Malaria remains a major threat, both for travellers and for military personnel deployed each year to endemic areas. In France in 2007, up to 4,400 cases of malaria were estimated to have been imported, including 333 cases in military personnel 1. Chemoprophylaxis combined with anti-vector devices, such as impregnated bed nets, repellents and long-sleeve clothes, are protective measures for non-immune people travelling in malaria-endemic areas 23456. The growing resistance of Plasmodium sp. to anti-malarial drugs has led to an increasing interest in anti-vector protection measures 2. Their efficacy in travellers has rarely been assessed and are usually only assessed by questionnaire data analysis 7. Only one double-blinded randomized study has directly shown the efficacy of insecticide-treated clothing in reducing the incidence of malaria among travellers 8. The main difficulty is in taking into account the exposure of the travellers to malaria transmission. Serological techniques have been used to indirectly estimate this exposure but accommodation conditions, duration and seasonality of travellers' stay in endemic areas lead to a lower exposure to malaria transmission than those of indigenous populations 910. Serologic methods probably underestimate the real exposure according to the personal protection measures of individuals 1112131415. For travellers, only one remote sensing model approach has been conducted that showed a relationship between environment and malaria risk, but no quantitative risk maps could be made16. Rombo et al suggested that entomological inoculation rate (EIR) be used as a predictor of risk for non-immune persons 9. However, the EIR that is the most commonly used measure of the intensity of malaria transmission 17 has never been estimated for travellers. It is possible to use previously published data, but reliable and timely quantitative data on area specific EIR within countries are incomplete 1718. In fact, travel medicine practitioners have two levels (low or high) of risk according to the type of location: urban or rural. For indigenous populations, malaria transmission in the same rural areas is very heterogeneous and malaria exposure varies between neighbouring villages or settlements 192021222324. These variations are usually not considered in the risk assessment for travellers. No studies have been conducted to determine if malaria exposure for different groups of travellers in the same rural area varies according to each group's specific itinerary in this area.

The aim of this study was to estimate the level of exposure to malaria transmission of different groups of travellers with different travel itineraries during the same period in the same rural area in western Côte d'Ivoire by entomological methods.

The findings of this study clearly show that the level of malaria transmission and malaria risk varies widely (varying by a factor of eleven) between groups of travellers travelling in the same region and at the same time. Currently, the World Health Organization 36 and travel medicine practitioners only consider travellers to rural areas to have a higher risk of exposure to disease vectors in their recommendations. They do not take into account the heterogeneity of transmission in urban or rural areas in the same district 3738. Differences between human individuals in their attractiveness to female mosquitoes 39, in chemoprophylaxis observance, and in compliance with the use of anti-vector devices are not taken into account for assessment of individual risk for malaria, although they are determining factors for clinical malaria in travellers.

The level of transmission varied greatly from one site to another. These results are consistent with studies conducted in other parts of Africa that showed an important heterogeneity in malaria transmission between rural areas 40414243 and within the same rural area 192021222324. In this study, EIR varied between study sites from less than one to approximately 100 infective bites per months during the dry season, and from less than nine to approximately 115 infective bites per months during the rainy season. During the duty, malaria transmission increased overall during the rainy season but was also very high in some places during the dry season like in Kahin due to the presence of An. nili that thrive on the border of the river. This study confirms that, in wet savannah areas or in mountainous forested areas of Côte d'Ivoire, the rainy season, which occurs for most months in the year, allows the breeding sites to retain water much longer and thus sustain more substantial residual vector populations capable of ensuring malaria transmission even during the dry season 444546. In all sites, CSP rates did not vary with the increase in rainfall. The variations in malaria transmission were essentially caused by the variations in HBR, probably because of variations in the availability of breeding sites. These results confirm the heterogeneity of malaria transmission in rural areas 43 and the seasonal variation in malaria in western Côte d'Ivoire 47. Travel medicine practitioners should be very prudent in counseling travellers. Travel during the dry season is not synonymous with low transmission levels and data extracted from literature to predict the malaria risk may not be relevant in another place in the same region. HBRs and EIRs were greater than previously reported in Danane 25 and Man areas 4546, respectively. It is difficult explain whether these difference are due to anthropic modifications of the landscape 474849, to geographical and ecological differences at the study sites, or the greater sensitivity of ELISA CSP in P. falciparum detection than the salivary glands detection that was used in previous studies.

Although An. gambiae s.s. and An. funestus s.s. were individually the only significant malaria vectors at three sites (Zoukougbeu, Logouale and Dieouzon), in the other sites, the transmission was carried out by a vector association of at least two anopheline species, as previously described in Côte d'Ivoire 44505152. The main vectors were principally An. gambiae s.s. and An. funestus s.s,. with An. nili s.s. in a few places. In Guezon, An. funestus s.s. was the main vector during both dry and rainy seasons, and An. gambiae s.s was involved in transmission only during the rainy season. In Bangolo, An. gambiae s.s. was only found infected during the rainy season. The few human landing collections conducted here during the dry season did not allow P. falciparum-infected specimens to be caught, but An. gambiae s.s. was probably involved in malaria transmission in the dry season too. On the other hand, only a few specimens of An. funestus s.s., including one infected specimen, were caught in Bangolo, and these were only caught during the rainy season. Anopheles funestus s.s. was also only involved in malaria transmission as a secondary vector during the rainy season. In Logouale, An. gambiae s.s., An. funestus s.s. and An. nili s.s. were present during the dry and rainy seasons, but only An gambiae s.s. seemed significantly involved in malaria transmission. Indeed, the low An. funestus s.s. and An. nili s.s HBRs did not allow them to play a significant role in malaria transmission. In Kahin, An. gambiae s.s., An. funestus s.s. and An. nili s.s. were involved in malaria transmission during the dry and rainy seasons. Anopheles gambiae s.s was the main vector during the rainy season. Anopheles nili s.s was the main vector during the dry season. During this period, the EIR was above 60 infective bites by An. nili s.s. Anopheles nili breeding sites were probably located on the borders of the local river. During the wet season, the increase in the river water level probably destroyed most An. nili larval habitats. This is probably the explanation for the An. nili EIR and overall EIR decrease in Kahin during the rainy season. Others species were caught such as An. leesoni, An. hancocki, An. pharoensis and An. ziemanni, but none of them were found to be infected. During the study, in most of sites, the three dominant vectors bit humans as often indoors as they did outdoors.

Most of An. nili s.l. bites occurred in the first part of the night. In Kahin, four of the ten infected An. nili were caught between 9.00 p.m. and midnight These findings (exophagic behaviour, biting cycles) could limit the impact of insecticide-treated mosquito nets for local populations or travellers when they are exposed to An. nili bites. Furthermore, this study showed that 4% of bites occurred between 6:00 a.m. and 7:00 a.m. At the four permanent sites, three of the 27 infected An. funestus and 10 of the 53 infected An. gambiae were caught 6:00 a.m. and 7:00 a.m. Consequently, measures for anti-vector protection used in French military from 6:00 p.m. to 6:00 a.m., such as the wearing of long-sleeve clothes or the use of repellents, have been prolonged till 7:00 a.m. daily. These results showed that malaria transmission also occurs in the first hours of the day. Entomological studies often stopped the captures on 6:00 a.m., and, in some parts of Africa, they are probably underestimating the level of transmission.

The M form was predominant in sites where vegetation cover was deeper, like in Logouale and Douekoue. The S form was predominant in mountainous areas where forest cover was highly degraded (Guezon and Bleni Mehouin). In other places, the S-part of the population increased with anthropic modifications of the environment. This result is consistent with data from Wondji et al in Cameroon 53. The variations in proportions of M and S forms between sites could be explained by different breeding conditions: The M-form is considered to be more adapted to colonize semi-permanent larval sites, while the S-form is more adapted to rain-dependent temporary breeding sites 54. Interestingly, there was no difference in An. gambiae s.s. population composition between the dry and rainy seasons, as described by Della Torre et al in Burkina Faso 55. This discrepancy could perhaps be attributed to a different repartition in chromosomal forms of An. gambiae s.s. between western Côte d'Ivoire and Burkina Faso.

Although exposure to malaria transmission is high, very few malaria cases occurred among the soldiers. Eleven cases were recorded: 54.5% of which occurred in Côte d'Ivoire and 45.5% after returning to France. These proportions are similar to those observed for the French Army from 1998 to 2006 56. The attack rates by platoon were not associated with the level of exposure and most of cases occurred in the less exposed platoons (platoons 3, 4 and 5). This result could be explained by the high anopheline aggressiveness in some places, as was the case in Logouale. Soldiers were very interested in this study and in human landing catches. They saw the results of the catches each night. In some places, they could have increased their compliance to the use of personal protection devices against vectors and chemoprophylaxis. This bias could explain the lower incidence of malaria in platoon 2. The impact of an increase in protective behaviors and their effects on malaria exposure has already been described in indigenous populations exposed to high anopheline aggressiveness when compared to less exposed populations in the same areas in Mali, Tanzania or Côte d'Ivoire 485758. Other studies have shown the importance of personal protection measures in decreasing malaria infection risk in soldiers 859. This study also suggests the important role of preventive measures and personal protection in mitigating the risk of disease. In areas of high transmission, these measures seem highly effective, as was observed in platoon 2. The improvement of the efficacy of the malaria prevention policy needs probably to improve the compliance with the preventive measures and the identification of the localities at higher risk of transmission in order to avoid the lasting deployment of troops in these settings.

Malaria transmission in rural area is highly heterogeneous. The malaria exposure of travellers is highly dependent on this heterogeneity. Physicians involved in travel medicine or supporting expatriated population or refugees should emphasize the importance of vector control measures in protecting these populations (expatriated population or refugees) or counsel travellers before their stay in malaria transmission area. In addition, anti-vectorial measures are actually the only recommended measures for protection against vectors that transmit other pathogens, such as arboviruses.

The authors declare that they have no competing interests.

EOP: Contributed substantially to conception, study design, data collection, analysis and interpretation, preparation of the manuscript CR: conceived of the study, and participated in its design and coordination, data collection and critically reading the manuscript for important intellectual content. BK and CP: carried out data collection and trained the field staff. FJ, RG, VM and MB: processed entomological specimens in the laboratory, compiled the data and carried out statistical analyses. JPB: Participation in study design, coordination and critically reading of the manuscript. FP: Contributed substantially to conception, study design, data collection, analysis and interpretation, performed the statistical analysis and helped to draft the manuscript. All authors read and approved the final manuscript.

This study was supported by Délégation Générale pour l'Armement (grant 02CO011, no. 010808) with the valuable assistance of the veterinary Chief Claire Dane.