburb
−1·51, 12·40a U Jul 1984–Dec 1984 NAc NAh 7·7 3 P:P:A Rossi et al., 1986
 Ouagadougou,
  Saint Camille suburb
−1·52, 12·36a U Jul 1984–Dec 1984 NAe NAh 5·6 1 P:P:A Rossi et al., 1986
 Ouagadougou, Saint Léon
  suburb
−1·52, 12·37a U Jul 1984–Dec 1984 NAe NAh 0·0 0 0:0:0 Rossi et al., 1986
 Pabré −1·58, 12·50a R Jul 1984–Dec 1984 NAe NAh 113·0 3 P:P:A Rossi et al., 1986
 Soumosso −4·05, 11·02a R Jan 1981–Dec 1981 NANA NANA 200·8 3 57:35:8 Carnevale & Robert, 1987
 Tago −4·38, 11·67a R Jan 1983–Dec 1983 NAe NAg 82·0 3 P:P:A Gazin et al., 1988
 Toukoro −4·25, 11·43b R Jan 1981–Dec 1981 NANA NANA 76·7 3 57:35:8 Carnevale & Robert, 1987
 VK1 −4·41, 11·35a I Jan 1981–Dec 1981 NANA NANA 0 3 72:25:3 Carnevale & Robert, 1987
 VK2 −4·41, 11·37a I Jan 1981–Dec 1981 NANA NANA 21·9 3 72:25:3 Carnevale & Robert, 1987
 VK3 −4·41, 11·38a I Jan 1981–Dec 1981 NANA NANA 62·1 3 72:25:3 Carnevale & Robert, 1987
 VK4 −4·42, 11·37a I Jan 1981–Dec 1981 NANA NANA 20·1 3 72:25:3 Carnevale & Robert, 1987
 VK4 −4·42, 11·37a I Jan 1983–Dec 1984 0.36 e 14000g 50·0 4 100:0:0 Robert et al., 1985
 VK4 −4·42, 11·37a I May 1985–Apr 1986 0–21 e 25857g 54·9 4 90:10:0 Robert & Carnevale, 1991
 VK5 −4·43, 11·38a I Jan 1981–Dec 1981 NANA NANA 36·5 3 72:25:3 Carnevale & Robert, 1987
 VK6 −4·44, 11·37a I Jan 1981–Dec 1981 NANA NANA 54·8 3 72:25:3 Carnevale & Robert, 1987
 VK6 −4·44, 11·37a I Jan 1983–Dec 1984 0–43 e 13900g 60·0 2 90:10:0 Robert et al., 1985
 Zagtouli −1·63, 12·33a R/I Jul 1984–Dec1984 NAe NAh 82 2 P:P:A Rossi et al., 1986
Burundi
 Gasange 29·60, −4, 32a NA Nov 1992–0ct 1993 3·14 e 8503 g 267·0 6 P:P:A Van Bortel et al., 1996
 Gihanga Mulira 29·29, −3·19b I Jan 1983–Dec 1983 0·48e 21499g 103·2 12 96:4:0 Coosemans, 1985
 Gisenga 29·67, −4, 44a NA Nov 1992–May 1994 3·85 e 6534 g 251·7 5 P:P:A Van Bortel et al., 1996
 Kazirabageni 29·63, −4·23a R Nov 1992–May 1994 2·02 e 1753 g 35·4 5 P:P:A Van Bortel et al., 1996
 Mugere 29·66, −4, 37a I Nov 1992–0ct 1993 1–02 e 59021 g 600·9 7 P:P:A Van Bortel et al., 1996
Cameroon
 Ebogo 11·47, 3·40b R Apr 1991–Mar 1992 0·93 e 38189g 355·0 6 14:0:86 Njan Nloga et al., 1993
 Edea, Bililang suburb 10·13, 3·80a U Jan 1990–Dec 1990 1·20e 319g 3·8 NA 100:0:0 Robert et al., 1993
 Edea, Pongo suburb 10·13, 3·80a U Jan 1990–Dec 1990 8·21 e 368g 30·2 NA 100:0:0 Manga et al., 1993
 Etoa 11·48, 3·77b R Feb 1996–May 1996 4·10 e 11571g 474·5 NA P:55:45 Manga et al., 1997b
 Mbébé 11·00, 4·15a R Apr 1989–Mar 1990 1–93 NA 10330g 200·0 5 36:9:55 Le Goff et al., 1992
 Nditam 11·26, 5·36a R May 1995–Mar 1996 9·40e 876g 82·1 1 100:0:0 Manga et al., 1997a
 Nsimalen, Ekoko 12·12, 3·82a R Apr 1991–Mar 1992 1·22 e 8724g 106·0 5–6 13:0:87 Manga et al., 1995
 Nsimalen, Nkol Mefou 11·57, 3·70a R Apr 1991–Mar 1993 2·33 e 2920g 68·0 6 100:0:0 Manga et al., 1995
 Sanaga river villages 11·00, 4·15a R Apr 1989–Mar 1990 1·77 e 10303g 182·1 6 43:0:57 Carnevale et al., 1992
 Yaoundé, Essos 11·00, 3·00a U Mar 1989–Feb 1990 20·3 e 64g 13·0 NA 100:0:0 Manga et al., 1993
 Yaoundé, Nkol Bikok
  quarter
11·52, 3·87a U Mar 1989–Mar 1990 5·00e 284·7g 14·2 1 100:0:0 Fondjo et al., 1992
Congo, Democratic Republic of
 Kinkolé 15·51, −4·36a R Feb 1990–Dec 1991 3·24 e 1241g 40·2 12 100:0:0 Karch et al., 1993
 Kinshasa, Kimbangu 3
  district
15·31, −4, 35·c U Sep 1988–Dec 1989 1·86f 1862 g 29·2 5–6 100:0:0 Coene, 1993
 Kinshasa, Kwamuthu
  district
15·27, −4, 46c R Sep 1988–Dec 1989 7·41 f 6132 g 454·4 5–6 86:2:12 Coene, 1993
 Kinshasa, rural area 15·30, −4·44a R Apr 1989–0ct 1990 NAe NAg 612·0 NA 100:0:0 Karch et al., 1992
 Kinshasa, semi–rural area 15·35, −4·36a R/U Apr 1989–0ct 1990 NAe NAg 198.7 NA 100:0:0 Karch et al., 1992
 Kinshasa, urban area 15·31, −4·31a U Apr 1989–Oct 1990 0·3e 953g 2·8 NA 100:0:0 Karch et al., 1992
Congo, Republic of
 Brazzaville 15·28, −4·26b U Oct 1982–May 1984 3·47e 650g 22·5 NA 100:0:0 Trape & Zoulani, 1987b
 Kulila 12·43, −4·17a R Oct 1981–Oct 1982 4·91 e 8103 g 397·9 5 97:3:0 Richard et al., 1988
 Linzolo 15·11, −4·41 b R Oct 1981–Jan 1984 2·11 e 11673g 246·0 9 NA Trape & Zoulani, 1987a
 Linzolo 15·11, −4·41 b R Oct 1983–Sep 1984 NANA NNg 234·6 9 96:4:0 Trape et al., 1987
 Makaba 12·38, −4·14b R Oct 1981–0ct 1982 6·85 e 1168 g 80·0 6 89:11:0 Richard et al., 1988
Côte d’Ivoire
 Alloukoukro −5·08, 7·80a R Jan 1991–Dec 1991 3·13 e 8507 g 266·5 6 77:23:0 Dossou-yovo et al., 1995
 Alloukoukro −5·08, 7·80a R Jan 1992–Dec 1992 3·39 e 5792 g 196·5 5 81:19:0 Dossou-yovo et al., 1995
Egypt
 El Zawya 30·85, 29·42a R Feb 1983–Feb 1984 0·00e 128g 0·0 0 0:0:0 El Said et al., 1986
Gambia, The
 Bakau −16·68, 13·48b U Jun 1988–May 1989 0·98f 128g 1·0 k NA 100:0:0 Lindsay et al., 1990
 Barokunda −15·32, 13·65b R NA 1988–NA 1988 2·97 f 3701i 110·0 5 100:0:0 Lindsay et al., 1993
 Biran Giddo Ya −15·85, 13·59a R Jun 1986–Dec 1986 2·67 e 412j 11·0 3 100:0:0 Lindsay et al., 1989
 Bwiam −16·09, 13·23c R Jan 1991–Dec 1991 2.47f 37h 0·9 5 P:A:P Thomson et al., 1994
 Dasilamia −14·27, 13·41 c R Jan 1991–Dec 1991 2·23f 54h 1·2 5 P:A:P Thomson et al., 1994
 Dasilamib −15·23, 13·48b R NA 1988–NA 1988 1·29 f 1160i 15·0 5 100:0:0 Lindsay et al., 1993
 Dongoro Ba −15·28, 13·38b R NA 1988–NA 1988 17·86 f 448i 80·0 5 100:0:0 Lindsay et al., 1993
 Jahally −14·97, 13·55c R Jan 1991–Dec 1991 0·95f 443h 4·2 5 P:A:P Thomson et al., 1995
 Jalangbereh −15·40, 13·38b R NA 1988–NA 1988 9·10 f 769i 70·0 5 100:0:0 Lindsay et al., 1993
 Jessadi −15·30, 13·63b R NA 1988–NA 1988 2·69 f 2380i 64·0 5 100:0:0 Lindsay et al., 1993
 Katamina −15·28, 13·55b R NA 1988–NA 1988 0·37 f 1076i 4·0 5 100:0:0 Lindsay et al., 1993
 Kerewan − 16·09, 13·49c R Jan 1991–Dec 1991 0·23f 211h 0·4 5 P:A:P Thomson et al., 1995
 Kulari −14·08, 13·40c R Jan 1991–Dec 1991 7·65f 0·28h 7·8 5 P:A:P Thomson et al., 1995
 Madina −15·25, 13·52a R NA 1988–NA 1988 7·45 f 2376i 177·0 5 100:0:0 Lindsay et al., 1993
 Male Kunda −15·30, 13·55b R NA 1988–NA 1988 2·60 f 2692i 70·0 5 100:0:0 Lindsay et al., 1993
 Niawodurulung −15·22, 13·46a R NA 1988–NA 1988 0·00f 1481i 0·0 5 100:0:0 Lindsay et al., 1993
 Pakali Ba −15·25, 13·50b R NA 1988–NA 1988 3·04 f 3253i 99·0 5 100:0:0 Lindsay et al., 1993
 Salikene −15·97, 13·48c R Jan 1991–Dec 1991 0·55f 360h 1·9 5 P:A:P Thomson et al., 1995
 Sare Alpha −13·98, 13·37b R Jan 1991–Dec 1991 6·07f 187h 11·2 5 P:A:P Thomson et al., 1995
 Saruja −14·90, 13·55c I Jan 1991–Dec 1991 2·17f 231h 5·0 5 P:A:P Thomson et al., 1995
 Saruja −14·90, 13·55c I Mar 1987–Jun 1988 NAf NAi 3·0 k NA 100:0:0 Lindsay et al., 1991
 Sibanor −16·20, 13·21b R Jan 1991–Dec 1991 2·88f 113h 3·2 5 P:A:P Thomson et al., 1994
 Sitahuma −15·40, 13·43a R NA 1988–NA 1988 2·23 f 3366i 75·0 5 100:0:0 Lindsay et al., 1993
 Sutukoba −14·02, 13·50c R Jan 1991–Dec 1991 2·94f 34h 1·0 5 P:A:P Thomson et al., 1994
 Turan −15·72, 13·58a R Jun 1986–Dec 1986 2·70e 890i 23·8 3 100:0:0 Lindsay et al., 1989
 Wellingara Ba −15·26, 13·41a R NA 1988–NA 1988 2·19 f 1553i 34·0 5 100:0:0 Lindsay et al., 1993
Kenya
 Ahero 34·92, −0·17c I May 1989–Jun 1990 NAf NAg 416·0 7 15:85:0 Githeko et al., 1993
 Fumbini 39·84, −3·61c NA Jan 1992–May 1993 2·30 f 131g 3·0 1 95:5:0 Mbogo et al., 1995
 Kambi ya Wari 39·84, −3·52c U/R Jan 1992–May 1993 10·54 f 37g 3·6 2 95:5:0 Mbogo et al., 1995
 Kaoyeni 39·90, −3·46c NA Jan 1992–May 1993 4·08 f 61g 2·5 1 95:5:0 Mbogo et al., 1995
 Kanyawegi 34·67, −0·92c R Jun 1991–May 1992 5·29 f 4917gh 259·9 NA 64:36:0 Oloo et al., 1996
 Karima 37·32, −0·69c NA Oct 1986–Jun 1987 2·65 f 657 g 17·4 NA 80:10:10 Ijumba et al., 1990
 Kibarani 39·85, −3·58c NA Jan 1992–May 1993 11·31 f 159g 18·0 4 95:5:0 Mbogo et al., 1995
 Kilifi Town 39·85, −3·63b U Dec 1990–Nov 1991 2·17 f 69gh 1·5 2 100:0:0 Mbogo et al., 1993b
 Kisian 34·67, −0·07b R Oct 1985–Jun 1988 5·98 e f 5001g 299·3 5–6 76:24:0 Beier et al., 1990
 Loboi swamp 36·06, 0·36a R NA 2·55 e 915 h 23·4 2 84:16:0 Aniedu, 1997
 Mikingirini 39·87, −3·56c NA Jun 1992–May 1993 0·00 f 46g 0·0 0 0:0:0 Mbogo et al., 1995
 Miwani 35·01, −0·59c R May 1989–Jun 1990 NAf NAg 91·0 3 80:20:0 Githeko et al., 1993
 Mtondia 39·90, −3·57c NA Jun 1992–May 1993 2·09 f 2844g 59·6 2 95:5:0 Mbogo et al., 1995
 Mukombe 39·86, −3·52c NA Jun 1992–May 1993 7·25 f 52g 3·8 2 95:5:0 Mbogo et al., 1995
 Mumias 34·49, 0·18c R May 1995–Mar 1996 7·22 f 647 h 46·7 7 63:37:0 Shililu et al., 1998
 Perkerra irrigation
  scheme
35·97, 0·47a I NA 3·08 e 533h 16·4 2 100:0:0 Aniedu, 1997
 Saradidi 34·38, −0·12a R Feb 1986–0ct 1987 NAef NAg 273·8 5 90:10:0 Beier et al., 1994
 Saradidi 34·38, −0·12a R Oct 1985–Jun 1988 17·1 e f 1387g 237·3 5 90:10:0 Beier et al., 1990
 Sokoke 39·82, −3·53b NA Dec 1990–Nov 1991 4·23 f 189gh 8·0 4 100:0:0 Mbogo et al., 1993b
 Ufuoni 39·93, −3·46c NA Jun 1992–May 1993 0·01 f 19g 0·0 1 95:5:0 Mbogo et al., 1995
 Zowerani 39·92, −3·51c NA Jun 1992–May 1993 2·35 f 68g 1·6 1 95:5:0 Mbogo et al., 1995
Madagascar
 Ambodifotatra, St Joseph,
  Maromandia
49·90, −17·00b R Nov 1988–0ct 1989 0·59 f 13761 g 82·0 12 P:P:P Fontenille et al., 1992
 Manarintsoa 47·42, −18·92c R Oct 1987–Jul 1988 0·71f 212 g 1·5 4–5 97:3:0 Lepers et al., 1991
 Manarintsoa 47·42, −18·92c R Oct 1988–Feb 1990 0·14 f 657 g 0·9 NA NA Fontenille et al., 1990
Mozambique
 Matola 32·93, −25·94a U/R Jan 1994–Jun 1995 NAf NAg 12 NA P:P:A Thompson et al., 1997
Senegal
 Aéré Lao −14·30, 16·40a R May 1982–Aug 1983 0·24 e 1600g 3·8 3–4 100:0:0 Vercruysse, 1985
 Affiniam, Diagobel,
  Tendimane
−16·30, 12·70a R Jan 1985–Nov 1985 0·03 e 22490 g 7·0 5 100:0:0 Faye et al., 1994
 Barkedji −14·87, 15·28d R Jul 1994–Mar 1996 NAf NAgh 111·1 2–3 100:0:0 Le Masson et al., 1997
 Boké Diallobé −14·00, 16·10a R May 1982–Aug 1983 1·2 e 200g 2·4 3–4 100:0:0 Vercruysse, 1985
 Dakar, around Grande
  Niaye marsh
−17·42, 14·75d NA Jan 1988–Dec 1988 NAe NAg 0·1 NA 100:0:0 Trape et al., 1992
 Dielmo −16·42, 13·72d R Apr 1990–Mar 1992 1·29f 14790g 191·5 5 69:31:0 Konaté et al., 1994
 Dielmo −16·42, 13·72b R Apr 1992–Mar 1993 NAf NAgh 222·8 6 23:77:0 Fontenille et al., 1997b
 Dielmo −16·42, 13·72b R Apr 1993–Mar 1994 NAf NAgh 78·5 4–5 74:26:0 Fontenille et al., 1997b
 Dielmo −16·42, 13·72b R Apr 1994–Mar 1995 NAf NAgh 135·2 4 66:34:0 Fontenille et al., 1997b
 Dielmo −16·42, 13·72b R Jan 1990–Dec 1990 NANA? NANA? 115·0 4 100:0:0 Rogier & Trape, 1993
 Dielmo −16·42, 13·72b R Jun 1990–May 1991 1·04e 9731 g 101·2 4 P:P:A Trape et al., 1994
 Dielmo −16·42, 13·72b R Jun 1991–May 1992 1.57e 17357 g 272·5 6 P:P:A Trape et al., 1994
 Diohine −16·52, 14·50b R Jan 1995–Dec 1995 1·95 f 680g 13·3 5 100:0:0 Robert et al., 1998
 Diomandou Dieri −14·44, 16·50a I Jun 1990–Nov 1991 0·00 e 2665g 0·0 0 0:0:0 Faye et al., 1993
 Diomandou Walo −14·43, 16·51a I Jun 1990–Nov 1991 0·09 e 5913g 5·2 1 100:0:0 Faye et al., 1993
 Kassack-Nord −16·03, 16·40a R Sep 1992–Nov 1994 0·00f NAg 0·0 0 0:0:0 Faye et al., 1995b
 Kotiokh −16·58, 14·48a R Jan 1995–Dec 1995 1·75 f 1558g 27·3 8 100:0:0 Robert et al., 1998
 Maka-Diama −16·40, 16·20a R Sep 1992–Nov 1994 0·00f NAg 0·0 0 0:0:0 Faye et al., 1995b
 Ndiop −16·42, 13·75b R Jan 1993–Dec 1993 NAf NAg 63·0 2 100:0:0 Fontenille et al., 1997a
 Ndiop −16·42, 13·75b R Jan 1994–Dec 1994 NAf NAg 17·0 2 100:0:0 Fontenille et al., 1997a
 Ndiop −16·42, 13·75b R Jan 1995–Dec 1995 NAf NAg 37·0 2 93:7:0* Fontenille et al., 1997a
 Ndiop −16·42, 13·75b R Jan 1996–Dec 1996 NAf NAg 7·0 1 100:0:0 Fontenille et al., 1997a
 Ndiop −16·42, 13·75b R May 1993–Dec 1996 NAf NAg 31·0 2 98:2:0 Fontenille et al., 1997a
 Ngayokheme −16·43, 14·53b R Jan 1995–Dec 1995 1·80f 512g 9·2 NA 100:0:0 Robert et al., 1998
 Pout region −17·07, 14·77a NA Aug 1988–Jul 1989 0·00e 23908 g 0·0 0 0:0:0 Faye, 1992
 Pikine −17·40, 14·75b U Oct 1979–Dec 1980 0·55e 7818 g 43·0 4 100:0:0 Vercruysse & Jancloes, 1981
 Thiaye, Ngadiaga,
 Diamballo
−17·05, 14·95a R Jul 1991–Jun 1992 0·48f 2305g 11·0 4 100:0:0 Faye et al0, 1995a
 Takème and Ousseuk −16·20, 12·80a NA Jan 1985–Nov 1985 0·36 e 15123 g 55·0 5 100:0:0 Faye et al., 1994
 Toulde Galle −14·60, 16·53a I Jun 1990–Nov 1991 0·00 e 438g 0·0 0 0:0:0 Faye et al., 1993
Sierra Leone
 Bayama −11·77, 8·00a R Nov 1990–Oct 1991 4·36 f 20294 g 884·2 7 100:0:0 Bockarie et al., 1995
 Bayama −11·77, 8·00a R Nov 1990–Oct 1991 3·90f 20732g 808·5 12 100:0:0 Bockarie et al., 1993
 Bayama −11·77, 8·00a R Aug 1993–Nov 1994 6·82f 6125g 417·6 NA NA E. B. Magbity, 1999, pers.
com.; Magbity et al., 1999
 Baoma −11·73, 8·04a R Aug 1993–Nov 1994 2·16f 3924g 84·8 NA NA E. B. Magbity, 1999, pers.
com.; Magbity et al., 1999
 Dandabu −11·64, 8·15a R Aug 1993–Nov 1994 5·38f 1507g 81·1 NA NA E. B. Magbity, 1999, pers,
com.; Magbity et al., 1999
 Gumahun −11·50, 8·18a R Jun 1992–Jul 1993 6·94f 175gi 12·1 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
 Jaiama −11·69, 8·16a R Aug 1993–Nov 1994 8·72f 2602g 226·9 NA NA E. B.Magbity, 1999, pers.
com.; Magbity et al., 1999
 Konjodorma −11·50, 8·15a R Jun 1992–Jul 1993 3·96f 818gj 32·4 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
 Kpetema −11·50, 8·13a R Jun 1992–Jul 1993 8·81f 1551gj 136·7 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
 Manjama −11·74, 8·02a R Aug 1993–Nov 1994 12·13f 2340g 283·7 NA NA E. B. Magbity, 1999, pers.
com.; Magbity et al., 1999
 Mendewa −11·48, 8·16a R Jun 1992–Jul 1993 4·74f 128gj 6·1 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
 Mendewa −11·48, 8·16b R Jan 1990–Apr 1991 NAf NANA 21·9 4–5 58:42:0 Bockarie et al., 1994
 Nengbema −11·68, 8·13a R Jun 1992–Jul 1993 4·42f 471gj 20·8 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
 Nengbema −11·68, 8·13a R Jan 1990–Apr 1991 NN NANA 21·5 4–5 97:3:0 Bockarie et al., 1994
 Ngalu −11·56, 8·11a R Jun 1992–Jul 1993 6·08f 712gj 43·3 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
 Njala-Komboya −11·54, 8·20a R Jan 1990–Apr 1991 NAf NANA 26·7 4–5 96:4:0 Bockarie et al., 1994
 Nyandeyama −11·66, 8·12a R Jun 1992–Jul 1993 4·05f 993gj 40·2 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
 Nyandeyama −11·66, 8·12a R Jan 1990–Apr 1991 NAf NANA 35·5 4–5 97:3:0 Bockarie et al., 1994
 Tondoya −11·64, 8·13a R Jun 1992–Jul 1993 7·30f 1424gj 102.0 NA 100:0:0 Magbity et al., 1997;
E. B. Magbity, 1999,
pers. com.
Tanzania
 Chasimba 38·82, −6·58c R Jan 1992–Dec 1992 NAf NAi 217·7 9 87:13:0 Shiff et al., 1995
 Idete 36·48, −8·10b R Jan 1992–Dec 1994 NAf NAi 584·0 12 P:P:A Charlwood et al., 1998
 Kongo 38·83, −6·53b R Jan 1992–Dec 1992 NAf NAi 576·7 12 81:19:0 Shiff et al., 1995
 Kaole 38·93, −6·45c R Jan 1996–Dec 1996 NAf NAi 124·3 12 66:34:0 Temu et al., 1998
 Kerege 39·03, −6·57b R Jan 1992–Dec 1992 NAf NAi 271·6 11 88:12:0 Shiff et al., 1995
 Kikwawila 36·75, −8·08a R Dec 1983–Aug 1984 2·49 e 11427 NA 284·0 3 69:31:0 Biro, 1987
 Kikwazu 38·82, −5·40a R Apr 1988–Dec 1989 NAe NAi 667·0 3 100:0:0 Mnzava, 1991
 Kongo 38·83, −6·53c R Jan 1996–Dec 1996 NAf NAi 306·3 12 83:17:0 Temu et al., 1998
 Kumbamtoni 38·76, −5·10a R Jan 1987–Dec 1988 7·14 f 6004i 428·8 NA P:P:P Magesa et al., 1991
 Kumbamtoni 38·82, −5·31a R Apr 1987–Mar 1989 NAe NAi 420·0 6 100:0:0 Mnzava, 1991
 Mapinga 39·07, −6·60b R Jan 1992–Dec 1992 NAf NAi 235·6 9 83:17:0 Shiff et al., 1995
 Matimbwa 38·87, −6·50b R Jan 1992–Dec 1992 NAf NAi 702·6 12 90:10:0 Shiff et al., 1995
 Matimbwa 38·87, −6·50c R Jan 1996–Dec 1996 NAf NAi 122·1 12 51:49:0 Temu et al., 1998
 Michenga 36·63, −8·12a R Jan 1990–Dec 1990 NAe NAi 547·5 4 100:NA:NA Babiker et al., 1997
 Michenga 36·63, −8·12a R Jan 1990–Dec 1991 NAf NAi 548·0 4 100:NA:NA Lyimo, 1993
 Namawala 36·40, −8·15e R Mar 1990–Jul 1991 1.37 f 24090i 329·0 6 P:P:A Smith et al., 1993
 Temgini and Enzi 38·76, −5·18c NA NA 1995–NA 1996 NAf NAgi 405·0 NA NA C. F. Curtis, 1998,
pers. com.
 Umba 38·87, −5,17a R Jan 1987–Dec 1988 4.68 f 11899i 556·7 NA P:P:P Magesa et al., 1991
 Yombo 38·85, −6·57b R Jan 1992–Dec 1992 NAf NAi 220·6 10 79:21:0 Shiff et al., 1995
 Zinga 38·98, −6·52b R Jan 1992–Dec 1992 NAf NAi 93·7 7 88:12:0 Shiff et al., 1995
1

Sources of latitude/longitude:

a

published maps

b

GeoName™ digital gazetteer CD-ROM;

c

correspondence with authors;

d

cited in the reference.

2

R, rural; U, urban; I, irrigated rice.

3

Sporozoite detection method:

e

dissection;

f

ELISA.

4

ma sampling technique:

g

human bait;

h

insecticide spray;

i

i light traps;

j

exit traps.

5

EIR values:

k

converted using (Port et al., 1980) average child-to-adult conversion factor, f, of 3·57.

6

Values in italics record the number of months in which 75% of transmission occurs. Other values record the length of the entire transmission season.

7

For Anopheles garnbiae s.l.: An. funestus: any other locally important vector. Where absolute values are unavailable P indicates presence only and A absence.

NA refers to data that were not available, or not applicable. Values in bold refer to calculated values.

Criteria for data exclusion

Annual EIR data measured before 1980 were excluded because it was not clear whether information collected over 20 years ago would be representative of the conditions today. In addition, the data were also extracted for comparison with contemporaneous meteorological satellite sensor data available from 1981 to date (D. J. Rogers et al., paper in preparation). Moreover, data before 1980 were more difficult to search using electronic abstracting databases, although such information would make a useful addition to those compiled here. Sites were also excluded if malaria control activities, local bednet and/or insecticide usage were reported. The possibility of unreported use of bednets, insecticide and repellents in the studies remains a problem, however, so these data are best interpreted as potential EIR values. Finally, data were not included if the sampling frequency and duration of observation were insufficient to record the EIR throughout the entire year or transmission season.

Methodological information

EIR sampling methods vary considerably and those used in obtaining the biting rate (human bait, pyrethroid spraying, light or exit traps) and the means of measuring the sporozoite index (dissection or ELISA) are noted in Table 1. Calculation of the sporozoite index was often complicated by the subdivision of the index into proportions due to different Plasmodium species. Only the sporozoite indices attributable to P. falciparum were used. Where the relative contribution to total transmission level for each species was not documented, the total was assumed to be due to P. falciparum.

Geo-referencing

Sites were geo-referenced using information from the original references, published maps and/or the GeoName™ digital gazetteer CD-ROM (GDE Systems Inc., San Diego, CA, USA). Sites for which co-ordinates were found are included in Table 1. Those sites which had conflicting latitude and longitude values from different sources were double-checked and the erroneous co-ordinates discarded. The method used to geo-reference each annual EIR value was therefore also recorded.

Data distribution

The information reviewed in this paper is available for downloading from both the Mapping malaria risk in Africa/Atlas du risque de la malaria en Afrique (MARA/ARMA URL; http://www.mara.org.za) and Scientists for Health And REsearch for Development (SHARED URL; http://www.shared.de) web sites as comma separated text files. Mechanisms for correcting existing and adding additional information will be staged in the near future. The Appendix, which provides an example of a completed data sheet used to abstract data in this study, is also available for downloading.

Results

Annual EIR data

Four hundred references were retrieved and searched for annual EIR data of which 91 satisfied the selection criteria. These papers contained 201 temporally distinct annual EIR measurements from 16 countries. Of these, 159 were spatially distinct. Table 1 contains all the 193 geo-registered sites from 15 countries with data collected after 1980. Table 2 contains annual EIR data that could not be geo-registered: 8 sites in 5 countries. The Figure shows the distribution of the study sites detailed in Table 1. The apparent disparity in the number of sites is due to the close proximity of many studies which could not be resolved on a map of Africa at the continental scale.

Table 2.

Locations in Africa for which annual EIR data were published that could not be geo-referenced

Country and site Location1
(Long., Lat.)
Land-
use2
Study date Sporozoite
index3 (%)
Biting rate4
(ma annual)
EIR5
(annual)
Seasonality6
(months)
Relative EIR
transmission by
species7 (%)
Citation
Cameroon
 Yaoundé, Nikol Bisson NA, NA R Mar 1989–Mar 1990 1·67e 1814g 30·3 3 100:0:0 Fondjo et al., 1992
Congo, Democratic Republic of
 Mbangu-mbamu NA, NA R Feb 1990–Nov 1990 2·50e 17·3g 157·0 12 100:0:0 Karch et al., 1993
 Mbansale NA, NA R Feb 1990–Nov 1990 3·50e 5001g 175·2 12 100:0:0 Karch et al., 1993
 Mbansalé NA, NA R Feb 1991–Nov 1991 4·80e 6826g 324·9 12 100:0:0 Karch et al., 1993
Côte d’Ivoire
 Bouaké, market garden
  districts
NA, NA R Jan 1992–Dec 1992 2·01 NA 4380g 88 8 100:0:0 Dossou-yovo et al., 1994
 Bouaké, rice fields district NA, NA I Jan 1992–Dec 1992 0–68 NA 18615g 126 9 100:0:0 Dossou-yovo et al., 1994
Egypt
 Abheet village NA, NA R Feb 1983–Feb 1984 0·59e 308g 1·8 1 0:0:100 El Said et al., 1986
Sudan
 Asar NA, NA R Oct 1990–Dec 1990 2·06e NAih 0·6 NA 100:0:0 Babiker et al., 1997
1

Sources of latitude/longitude:

a

published maps;

b

GeoName™digital gazetteer CD–ROM;

c

correspondence with authors;

d

cited in the reference.

2

R, rural; U, urban; I, irrigated rice.

3

Sporozoite detection method:

e

dissection;

f

ELISA.

4

ma sampling technique:

g

human bait;

h

insecticide spray;

i

light traps;

j

exit traps.

5

EIR values:

k

converted using (Port et al., 1980) average child–to–adult conversion factor, f, of 3·57.

6

Values in italics record the number of months in which 75% of transmission occurs. Other values record the length of the entire transmission season.

7

For Anopheles gambiae s.l.: An. funestus: any other locally important vector. Where absolute values are unavailable P indicates presence only and A absence.

NA refers to data that were not available, or not applicable. Values in bold refer to calculated values

Figure.

Figure

A map showing the geo-referenced locations in Africa for which annual EIR data were published. The top left corner is 40°N, 20°W. Each grid square is 5 × 5 degrees and north is to the top of the page.

These studies collectively demonstrate that Africa has substantial cross-continent variability in annual EIR. The mean annual EIR value for the 159 spatially distinct sites was 121 infected bites per annum, although exposure ranged from a maximum of 884 to a minimum of 0. The local land-use also had a major effect on annual EIR. The ‘rural’ class had an overall mean of 146 (ranae 0–884) while those surrounded by irrigated rice were less exposed with a mean of 99 (range 0–601) and those in urban areas receiving significantly lower exposure with a mean of 14 (range 0–43). The variance in annual EIR expressed as the [(standard error/mean) × 100] is shown by country in Table 3. Spatially distinct rural sites only (n = 133) were used, to help control for major agricultural or demographic impact, and only countries with at least 10 spatially distinct study sites were included. Tanzania showed the least variance at 10·9%, and Senegal the highest at 42%. Finally, of the 133 sites for which seasonality information could be gathered, 30% showed acute seasonal variation in annual EIR (i.e., with 75% of annual transmission occurring in 1–3 months).

Table 3.

Variance in EIR values within countries for all spatially unique rural sites (133 sites total with temporal duplicates averaged)

Burkina
Faso
Kenya Senegal Sierra
Leone
Tanzania Gambia,
The
All
Africa
Mean1 156·8 64·3 28·7 129·0 367·0 42·1 134·2
Median 133 17·4 9·2 62·2 405 11 76·7
Maximum 441·6 299·3 159·5 703·4 667 177 703·4
Minimum 55 0 0 12·1 93·7 0 0
Range 386·6 299·3 159·5 691·3 573·3 177 703·4
Standard deviation 96·5 102·3 46·8 184·5 164·6 52·3 162·2
Standard error of mean 24·9 24·8 12·1 49·3 39·9 12·7 14·1
Variation 15·9 38·6 42·0 38·2 10·9 30·1 10·5
Number of sites 15 17 15 14 17 17 133
1

The arithmetic mean is used as a measure of central tendency in the data (Dossou-Yovo et al., 1994). Median values are also presented as they are less sensitive to non-normal distributions.

Study methodology

Biting rates were determined primarily by human-bait samples (n = 111), followed by light-traps (n = 31), pyrethrum spray-catches (n = 22) and exit-traps (n = 2) measurements. Combinations of the above techniques were used in 17 studies, whilst 18 more provided no information on how the biting rate estimate was obtained. The methods used to evaluate the sporozoite index were roughly even, with 82 determined by dissection and 99 by ELISA. Three records were calculated using the average of dissection and ELISA and 17 did not record the methodology used.

Geo-referencing

Three of the 201 annual EIR values were published with an accurate latitude and longitude of the study area. Correspondence with authors provided co-ordinate details for a further 33 sites. The largest source of geo-referencing information was obtained from the GeoName™ digital gazetteer CD-ROM which geo-referenced 100 annual EIR values. Finally, published maps were used to uncover the co-ordinates of a further 57 sites. This left the 8 unlocated study sites shown in Table 2.

Discussion

Annual EIR heterogeneity

It has been observed that Africa can support a very wide range of EIRs (Gilles, 1993). More recently, however, variation in infection risk has been linked to very different clinical patterns and public health burdens (Snow et al., 1997, 1998b; Snow & Marsh, 1998; Gupta et al., 1999a, 1999b). Although this study cannot claim to have identified every study of estimated annual EIR in Africa since 1980, the results of the search do support the claim for a diverse transmission pattern for the continent. Of particular interest in this respect is the enormous apparent variation within countries such as Senegal and Kenya. If this variation reflects sub-regional ecological heterogeneity, rather than sampling biases in the distribution of studies, it has important implications for disease management.

The results demonstrate a marked demographic influence on annual EIR values. The relatively small annual EIR in urban versus rural settings was not unexpected and is exemplified in the series of studies around Brazzaville in The Congo (Trape & Zoulani, 1987b) and neighbouring Kinshasa in Zaire (Coene, 1993). The impact of irrigated rice farming on surrounding EIRs is complicated. It has been shown to increase (Coosemans, 1985), have little overall effect upon (Robert et al., 1985; Githeko et al., 1993; Dossou-yovo et al., 1994) and also decrease (Robert et al., 1985; Githeko et al., 1993; Dossou-yovo et al., 1994) malaria transmission depending on the location. This variation can be due to many factors, such as relative effects of irrigation on species abundance and sporozoite rate, the number of rice harvests, surrounding human population numbers and levels of breeding site contamination, the number of cattle in the locality, as well as the degree of local immunity. The data collated here are insufficient to explore any of these mechanisms in detail, but it is interesting to note that when populations near irrigated rice areas were compared to those from the sample of rural Africa as a whole they were, on average, less exposed.

Year-to-year variation in annual EIR is also very important, particularly in naturally seasonal areas. Too few investigations published data over multiple years, however, to make reliable generalizations. The following studies from Senegal should be considered when interpreting single annual EIR estimates from a site, since the annual EIR ranged from 89 to 238 in Dielmo over a 3-year monitoring period (Fontenille et al., 1997b) and from 7 to 63 infected bites per year over a 4-year monitoring period in Ndiop (Fontenille et al., 1997a).

The Figure shows that entomological studies are preferentially conducted where malaria is known to exist and is often a recognized local health problem. For example, the range of estimates for Burundi (Coosemans, 1985; Van Bortel et al., 1996) suggests a country of intense transmission, whilst most of Burundi is at high altitude and free from malaria (Van der Stuyft et al., 1993). The Figure also shows that studies are more likely to be conducted in locations where malaria research has a strong historical basis. In Kenya, for example, the sites for most data neighbour the Kenyan Medical Research Institute (KEMRI) research centres on the coast and Lake Victoria. The annual EIR statistics presented for each country must therefore be interpreted with this caveat. It would also be of value if an international network could be developed to map EIR using standardized methods in a grid-based system across Africa to record more closely the spatial distribution of malaria transmission on the continent, rather than the distribution of medical research centres and their accessible field sites.

Information technology issues

The data presented in Table 1 have been staged on the internet for public access (see Materials and Methods) along with predicted maps of annual EIR for the African continent (D. J. Rogers et al., paper in preparation). This follows an objective to initiate the provision of web-based information to guide malaria control by the year 2000 (MARA/ARMA, 1998). There is a growing emphasis in many sectors on the potential benefits of the rapid internet-based supply of quality information. The greatest value of this increased information flow should be the possibility for iterative information updates with new, missed or corrected information. The most obvious problems to resolve are those of data-quality control and the provision of information in a manner accessible to a variety of users. The range of issues pertaining to data supply for malaria control planning is far from being resolved and is not appropriately addressed in this article. The authors have simply sought to make information available that might be of use to those collating data on malaria risk. Furthermore, these data are presented in such a way that those who may take issue with the protocol adopted can consult the original sources at the internet sites specified in Materials and Methods.

Data quality issues

It is evident that considerable resources have been expended by researchers, institutes and donors to provide annual EIR information across a range of sites in Africa over the past 2 decades. One of the major problems involved in comparing this information is the absence of data standardization between studies. An attempt has been made in this paper to highlight these issues.

The second major problem relates to the comprehensiveness of the reporting of annual EIR information. Most studies were incomplete in the range of information recorded. Similar problems have been experienced in parallel exercises to collate parasite rate (MARA/ARMA, 1998) and helminth infection data (Brooker et al., 2000) in Africa. The following data are suggested as the minimum requirement for future peer-reviewed reporting of annual EIR. First, all methodological information should be identified including exactly how, when and for what duration biting rate and sporozoite indices were determined. It is also important that accurate names and co-ordinates are given for each of the study sites. Brief details about the nature of the surrounding land-use are useful in the determination of the extent of agricultural and demographic impacts. Where ELISA techniques are used it is helpful to record the proportion of infective bites that can be attributed to each Plasmodium species and the vector species by which malaria is locally transmitted. If future reporting completed the list of information indicated in the Appendix, the foundation for a centralized, high-quality information database of annual EIR estimates across Africa would be assured.

The third problem is that, since accurate measurement of the EIR is labour intensive and thereby costly, estimates are spatially and temporally infrequent and unavailable for many settings. Another part of the study attempted to overcome this paucity of ground data by predicting annual EIR values across Africa using the information reviewed here and environmental data derived from meteorological satellite sensors (D. J. Rogers et al., paper in preparation).

Reviews are becoming increasingly important within the arena of evidence-based planning for disease control and prevention (Bero & Rennie, 1995; Bero, 1996). Furthermore, improved internet access will facilitate the use of central data resources by a wider spectrum of the research and control community. It is hoped the presentation and synthesis of work here, and in the public domain, will expedite future information gathering required for rationalization of malaria control in space and time.

Acknowledgements

The following people provided substantial help in both the search for and assessment of the accuracy of the information presented in this review: John Beier, Pierre Carnevale, José Coene, Chris Curtis, Pierre Fontenille, Pierre Gazin, Andrew Githeko, S. Karch, Steve Lindsay, L. Manga, Louis Molineaux, Jean Mouchet, Eskild Petersen, J. Pull, Vincent Robert, Clive Shiff, Tom Smith, Marcel Tanner, Emanuel Temu, Madeleine Thompson and Peter Trigg. All remaining errors are entirely the responsibility of the authors. We are grateful to Joseph Lines, David Kelly, Bill Snow and Mike Packer for their comments on the manuscript. We thank the Sir Halley Stewart Trust for providing salary support to J.F.T. This publication is also an output from a research project funded by the Department for International Development (DFID) of the UK, project ZC0012. However, the DFID can accept no responsibility for any information provided, or views expressed. S.I.H. is an Advanced Training Fellow with the Wellcome Trust (#056642). R.W.S. is a senior Wellcome Trust Fellow in Basic Biomedical Sciences (#033340).

Appendix

An example of a completed proforma used to abstract information from references on P. falciparum entomological inoculation rates (EIR) in this study, using data from Oloo et al., 1996

Parameter Unit Value
Start date mm-yy June 1991
End date mm-yy May 1992
Sampling frequency d-w-m-y Monthly
Country No units Kenya
District No units Siaya
Town No units Kisumu (0° 06′ S, 34° 45′ E)
Site No units Kanyawegi (25 km NW of Kisumu)
Paper latitude dd Not reported
Paper longitude dd Not reported
Retrieved latitude dd 00·9233 S (Map)
Retrieved longitude dd 34·67328 E (Map)
Mosquito species No units An. gambiae s.l., An. funestus
Mosquito collection method No units Human bait and spray
Mean annual biting rate b/p/night 8·42,5·05a
Sporozoite rate determination method No units ELISA
Mean annual P. falciparum sporozoite infection rate % 0·05,5·05a
Mean annual EIR bi/p/year 166·1,93·8 (259·9)a
Mean annual EIR bi/p/night 0·455,0·257 (0·712)a,b
Transmission season Months NA
Ecotype (rural, urban or irrigated rice) R/U/I R

d, Day; w, week; m, month; y, year; dd, decimal degrees; bi, infectious bite; p, person; NA, not available or not applicable.

a

Data are presented for An. gambiae s.l., An. funestus and (total).

b

Values calculated from (bi/p/year)/365 (and match ma × s table values product), not the rounded daily values quoted in Table 1.

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