The burden of malaria remains high in many African countries, despite increasing effort to control the disease due to a persistent high level of transmission 1 2 . Determining the intensity of transmission by mosquito populations is a key component of epidemiologic studies of malaria. This is usually estimated using the entomological inoculation rate (EIR), an index which provides the most direct measure of the risk of human exposure to the bites of infective anopheline vectors 3 . The EIR is also highly valuable for monitoring the suitability of vector control operations 4 . The risk of human exposure to infectious bites of vectors in Africa is however, not uniform 5 . Indeed, the transmission pattern may even vary greatly from region to region and even from village to village in the same district 6 7 8 .

In most malarious regions of the world, there is little baseline information on vector populations and variation in the intensity of malaria transmission. Consequently, currently used vector control methods via indoor residual sprays (IRS) and Insecticide treated bed nets (ITNs) are applied without regard to the local epidemiology of the disease, especially the relationship to vector behavior and intensity of transmission. Optimum effectiveness of these control strategies presumably depends on vectors biting at hours when most people are in bed 9 . However, behavioral heterogeneity of Anopheles species in ecologically different localities is widespread 10 and could dictate the success of these strategies. For example, The Garki malaria control project in Nigeria in the 1970s failed largely because of failure to recognize persistent malaria transmission by exophilic outdoor-resting mosquitoes, despite widespread indoor residual insecticide spraying 11 12 13 .

Therefore, the implementation of effective vector control strategies requires information on the main vectors, their population structure, distribution and efficiency in malaria transmission and variation even within local scales 1 . Moreover, entomological parameters to identify the main vectors for selecting suitable vector control options 9 14 15 are required in several communities in Africa, including Ghana, where malaria remains endemic in several communities 16 17 .

Efforts are being made to achieve malaria elimination and eradication worldwide 18 19 . The strategies being adopted include improved vector control, chemotherapy and possible vaccination. Kpone-on-Sea, a coastal fishing village in southern Ghana, is being developed as a possible site for testing various malaria control strategies with the hope to improve case management, control and prevention of the disease. To achieve effectiveness in these strategies, all aspects of malaria epidemiology need to be well understood including a better understanding of the vector transmission indices. Thus, this paper describes an entomological study on the vectors of malaria and their relative contributions to Plasmodium falciparum transmission at Kpone-on-Sea. The study was conducted over a period of six months (November 2005 - April 2006) during the dry season especially where vectors are likely to be confronted with highly variable and challenging climatic conditions with likely change on the disease transmission patterns.

Anopheles gambiae s.s. was the dominant anopheline mosquito and the main sibling species of the An. gambiae complex in the surveyed area. This concurs with findings in coastal savanna areas of Ghana 30 31 and elsewhere in Africa 32 . Anopheles melas was absent amongst the human biting populations of the An. gambiae complex, although it is known to be a vector along the coast of West Africa 33 34 35 . This species therefore, appears not to play a role in malaria transmission in the coastal areas of Ghana. Other anophelines recorded but in reduced numbers included An. funestus and An. pharoensis, a finding which is similar to earlier work in the nearby coastal savanna area of Prampram 30 .

The Anopheles gambiae s.s. M molecular form was dominant and the main malaria vector identified. The molecular M-form has been reported to have a higher vectorial capacity 36 . This fact is corroborated in our findings where a high circumsporozoite antigen rate was recorded and which was relatively higher compared to earlier studies in coastal savanna areas of Ghana 30 31 . The M-form is known to breed in permanent and semi-permanent water swamps in floodable river banks favorable to its development 37 . The low occurrence of the molecular S-form in our study during the dry season is also not surprising as this form is known to be well adapted to rainfall breeding sites 38 39 . Although these molecular forms are thought to represent incipient species 40 41 the factors underlining their co-existence in this area are still unclear, and deserve further investigation.

Peak indoor infective bites were also observed between the hours of 12.00 - 01.00 am when most of the inhabitants were in bed; hence suggesting that if inhabitants slept under impregnated bednets, human-vector contact and thus the risk of infective bites could be reduced. However, the majority of the infective bites occurred outdoors between the hours of 12.00-01.00 am and 03.00-04.00 am which coincides with peak biting densities of this vector. An interesting finding was the distribution of sporozoite-laden bites during the night, which indicated that malaria transmission in the study site occurred in the evenings after 09.00 pm till near daybreak (04.00 - 05.00 am). This has important epidemiologic implications as some of the inhabitants, involved in domestic and other activities are already out of bed (by 04.00 am) so therefore, increases their exposure to infective inoculations.

The high degree of outdoor behavior contrasts with a greater endophilic tendency displayed by this species in earlier studies in nearby coastal area of Ghana 30 . This observation is however, not surprising as heterogeneity in biting pattern is common place even within local scales. Equally, early evening biting activity by An. gambiae s.s. was evident, a pattern which contrasts with that reported by 30 where no biting occurred during the early hours of the morning. The basis for the combined high degree of outdoor biting and early biting populations remains unclear, but could be in response to prolonged use of insecticides indoors 9 . Such characteristics would tend to reduce the impact of control strategies directed towards the indoor biting fraction of the population.

This variation in feeding behavior within vector species may have a genetic basis 11 , which raises the possibility that vector control measures using insecticides could select for genotypes which are least likely to encounter the intervention. The early and outdoor biting populations may represent behavioral shifts as a consequence of phenotypic plasticity or evolutionary change within vector populations which remains unclear 42 . Regardless of the mechanism, such behavioral plasticity limits contact between vectors and insecticides, thus diminishing the effectiveness of the interventions that use them 9 43 . This outdoor biting activity has been linked to persistent malaria transmission in the face of mounting control measures indoors 42 . Thus there have been calls for innovative measures to develop new tools to fight malaria transmission by exploiting the ecology of the vector through an integrated approach to complement current vector control strategies 19 42 44 .

No infected An. funestus was recorded during the study period. Previous studies in other coastal areas of Ghana 30 31 have implicated this species as the second most important vector after An. gambiae s.l. Therefore, the absence of infection in An. funestus might be attributable to the low numbers caught biting; hence a longer period of survey in the rainy season is required to ascertain whether An. funestus is a significant vector at Kpone-on-Sea.

The observed parity rates for the anophelines were high. This indicates that older populations of mosquitoes tend to accumulate with time. This allows for increased feeding frequencies and thus, increased chances of the vectors becoming infected or even re-infected during subsequent feeding 45 46 . Our analysis showed that a higher proportion of the engorged An. gambiae had fed on humans. This finding confirms the anthropophilic tendencies displayed by this species throughout most of its distribution 47 , making it the most efficient malaria vector in Africa. Host blood meal detection assay was, however, conducted on only the few number of anophelines sampled using PSC. Inefficiencies of collectors could introduce bias in the number of mosquitoes that have fed on humans using human landing catches. As such, only anopheline mosquitoes collected by PSC were assayed for blood meal analyses. Few anophelines were captured using this strategy although the reason for the low captures is unclear given adequate sampling regimes that were conducted. It has been noted however, that during the dry season, as a survival strategy, adults may hide in shelters such as rodent burrows, abandoned houses and wells, thereby minimizing the chances of their detection through pyrethrum spray collections 48 .

Malaria transmission dynamics have been shown to vary greatly across Africa with inoculation rates varying from as low as 0.1 to over 1000 infective bites/person/year (ib/p/y) 7 49 . Generally in Africa, when the EIR <10, the area is considered to have unstable malaria and where EIR > 100, malaria is said to be stable 50 . Our results therefore suggest that malaria endemicity at Kpone-on-Sea remains variable, which depends on environmental and demographic conditions such as rainfall, vegetation cover, human population density and land use patterns. The observed annual inoculation rates in Kpone-on-Sea is, however, higher relative to other coastal areas of Ghana 30 31 probably due to a multiplier effect of the density and high biting rate of this species.

The findings provide a baseline for evidence-based planning and implementation of malaria control activities targeting vectors. An. gambiae s.s. is responsible for maintaining the status quo of malaria in the study site during the study period. The plasticity observed in biting patterns, especially the combined outdoor and early biting behavior of the vector has important consequences for the success of the widely used insecticide-based strategies using ITN and IRS. New or improved interventions should be informed by the local malaria epidemiology as it relates to vector behavior.

The authors declare that they have no competing interests.

DPT IAQ MDW DAB conceived and designed experiments. DPT conducted the experimental work. DPT analyzed the data. DPT IAQ EAA MDW MAA CAB DAB contributed to the manuscript. All authors approved the final version of the manuscript.