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Entomological indices of malaria transmission in Chikhwawa district, Southern Malawi.
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PMID:  23171123     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
BACKGROUND: Although malaria is highly prevalent throughout Malawi, little is known of its transmission dynamics. This paper describes the seasonal activity of the different vectors, human biting indices, sporozoite rates and the entomological inoculation rate in a low-lying rural area in southern Malawi.
METHODS: Vectors were sampled over 52 weeks from January 2002 to January 2003, by pyrethrum knockdown catch in two villages in Chikhwawa district, in the Lower Shire Valley.
RESULTS: In total, 7,717 anophelines were collected of which 55.1% were Anopheles gambiae sensu lato and 44.9% were Anopheles funestus. Three members of the An. gambiae complex were identified by PCR: Anopheles arabiensis (75%) was abundant throughout the year, An. gambiae s.s. (25%) was most common during the wet season and Anopheles quadriannulatus occurred at a very low frequency (n=16). An. funestus was found in all samples but was most common during the dry season.Anopheles gambiae s.s. and An. funestus were highly anthropophilic with human blood indices of 99.2% and 96.3%, respectively. Anopheles arabiensis had fed predominantly on humans (85.0%) and less commonly on cattle (10.9%; 1.2% of blood meals were of mixed origin). Plasmodium falciparum (192/3,984) and Plasmodium malariae (1/3,984) sporozoites were detected by PCR in An. arabiensis (3.2%) and An. funestus (4.5%), and in a significantly higher proportion of An. gambiae s.s. (10.6%)(p<0.01). All three vectors were present throughout the year and malaria transmission occurred in every month, although with greatest intensity during the rainy season (January to April). The combined human blood index exceeded 92% and the P. falciparum sporozoite rate was 4.8%, resulting in estimated inoculation rates of 183 infective bites/ person per annum, or an average rate of ~15 infective bites/person/month.
CONCLUSIONS: The results demonstrate the importance of An. gambiae s.s., An. arabiensis and An. funestus in driving the high levels of malaria transmission in the south of Malawi. Sustained and high coverage or roll out of current approaches to malaria control (primarily insecticide-treated bed nets and indoor residual house spraying) in the area are likely to reduce the observed high malaria transmission rate and consequently the incidence of human infections, unless impeded by increasing resistance of vectors to insecticides.
Authors:
Themba Mzilahowa; Ian M Hastings; Malcolm E Molyneux; Philip J McCall
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't     Date:  2012-11-21
Journal Detail:
Title:  Malaria journal     Volume:  11     ISSN:  1475-2875     ISO Abbreviation:  Malar. J.     Publication Date:  2012  
Date Detail:
Created Date:  2013-01-04     Completed Date:  2013-06-06     Revised Date:  2013-07-11    
Medline Journal Info:
Nlm Unique ID:  101139802     Medline TA:  Malar J     Country:  England    
Other Details:
Languages:  eng     Pagination:  380     Citation Subset:  IM    
Affiliation:
Liverpool School of Tropical Medicine, Liverpool, Pembroke Place, UK. tmzilahowa@gmail.com
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MeSH Terms
Descriptor/Qualifier:
Animals
Anopheles / parasitology
Anopheles gambiae / parasitology
Cattle
Female
Humans
Insect Vectors / parasitology
Malaria / epidemiology,  prevention & control,  transmission*
Malawi / epidemiology
Mosquito Control
Oocysts
Plasmodium / isolation & purification
Rural Population
Seasons
Sporozoites
Comments/Corrections

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Journal ID (nlm-ta): Malar J
Journal ID (iso-abbrev): Malar. J
ISSN: 1475-2875
Publisher: BioMed Central
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Copyright ©2012 Mzilahowa et al.; licensee BioMed Central Ltd.
open-access:
Received Day: 8 Month: 8 Year: 2012
Accepted Day: 8 Month: 11 Year: 2012
collection publication date: Year: 2012
Electronic publication date: Day: 21 Month: 11 Year: 2012
Volume: 11First Page: 380 Last Page: 380
PubMed Id: 23171123
ID: 3536595
Publisher Id: 1475-2875-11-380
DOI: 10.1186/1475-2875-11-380

Entomological indices of malaria transmission in Chikhwawa district, Southern Malawi
Themba Mzilahowa123 Email: tmzilahowa@gmail.com
Ian M Hastings1 Email: hastings@liv.ac.uk
Malcolm E Molyneux123 Email: mmolyneux999@gmail.com
Philip J McCall1 Email: mmolyneux999@gmail.com
1Liverpool School of Tropical Medicine, Liverpool, Pembroke Place, UK
2Malawi-Liverpool Wellcome Trust Clinical Research Programme, Blantyre, Malawi
3Malaria Alert Centre (MAC), P/ Bag 360, Chichiri, Blantyre 3, Malawi

Background

Malaria is highly endemic and prevalent throughout Malawi, with two thirds of the total population of 12 million people at direct risk of the infection [1,2]. Over 18% of hospital deaths of children less than five years old, and over one third of all outpatient visits, are attributed to malaria [1], the most highly prevalent parasite species being Plasmodium falciparum. While there has been substantial research in Malawi on the pathology and chemotherapy of malaria, little is known of malaria transmission dynamics and of the vectorial roles of various anopheline species known to occur in Malawi.

Malaria transmission in Africa is dominated by the Anopheles gambiae species complex and the Anopheles funestus group of mosquitoes. Studies on malaria transmission in Malawi began in 1911, when two vectors were identified: An. funestus which was the most common, and An. gambiae s.l. (referred to as Anopheles costalis) [3]. Later studies by Lamborn [4] and Berner [5] recorded that while An. funestus was abundant throughout the year, An. gambiae s.l. was found only in the wet season. In 1992, Hawley et al.[6] surveyed districts in the southern (Nsanje and Mangochi) and central (Dowa) regions of Malawi over a four month period in dry and wet seasons. They confirmed the presence of An. funestus and A. gambiae s.l. and, for the first time, identified both An. gambiae sensu stricto (s.s.) and Anopheles arabiensis in all three sites. Spiers et al.[7] recorded An. gambiae s.s., An. arabiensis, Anopheles merus and Anopheles quadriannulatus (the latter two species in very low numbers); An. arabiensis was the predominant anopheline species, comprising over 80% of adult collections. Plasmodium falciparum sporozoites have been detected in An. gambiae s.s., An. arabiensis and An. funestus[6,8] and all three mosquitoes have been shown also to be vectors of Wuchereria bancrofti in Malawi [9]. The absence of more comprehensive transmission data for Malawi remains an obvious knowledge gap [10], particularly in an era when reducing transmission is increasingly recognized as an important component of malaria control and a necessary step toward eventual elimination of the infection [11].

Malawi’s malaria control programme is supported by the Global Fund and the US government’s President’s Malaria Initiative (PMI). An initial aim was to reach universal coverage with indoor residual spraying (IRS) by 2011 [12], since such a strategy has proved to be highly effective in some settings [13]. Any optimism must, however, be tempered by reports of resistance to pyrethroids in populations of both the An. gambiae and An. funestus complexes in southern Malawi [14,15].

To provide basic data on the entomological indices of malaria transmission in southern Malawi, we undertook a study on malaria transmission in the Lower Shire Valley, an area of intense year round malaria transmission. Insight into the genetic structure of P. falciparum using data derived from infections in vectors collected from that study has already been reported [16], and here we describe the relative importance of different mosquito species in transmission, their human biting rates and the entomological inoculation rate for malaria in a rural area in southern Malawi.


Methods
Study area

The study was carried out in Chikhwawa district (16° 1' S; 34° 47' E), in the Lower Shire Valley, southern Malawi. Based on hospital in-patient records, this area was assumed to have perennial malaria transmission. The area has been a focus of studies investigating malaria-related anaemia, P. falciparum genetics and lymphatic filariasis [9,17-19]. Chikhwawa is approximately 70m above sea level, divided throughout its length by the Shire River (the largest river in Malawi) and prone to flooding in the wet season. The area has a tropical climate with a mean annual temperature of 26°C, a single wet season from November to April, and annual rainfall of approximately 770 mm (Malawi Meteorological Office, Chileka, Blantyre). There are extensive rice and sugarcane irrigation schemes.

Following consultation with the District Environmental Health Officer (DEHO) and on the basis of accessibility by road and similarity to other villages in the area, two study villages were selected: Chipula (15° 59' 33" S; 34° 47' 22" E) to the west of the Shire River and north of Chikhwawa District centre (with 109 households), and Kela (16° 02' 72" S; 34° 50' 52" E) (with 117 households) situated to the east of the Shire river. At both villages, the human population lived in thatched brick homes and engaged in subsistence agriculture. Two crops of maize were grown per year. Fishing was common, particularly in Kela, which is situated on the edge of a lake. Some households owned cattle (approximately 50 and 20 animals at Chipula and Kela, respectively) or goats, the majority of which were held in large communal kraals overnight.

At the time of the study, organized vector control in this area had not yet been implemented; few households used insecticide-treated bed nets and indoor residual spraying or other approaches were not in use.

Mosquito collection

Sampling of adult anopheline mosquitoes was carried out weekly over a period of 52 weeks, from January 2002 to January 2003. Permission to work in the villages was first sought from the village chiefs and householders during community sensitization meetings. Later and during each visit, collections were only carried out after the household owner had provided their free and informed consent to do so. All households were listed at the beginning of the study. Microsoft Excel was used to randomly select individual households, with replacement. To avoid excessive intrusion into the participants’ homes, three new houses were selected at each subsequent visit and a house could be re-visited only after a month. The right to refuse or withdraw at any time was respected. Where permission to enter a house was refused, an alternative house in the immediate proximity was selected.

Collection of mosquitoes was carried out between 6:00 a.m. and 8:00 a.m. Preparation of houses and mosquito collection followed standard procedures [20]. The owners were requested to refrain from sweeping until after collection, and all food items, cooking utensils and water were moved outside during the exercise. White sheets were laid in the rooms to cover the floors, beds and other flat surfaces. The interior space of the house was then sprayed with a pyrethroid-based household insecticide aerosol ‘Doom’ (Dichlorvos, Tetramethrin and d-Phenothrin; Robertsons Homecare Ltd, South Africa) [20]. The eaves and windows were sprayed simultaneously on the outside. The house was exited and doors closed. The sheets were removed for inspection after 10–15 minutes.

All mosquitoes were collected and placed in petri dishes lined with a damp filter paper. Mosquito samples from the 3 houses were kept in separate petri dishes and labeled accordingly. Petri dishes were put in a cooler box and transported to the laboratory in Blantyre for processing.

Mosquito analyses

Freshly collected anophelines were identified initially using a morphological key [21,22]. Mosquito blood meals were taken from a sub-sample of up to 20 freshly fed mosquitoes per house for subsequent host identification, by crushing individual abdomens onto Whatman® filter papers (No. 3; 110 mm diameter), that were stored in plastic bags containing silica gel until analysis. Abdomens of remaining mosquitoes were then removed and dissected to examine midguts for oocysts [16]. Heads and thoraces of all collected mosquitoes were stored in 1.5 μl eppendorf tubes and stored over silica gel for subsequent examination for Plasmodium spp. sporozoites.

Identification of species within the An. gambiae species complex

Anopheles cytospecies were identified using ribosomal DNA extracted with LIVAK lysis buffer from individual mosquitoes followed by DNA amplification by the standard rDNA-PCR method [23,24]. If the initial PCR test failed to amplify for a sample, it was repeated twice until successful amplification occurred, or was scored as unknown. Reactions included negative and positive controls. All reactions were carried out using the GeneAmp® PCR System 2700 (Applied Biosystems, UK).

Analysis of blood meals

A direct enzyme-linked immunosorbent assay (ELISA) was used to identify the source of mosquito blood meals [25]. Each sample was analysed by two separate assays: one for human blood and one for bovine blood. Positive results were read visually, 30 minutes after adding the substrate ABTS (2,2′ azino-bis 3-ethylbenzthiazoline-6-sulphonic acid) and hydrogen peroxide (Kirkegaard and Perry Laboratories, USA) in a 1:1 mixture ratio. A sub-sample of 100 positive specimens was re-run for each test to confirm the test.

Detection of Plasmodium spp. sporozoites

Plasmodium spp. sporozoites were detected and identified by PCR on DNA extracts from heads and thoraces of individual female mosquitoes [26]. DNA was extracted [23] and diluted to 10% in water for optimal PCR amplification. PCR reactions were performed using the Applied Biosystems, GeneAmp® PCR System 2700 thermocycler. The cycling parameters were: step 1, 95°C for 5 minutes; step 2, 94°C for 1 minute, annealing at 58°C for 2 minutes, extension at 72°C for 2 minutes; step 3, final extension at 72°C for 5 minutes.

Climate data

Climate data measured at Kasinthula irrigation project (16° 5' S, 34° 49' E; the nearest weather station to the study sites) was obtained from the central meteorological offices at Chileka International Airport in Blantyre. Monthly mean rainfall, and daily mean, maximum and minimum temperatures were recorded (Figure 1).

Data analyses

Data were analysed using SPSS version 12.0.1. Proportions were compared using Chi-Square test, or Fisher’s Exact test where appropriate. The entomologic inoculation rate (EIR) was calculated from the mean numbers of blood-fed mosquitoes (number of mosquitoes divided by the number of house occupants, multiplied by the human blood index for each mosquito species) multiplied by the sporozoite rates. The estimated annual EIR was the sum of the monthly means for the three vectors.

Ethical issues

Ethical permission for the study was provided by the Research Ethics Committees of the College of Medicine Blantyre (COMREC) and the Liverpool School of Tropical Medicine. Permission to work in specific villages was granted by each village chief following an initial briefing meeting at which the nature and objectives of the study were explained to all members of the community in the local language, Chichewa. Written informed consent was obtained at the beginning of the study. On the day of mosquito sampling, the purpose of the work was again explained to each householder, and permission to enter the house was sought.


Results
Anopheline species composition

A total of 7,717 anopheline mosquito adults were collected from houses in the two study villages between January 2002 and January 2003. Only members of the An. gambiae species complex (n=4,253; 55.1%) and An. funestus group of mosquitoes (n=3464, 44.9%) were found (Table 1). The proportions of An. gambiae s.l. were slightly higher than An. funestus s.l at both study villages (χ2 = 6, df = 1, p<0.013). An. gambiae s.l. comprised 57.2% (n=1,407) and An. funestus 42.8% (n=1,054) of the anophelines collected at Chipula, the equivalent proportions for Kela being 54.1% (n=2,846) and 45.9% (n=2,410).

A total of 3,410 individual mosquitoes of the An. gambiae complex were identified to species level by PCR and three species were identified: An. arabiensis (n=2,473), An. gambiae s.s. (n=795) and An. quadriannulatus (n=14). The proportions of these species were similar at both villages: An. arabiensis comprised 76.5% (n=831), An. gambiae s.s. 22.8% (n=248) and An. quadriannulatus 0.7% (n=8) at Chipula, and 74.8% (n=1,642), 24.9% (n=547) and 0.3% (n=6) respectively at Kela.

No formal identifications of the An. funestus were carried out in this study. However, all 30 specimens in a sample of this species (collected from Kela village) were identified as An. funestus s.s. (Besansky, pers. communication) [27].

Seasonal variation in species abundance

The three most common anophelines, An. funestus, An. arabiensis and An. gambiae s.s., were present throughout the entire collection period, although the relative abundance varied markedly in different months (Figure 2).

Anopheles arabiensis was the predominant species in all catches, in Chipula ranging from 59% (n=121/205) of the total monthly catch (i.e. total number of all anopheline mosquitoes caught) in January 2002 to 97% (n=34) in August 2002, and in Kela ranging from 44% (n=52) in March 2002 to 93.6% (n=204) in September 2002 for Chipula and Kela respectively.

Anopheles gambiae s.s. proportions in Chipula varied from 2.9% (n=1) in August 2002 to 42% (n=88) in January 2002, and in Kela varied from 6.4% (n=14) in September 2002 to 56% (n=66) in March 2002. Unlike An. arabiensis and An. funestus, more adults of An. gambiae s.s. were found during the wet season when both minimum and maximum temperatures were also high (Figure 1).

The proportions of adult female An. funestus in monthly catches ranged from 76% (n = 193) in April 2002 to 2.9% (n = 4) in January 2003 in Chipula, and from 74% (n = 129) in April 2002 to 2% (n = 2) in November & December 2002 in Kela (Figure 2). At both sites An. funestus s.l. numbers were greatest during the drier months (March to September) when temperatures were also slightly lower (Figure 1).

Sources of mosquito blood meals

A total of 2,300 mosquito blood meal specimens were collected for identification, a random sample of which (n=883; 38.4%) were analysed by ELISA to identify the source of the ingested blood (Table 2). Humans were the most common blood meal source for all mosquitoes (92.8%; n=819), with only 5.3% (n=47) feeding on cattle. Only 5/883 (0.6%) blood meals were from both hosts. Twelve samples (1.4%) were negative in both ELISAs, either because the reactions failed or possibly because the blood was derived from other hosts. A random sample of 100 specimens that tested positive for human blood was re-tested with similar results.

Figure 3 shows the human blood indices of the three vector species at both sites. Indices were very high for An. gambiae s.s (0.983 and 1) and for An. funestus (0.959 and 0.969) and slightly less for An. arabiensis (0.838, 0.861). In all cases, significantly more blood meals were taken from humans than from cattle, although this preference was significantly lower in An. arabiensis than in the other two species (Fisher’s exact test, df = 6, p<0.000).

Plasmodium sp. oocyst rates

A total of 7,717 adult female anophelines were dissected and oocysts were found on the midguts of 100 (1.3%). Significantly more mosquitoes (84%) infected with oocysts were collected from Kela village (90.5%; n = 656) than from Chipula (9.5%; n = 69) (χ2 = 11.78; P<0.001). The estimated number of oocysts per midgut varied from 1 to 63 and the mode was 1 oocyst per midgut; data were heavily skewed with a mean of 6 and a very large variance of 66, and were described best by a negative binomial distribution. Oocyst rates were significantly higher in An. funestus (1.79%; n=3519) than in An. gambiae s.l (0.88%; n=4198). The mean oocyst load was slightly higher for adult female An. gambiae s.l (7.9; 95% CI 4.3 – 11.5) was slightly but not significantly higher than An. funestus (6.9; 95% CI 4.6 – 9.2). The dissected oocysts were processed and used for a related study, as reported previously [16].

Plasmodium spp. sporozoite rates

Screening of 3,958 mosquitoes (2,315 An. arabiensis, 718 An. gambiae s.s. and 925 An. funestus) for Plasmodium spp. sporozoites using PCR (Table 3), identified 192 (4.85%) as infected. All carried P. falciparum alone, except for a single An. gambiae s.s. that was infected with both P. falciparum and Plasmodium malariae. No Plasmodium ovale parasites were detected. Infective mosquitoes were found in every month with highest prevalence occurring in April. Anopheles gambiae s.s. had the highest year-round sporozoite rate of 10.6%, significantly greater than both An. funestus (4.5%) and An. arabiensis (3.2%) (p<0.05). The inoculation rates of An. arabiensis, An. gambiae s.s and An. funestus were 5.82, 6.71 and 2.33 infective bites/person/month at Chipula and 5.45, 8.09 and 2.14 infective bites/person/month at Kela. The community in this area therefore received an estimated average of 183 infective bites/ person/ annum. In both villages, An. gambiae s.s was the most important malaria vector, being responsible for 44% and 52% of the transmission at Chipula and Kela respectively.


Discussion

The results demonstrate the importance of all three of the abundant anophelines in the region, An. gambiae s.s, An. arabiensis and An. funestus, as malaria vectors driving the high levels of malaria transmission in the south of Malawi. All three vectors were present throughout the year and malaria transmission occurred in every month. The combined human blood index exceeded 92% and the P. falciparum sporozoite rate was 4.8%, resulting in inoculation rates of 183 infective bites/ person per annum, or a monthly rate of ~15 infective bites/ person.

This EIR is higher than some previous estimates from Malawi where annual values of only 46.1 and 27.7 infective bites/ person/ year were recorded for Mangochi and Nsanje respectively [6] but close to those recorded in Nkhotakota in the central region of Malawi where Chiphwanya (2003) recorded sporozoite rates of 6.5%, 5.2% and 4.5% in An. gambiae, An. funestus and An. arabiensis respectively, an average sporozoite rate of 5.4% in all three vectors [8].

These rates are higher than in neighbouring Mozambique, where inoculation rates of 20 and 27 infective bites/ person/ year were recorded [28,29]. Indeed, the EIRs in this study are closer to those recorded further north in Tanzania or Kenya where values of this order or higher are common [30]. These high rates may be due, in part at least, to the use of the highly sensitive PCR method to screen mosquitoes for sporozoites as compared with microscopic examination [31]. Previously in Malawi, Chiphwanya [8] recorded comparable rates to these using ELISA. Such problems when comparing EIRs from different localities or dates have been noted previously [30,32]. Notably, the oocyst rate (1.3%) is lower than the sporozoite rate (4.85%). While PCR may have been highly sensitive in detection of sporozoites, it is also likely that detection of oocysts by microscopy failed to detect light infections at this stage of development, though this alone would be unlikely to account for the discrepancy between the rates.

Thus, the high EIR reported here may derive from a combination of factors, both genuine (the presence of three efficient vector species and the absence of vector control, the low availability of hosts other than humans) and potentially confounding (sensitivity of PCR in detection of Plasmodium sp. DNA, collection of endophilic anthropophilic mosquitoes only). Conversely, relying on the pyrethrum knockdown catch method meant that the study sampled only indoor resting mosquitoes. Exophilic mosquitoes would not have been caught, an important consideration if those mosquitoes were also partly zoophilic, and one that would have contributed to an underestimate of EIR. Thus in parts of Kenya, HBI values for An. arabiensis can be as low as 0.22 [33] or even 0.095 [34]. It is somewhat reassuring that HBI values comparable to ours (0.85) have been recorded in similar localities in neighbouring Zambia (0.872, 0.923, 0.94/0.96) [35-37] and Mozambique (0.985) [29]. One Zambian study found that although the relative proportions of the An. arabiensis population exhibiting endo or exophilic behaviour varied between different years, the HBI values remained high [37]. These various studies are consistent with the data presented here and indicate a strong anthropophilic tendency in An. arabiensis populations in southern Malawi and neighbouring areas.

The three species: An. funestus, An. arabiensis and An. gambiae s.s. fed predominantly on humans (92.8%) as confirmed by ELISA test results. The other blood meals were taken from cattle, and were mainly in An. arabiensis (10.9%). Few feeds (1.2%) were of mixed origin of these two hosts. Although cows were available in both villages, they were not common (only 70 in total in both villages) and were herded into open corrals (or bomas) at night; we did not collect mosquitoes from there, although it might be expected that numbers of An. arabiensis feeding on those animals could be very high. However, any potential zooprophylactic effect was not sufficient to prevent An. arabiensis from feeding on humans at a frequency that resulted in a sporozoite rate of 3.2% in the endophilic proportion of its population.

The high inoculation rates reported here result, at least in part, from the presence of three competent vector species that are present all year round permitting transmission in every calendar month. The importance of An. gambiae s.s. and An. funestus in Malawi is recognised but the demonstration of high levels of transmission by An. arabiensis is in sharp contrast with current reports that it does not play a significant role in malaria transmission [12]. In fact, An. arabiensis, along with An. funestus, are important in extending the duration of the malaria transmission period well beyond the wetter months. The most efficient vector, An. gambiae s.s was highly infective during the wet season i.e. between January and May 2002 with peak sporozoite rates experienced in April 2002 (late rainy season; results not shown) when high numbers of this species were found.

High EIRs, like those reported here, are associated frequently with correspondingly high prevalence rates of parasitaemia in human populations [38]. High inoculation rates can have important implications for malaria epidemiology, including the likely age at first malaria infection and the clinical disease pattern. Under high transmission intensity, children are likely to be infected at an early age, as demonstrated in a study in Mangochi in central Malawi [39] where malaria transmission is also intense, in which 60-80% of infants under 10 months of age in a cross-sectional survey were parasitaemic, the prevalence varying with season. When a high EIR causes P. falciparum infections to occur early in life, severe anaemia is common and is the predominating form of severe malaria among infants and toddlers [40], while those surviving to adulthood have acquired partial immunity and rarely suffer severe complications from P. falciparum infections.

The biggest challenge in malaria control is to reduce the observed high EIRs in order to achieve reductions in human infection rates sufficient to reduce the burden of disease. Simple but effective technologies directed against vectors – long-lasting insecticide-impregnated bed nets (LLINs) and indoor residual spraying (IRS) – can dramatically reduce malaria transmission intensity. The high levels of indoor biting reported here indicate that, subject to the vectors remaining susceptible to pyrethroid and other insecticides [15,41-44], the currently planned [12] roll-out of both LLINs and IRS is likely to reduce the EIR in the Lower Shire valley. The recently reported success achieved further north in the Nkhotakhota district of Malawi supports this prospect [13]. As elsewhere in Africa, the relative importance of exophilic An. arabiensis as a malaria vector is likely to increase gradually as a consequence of control measures that reduce the contribution of endophilic mosquitoes, and the development of new measures to combat these more elusive vectors will become increasingly important.


Competing interests

The authors declare that they have no competing interests.


Authors’ contributions

Conceived, designed and managed the study protocol: PJM, TM, IH, MM. Undertook the field collections and laboratory analyses: TM. Interpreted, analysed data and wrote the paper: TM, PJM, IH. All authors read, edited and approved the final version of the manuscript.


Acknowledgements

The authors thank the villagers and village chiefs for permitting this study to be carried out in their homes, and Fred Malikebu and Ireen Chaguluka for assistance in the field. We also thank Angus Spiers, Andrea Durutti Verardi, Bob Wirtz, John Gimnig, Martin Donnelly and Harold Townson for advice and technical assistance. TM was supported by awards from the Gates Malaria Partnership (GMP) and UNICEF/ UNDP/ World Bank/ WHO Special Programme for Research and Training in Tropical Diseases (TDR).


References
Malawi Demographic and Health Survey 2010Year: 2011Zomba: National Statistical Office http://www.measuredhs.com/pubs/pdf/PR4/PR4.pdf.
Kazembe LN,Kleinschmidt I,Sharp BL,Patterns of malaria-related hospital admissions and mortality among Malawian children: an example of spatial modelling of hospital register dataMalar JYear: 200659317067375
Davey JB,Newstead R,Mosquitoes and other blood-sucking arthropods of the Upper Shire RiverNyasaland Ann Trop Med ParasitolYear: 192115457462
Lamborn WA,The seasonal habit of the common anophelines of Nyasaland, with a note on its relation to the seasonal incidence of malariaBull Entomol ResYear: 192415361376
Berner WA,Mosquitoes of the Shire River system, NyasalandAnn Entomol Soc AmericaYear: 195548214218
Hawley WA,Sexton JD,Tambala P,Macheso A,Zimba C,Chitsulo L,Nyanwayu D,Nyasulu Y,Franco C,Kazembe P,Malaria vector assessment, Malawi: Oct 1991 - Sept 1992Year: 1992USAID, Lilongwe, Malawi: Unpublished report
Spiers AA,Mzilahowa T,Atkinson D,McCall PJ,The malaria vectors of the Lower Shire Valley, MalawiMalawi Med JYear: 20021447
Chiphwanya JA,Evaluation of insecticide susceptibility in malaria vector mosquitoes and their role in malaria transmission in central MalawiYear: 2003Johannesburg: University of Witwatersrand
Merelo-Lobo AR,McCall PJ,Perez MA,Spiers AA,Mzilahowa T,Ngwira B,Molyneux DH,Donnelly MJ,Identification of the vectors of lymphatic filariasis in the Lower Shire Valley, southern MalawiTrans R Soc Trop Med HygYear: 20039729930115228246
Wilson ML,Walker ED,Mzilahowa T,Mathanga DP,Taylor TE,Malaria elimination in Malawi: research needs in highly endemic, poverty-stricken contextsActa TropYear: 201212121822622100546
Mathanga DP,Walker ED,Wilson ML,Ali D,Taylor TE,Laufer MK,Malaria control in Malawi: current status and directions for the futureActa TropYear: 201212121221721763670
President's Malaria InitiativeMalaria Operational Plan (MOP) Malawi Year Five - FY2011Year: 2011 http://www.pmi.gov/countries/mops/fy11/malawi_mop-fy11.pdf.
Skarbinski J,Mwandama D,Wolkon A,Luka M,Jafali J,Smith A,Mzilahowa T,Gimnig J,Campbell C,Chiphwanya J,Impact of indoor residual spraying with lambda-cyhalothrin on malaria parasitemia and anemia prevalence among children less than five years of age in an area of intense, year-round transmission in MalawiAmJTrop Med HygYear: 2012869971004
Mzilahowa T,Ball AJ,Bass C,Morgan JC,Nyoni B,Steen K,Donnelly MJ,Wilding CS,Reduced susceptibility to DDT in field populations of Anopheles quadriannulatus and Anopheles arabiensis in Malawi: evidence for larval selectionMed Vet EntomolYear: 20082225826318816274
Hunt R,Edwardes M,Coetzee M,Pyrethroid resistance in southern African Anopheles funestus extends to Likoma Island in Lake MalawiParasit VectorsYear: 2010312221192834
Mzilahowa T,McCall PJ,Hastings IM,"Sexual" population structure and genetics of the malaria agent P. falciparumPLoS OneYear: 20072e61317637829
Brabin B,Prinsen-Geerligs P,Verhoeff F,Kazembe P,Reducing childhood mortality in poor countries: anaemia prevention for reduction of mortality in mothers and childrenTrans R Soc Trop Med HygYear: 200397363812886802
Calis CJ,Phiri KS,Faragher EB,Brabin BJ,Bates I,Cuevas LE,de Haan RJ,Phiri AI,Malange P,Khoka M,Factors associated with severe anaemia in Malawian childrenNew Engl J MedYear: 200835888889918305266
Nielsen NO,Makaula P,Nyakuipa D,Bloch P,Nyasulu Y,Simonsen PE,Lymphatic filariasis in Lower Shire, southern MalawiTrans R Soc Trop Med HygYear: 20029613313812055799
WHOManual on Practical Entomology. Part II. Methods and TechniquesYear: 1975Geneva: World Health Organization
Gillies MT,Coetzee M,A supplement to the Anophelinae of Africa south of the Sahara (Afrotropical Region)Year: 1987Johannesburg: The South African Institute for Medical Research
Gillies MT,de Meillon B,The Anophelinae of Africa south of the Sahara (Ethiopian Region)Year: 1968Johannesburg: The South African Institute for Medical Research
Collins FH,Mendez MA,Rasmussen MO,Mehaffey PC,Besansky NJ,Finnerty V,A ribosomal RNA gene probe differentiates member species of the Anopheles gambiae complexAmJTrop Med HygYear: 1987373741
Scott JA,Brogdon WG,Collins FH,Identification of single specimens of the Anopheles gambiae complex by the polymerase chain reactionAmJTrop Med HygYear: 199349520529
Beier JC,Perkins PV,Wirtz RA,Koros J,Diggs D,Gargan TP 2nd,Koech DK,Blood meal identification by direct enzyme-linked immunosorbent assay (ELISA), tested on Anopheles (Diptera: Culicidae) in KenyaJ Med EntomolYear: 1988259163357176
Snounou G,Viriyakosol S,Zhu XP,Jarra W,Pinheiro L,do Rosario VE,Thaithong S,Brown KN,High sensitivity of detection of human malaria parasites by the use of nested polymerase chain reactionMol Biochem ParasitolYear: 1993613153208264734
Koekemoer LL,Lochouarn L,Hunt RH,Coetzee M,Single-strand conformation polymorphism analysis for identification of four members of the Anopheles funestus (Diptera: Culicidae) groupJ Med EntomolYear: 19993612513010083746
Mendis C,Jacobsen JL,Gamage-Mendis A,Bule E,Dgedge M,Thompson R,Cuamba N,Barreto J,Begtrup K,Sinden RE,Høgh B,Anopheles arabiensis and An. funestus are equally important vectors of malaria in Matola coastal suburb of Maputo, southern MozambiqueMed Vet EntomolYear: 20001417118010872861
Thompson R,Begtrup K,Cuamba N,Dgedge M,Mendis C,Gamage-Mendis A,Enosse SM,Barreto J,Sinden RE,Hogh B,The Matola malaria project: a temporal and spatial study of malaria transmission and disease in a suburban area of Maputo, MozambiqueAmJTrop Med HygYear: 199757550559
Hay SI,Rogers DJ,Toomer JF,Snow RW,Annual Plasmodium falciparum entomological inoculation rates (EIR) across Africa: literature survey, Internet access and reviewTrans R Soc Trop Med HygYear: 20009411312710897348
Wilson MD,Ofosu-Okyere A,Okoli AU,McCall PJ,Snounou G,Direct comparison of microscopy and polymerase chain reaction for the detection of Plasmodium sporozoites in salivary glands of mosquitoesTrans R Soc Trop Med HygYear: 1998924824839861357
Kelly-Hope LA,McKenzie FE,The multiplicity of malaria transmission: a review of entomological inoculation rate measurements and methods across sub-Saharan AfricaMalar JYear: 200981919166589
Githeko AK,Service MW,Mbogo CM,Atieli FK,Juma FO,Plasmodium falciparum sporozoite and entomological inoculation rates at the Ahero rice irrigation scheme and the Miwani sugar-belt in western KenyaAnn Trop Med ParasitolYear: 1993873793918250629
Ijumba JN,Mwangi RW,Beier JC,Malaria transmission potential of Anopheles mosquitoes in the Mwea-Tebere irrigation scheme, KenyaMed Vet EntomolYear: 199044254322133010
Fornadel CM,Norris DE,Increased endophily by the malaria vector Anopheles arabiensis in southern Zambia and identification of digested blood mealsAmJTrop Med HygYear: 200879876880
Kent RJ,Thuma PE,Mharakurwa S,Norris DE,Seasonality, blood feeding behavior, and transmission of Plasmodium falciparum by Anopheles arabiensis after an extended drought in southern ZambiaAmJTrop Med HygYear: 200776267274
Fornadel CM,Norris LC,Glass GE,Norris DE,Analysis of Anopheles arabiensis blood feeding behavior in southern Zambia during the two years after introduction of insecticide-treated bed netsAmJTrop Med HygYear: 201083848853
Beier JC,Killeen GF,Githure JI,Entomologic inoculation rates and Plasmodium falciparum malaria prevalence in AfricaAmJTrop Med HygYear: 199961109113
Slutsker L,Khoromana CO,Hightower AW,Macheso A,Wirima JJ,Breman JG,Heymann DL,Steketee RW,Malaria infection in infancy in rural MalawiAmJTrop Med HygYear: 1996551 Suppl7176
Snow RW,Bastos de Azevedo I,Lowe BS,Kabiru EW,Nevill CG,Mwankusye S,Kassiga G,Marsh K,Teuscher T,Severe childhood malaria in two areas of markedly different falciparum transmission in east AfricaActa TropYear: 1994572893007810385
Chanda E,Hemingway J,Kleinschmidt I,Rehman AM,Ramdeen V,Phiri FN,Coetzer S,Mthembu D,Shinondo CJ,Chizema-Kawesha E,Kamuliwo M,Mukonka V,Baboo KS,Coleman M,Insecticide resistance and the future of malaria control in ZambiaPLoS OneYear: 20116e2433621915314
Cuamba N,Morgan JC,Irving H,Steven A,Wondji CS,High level of pyrethroid resistance in an Anopheles funestus population of the Chokwe District in MozambiquePLoS OneYear: 20105e1101020544036
Kloke GR,Nhamahanga E,Hunt RH,Coetzee M,Vectorial status and insecticide resistance of Anopheles funestus from a sugar estate in southern MozambiqueParasit VectorsYear: 201141621306631
Morgan JC,Irving H,Okedi LM,Steven A,Wondji CS,Pyrethroid resistance in an Anopheles funestus population from UgandaPLoS OneYear: 20105e1187220686697

Figures

[Figure ID: F1]
Figure 1 

Monthly rainfall (black bars), maximum temperature (dotted line) and minimum temperature (dashed line) measured at Kasinthula weather station in Chikhwawa, close to the study sites and covering the same period as the data presented in Figure1.



[Figure ID: F2]
Figure 2 

Monthly abundance ofAn. arabiensis, An. gambiae s.s.andAn. funestusfrom Chipula and Kela in the lower Shire valley, as measured by pyrethroid knockdown catch (Jan 2002 - Jan 2003).



[Figure ID: F3]
Figure 3 

Human blood indices (HBI) for the three malaria vectors in two villages, Chipula and Kela, in the lower Shire valley, Malawi (Jan 2002 - Jan 2003). Numbers of blood meals analysed were: An. gambiae s.s., 115, 131; An. arabiensis 160, 180; An. funestus 122, 175; from Chipula and Kela respectively.



Tables
[TableWrap ID: T1] Table 1 

Total numbers ofAnophelesadult female mosquitoes collected by pyrethrum knockdown catch from houses in two villages, Chipula and Kela, in the lower Shire valley, Malawi, between January 2002 and January 2003


Species Chipula Kela Total
An. funestus
1054
2410
3464
An. gambiae s.l.
1407
2846
4253
An. arabiensis
831 (76.5%)
1642 (74.8%)
2437
An. gambiae s.s.
248 (22.8%)
547 (24.9%)
795
An. quadriannulatus 8 (0.7%) 6 (0.3%) 14

[TableWrap ID: T2] Table 2 

Identity of blood meals fromAn. arabiensis, An. gambiae s.s.andAn. funestusadult female mosquitoes collected by pyrethrum knockdown catch in the lower Shire valley (Jan 2002 - Jan 2003)


Species Blood meal source
No. tested Bovine Human Bovine/Human Other
An. arabiensis
 
37 (10.9)
289 (85.0)
4 (1.2)
10 (2.9)
An. gambiae s.s.
246
1 (0.4)
244 (99.2)
1 (0.4)
0
An. funestus
297
9 (3.0)
286 (96.3)
0
2 (0.7)
Total 883 47 (5.3) 819 (92.8) 5 (0.6) 12 (1.4)

Blood meals were screened twice by ELISA for either human blood or for bovine blood, and blood meals that were negative in both screens were classed as ‘other’.


[TableWrap ID: T3] Table 3 

The total numbers of mosquitoes examined and sporozoite rates forAn. arabiensis,An. gambiae s.s. andAn. funestuscollected by pyrethrum spray catch from Chipula and Kela villages in the lower Shire valley Malawi, between January 2002 and January 2003


  Total Chipula village
Kela village
No. tested No. positive Sporozoite rate (95% CI) No. tested No. positive Sporozoite rate (95% CI)
An. arabiensis
2315
792
35
4.4 (2.9 – 5.8)
1523
39
2.6 (1.8 – 3.4)
An. gambiae s.s.
718
219
27
12.3 (10.2–4.4)
499
49
9.8 (8.2–11.5)
An. funestus
925
411
21
5.1 (0.8 – 9.5)
514
21
4.1 (1.5 – 6.7)
Total 3958 1422 83 5.8 (4.6-7.0) 2536 109 4.3 (3.5 – 5.1)


Article Categories:
  • Research

Keywords: Malaria, Africa, Malawi, Plasmodium, Anopheles, falciparum, Malariae, Gambiae, Transmission, EIR.

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