Light Traps Fail to Estimate Reliable Malaria Mosquito Biting Rates On
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Overgaard et al. Malaria Journal 2012, 11:56 http://www.malariajournal.com/content/11/1/56 RESEARCH Open Access Light traps fail to estimate reliable malaria mosquito biting rates on Bioko Island, Equatorial Guinea Hans J Overgaard1,5*, Solve Sæbø2, Michael R Reddy3, Vamsi P Reddy4, Simon Abaga5, Abrahan Matias6 and Michel A Slotman4 Abstract Background: The human biting rate (HBR), an important parameter for assessing malaria transmission and evaluating vector control interventions, is commonly estimated by human landing collections (HLC). Although intense efforts have been made to find alternative non-exposure mosquito collection methods, HLC remains the standard for providing reliable and consistent HBRs. The aim of this study was to assess the relationship between human landing and light trap collections (LTC), in an attempt to estimate operationally feasible conversion factors between the two. The study was conducted as part of the operational research component of the Bioko Island Malaria Control Project (BIMCP), Equatorial Guinea. Methods: Malaria mosquitoes were collected indoors and outdoors by HLCs and LTCs in three villages on Bioko Island, Equatorial Guinea during five bimonthly collections in 2009. Indoor light traps were suspended adjacent to occupied long-lasting, insecticide-treated bed nets. Outdoor light traps were placed close to the outer wall under the roof of the collection house. Collected specimens were subjected to DNA extraction and diagnostic PCR to identify species within the Anopheles gambiae complex. Data were analysed by simple regression of log- transformed values and by Bayesian regression analysis. Results: There was a poor correlation between the two collection methods. Results varied by location, venue, month, house, but also by the statistical method used. The more robust Bayesian analyses indicated non-linear relationships and relative sampling efficiencies being density dependent for the indoor collections, implying that straight-forward and simple conversion factors could not be calculated for any of the locations. Outdoor LTC:HLC relationships were weak, but could be estimated at 0.10 and 0.07 for each of two locations. Conclusions: Light trap collections in combination with bed nets are not recommended as a reliable method to assess human biting rates on Bioko Island. Different statistical analyses methods give variable and inconsistent results. Substantial variation in collection methods prevents the determination of reliable and operationally feasible conversion factors for both indoor and outdoor data. Until improved mosquito collection methods are developed that can provide reliable and unbiased HBR estimates, HLCs should continue to serve as the reference method for HBR estimation. Background humans, and frequency of feeding. The product of the Assessing the success of malaria vector control interven- HBR and the sporozoite rate, i.e. the percentage of tions requires a robust and accurate entomological mon- infectious mosquitoes, provides the entomological itoring system. To assess if vector control interventions inoculation rate (EIR). The EIR is the average number have an impact on malaria transmission, the human bit- of infectious mosquito bites a person receives per time ing rate (HBR), i.e. the density of mosquitoes engaged in unit, a measure that best represents transmission inten- blood feeding, is an essential parameter. The HBR is a sity [1-3]. The widely accepted standard for estimating function of overall mosquito density, propensity to bite the HBR is the human landing collection method (HLC). Human landing collections are typically conducted by * Correspondence: [email protected] volunteers trained to collect host-seeking mosquitoes 1 Department of Mathematical Sciences and Technology, Norwegian that land on exposed body parts during the evening and University of Life Sciences, Ås, Norway Full list of author information is available at the end of the article night hours when anopheline vectors are most active. © 2012 Overgaard et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Overgaard et al. Malaria Journal 2012, 11:56 Page 2 of 14 http://www.malariajournal.com/content/11/1/56 However, HLCs are logistically difficult and expensive to reported as having any effect on the relative sampling carry out, because volunteers need to be recruited and efficiency in these studies. trained; continuous supervision is essential to avoid loss Several studies have compared various types of and attrition of volunteers throughout the night; sorting human-baited tent trap designs with LTCs and HLCs and identification of collected non-target species is for estimating relative sampling efficiencies [15-19]. time-consuming. These are all issues that impact the These studies also report widely varying 1:1 LTC:HLC quality of HLCs to estimate HBR and increase the cost relative sampling efficiencies; 0.02 in urban Dar es Sal- per mosquito collected. The results are not always reli- aam, Tanzania [19]; 0.33 in the Kilombero valley, Tanza- able and consistent, due to differential attractiveness of nia [17]; 0.56 in Ahero, western Kenya [16]; 1.3 also in individual collectors to mosquitoes, fatigue and ineffec- Kilombero, Tanzania [18]; and 1.86 in Lwanda, western tiveness and/or misconduct of collectors. Furthermore, Kenya [15]. It was suggested that the extremely low effi- HLC is considered unethical by some, because collectors ciency of light traps in Dar es Salaam was caused by the are exposed to potentially lethal mosquito bites [4]. On highly illuminated, urban environment reducing the effi- the other hand, even though volunteers are intentionally cacy of the light source in the traps [19]. The large dif- exposed to potential bites during HLCs, they would be ference between the two Kenyan studies was explained exposed anyhow, given they live in endemic areas. by the use of bed nets that repelled the mainly zoophilic Further, it could be argued that proper training of Anopheles arabiensis to seek alternative hosts outdoors volunteers results in increased vigilance to host-seeking and differences due to the presence/absence of cattle mosquitoes and therefore may in fact have a protective between sites [11,12]. The reason for the even larger dif- effect. ferences between the Tanzanian studies [17,18], con- The World Health Organization recommends avoiding ducted in the same village just a few years apart, were HLC unless absolutely essential, especially if safer tech- suggested to be effects of minor, uncontrollable factors, niques are available that can provide proxy estimates of such as location, vector control interventions, season, human biting rates [5]. Despite numerous attempts to weather and house type, all of which may vary through find equivalent sampling methods, however, HLC pre- space and time. vails as the only reliable method to determine HBR Light traps have generally been found to underesti- [6-8]. Light traps (LT), window exit traps, indoor pyre- mate the abundance of host-seeking anophelines thrum knockdown catches, and outdoor pit traps do not [20-22]. Along the Kenyan coast, light traps performed provideadirectestimateoftheHBRbecausetheydo poorly at lower densities and the sporozoite rate was not specifically capture mosquitoes engaged in host- significantly higher in mosquitoes collected using light seeking. traps compared to human landing collections [21]. On Light traps in combination with occupied mosquito this basis, it was recommended that light traps should bed nets, where a person under the net functions as a not be used as a substitute to human landing collections mosquito attractant, have been proposed as a compar- in areas with low malaria mosquito densities. However, able and unbiased alternative to HLC in sampling blood Smith [23] emphasized that the log (x + 1) transforma- meal-seeking mosquitoes [9]. Many studies have been tion of data, commonly used in trap comparison studies conducted in Africa and elsewhere, to assess how well [e.g. [9,21]], is highly dependent on mosquito density, light traps provide reliable estimates of human biting particularly at low values of x. Therefore this transfor- rates compared to other methods [7,9-22]. In Africa, the mation may not be valid in areas with sparse mosquito ratio of the number of Anopheles gambiae s.l. collected counts, such as in Kenya. Poisson regression techniques by indoor LTs versus indoor HLC varied between 1.06- would be more appropriate for such analyses, because 1.91. However these numbers are based on varying equivocal data transformations are avoided and dispro- ratios of light traps and human collectors (LTC:HLC); portionally influential low mosquito counts are managed such as 3:2 [9,12], 1:2 [10], or 1:1 [11,13], and cannot, by weighting low density observations [23]. Conse- therefore, be compared directly. After adjusting for the quently, Hii et al. [14] in Papua New Guinea used a varying number of light traps and human collectors, the novel statistical approach based on parameterizing the one-to-one LTC:HLC ratio for An. gambiae s.l. ranged negative binomial as a gamma mixture of Poisson distri- from 0.59 in Bagamoyo, Tanzania [10], 1.08 in Burkina butions to model agreement between sampling methods, Faso [11], 1.59 in Sierra Leone [12], 1.6 in Muheza, Tan- assuming both proportionality and non-linear relation- zania [9], to 1.91 in Zambia [13]. The two Tanzanian ships. These results showed that light traps underesti- ratios differed by 2.7-fold even though the collections mated the abundance of Anopheles punctulatus and were carried out close in time (five to seven years) and Anopheles farauti s.l. at high densities. On the other space (140 km). The density of mosquitoes was not hand, the LTC:HLC ratio for Anopheles koliensis and Overgaard et al. Malaria Journal 2012, 11:56 Page 3 of 14 http://www.malariajournal.com/content/11/1/56 Anopheles karwari increased with increasing mosquito © £ © ¦ ¤ density.