Does fluoride influence oviposition of Anopheles stephensi in stored water habitats in an urban setting?

Malaria Journal Thomas et al. Malar J (2016) 15:549 DOI 10.1186/s12936-016-1594-x Open Access RESEARCH Does fluoride influence oviposition of Anopheles stephensi in stored water habitats in an urban setting? Shalu Thomas1†, Sangamithra Ravishankaran1†, N. A. Johnson Amala Justin1, Aswin Asokan1, T. Maria Jusler Kalsingh1, Manu Thomas Mathai2, Neena Valecha3 and Alex Eapen1* Abstract Background: The physico-chemical characteristics of lentic aquatic habitats greatly influence mosquito species in selecting suitable oviposition sites; immature development, pupation and adult emergence, therefore are considerations for their preferred ecological niche. Correlating water quality parameters with mosquito breeding, as well as immature vector density, are useful for vector control operations in identifying and targeting potential breeding habitats. Methods: A total of 40 known habitats of Anopheles stephensi, randomly selected based on a vector survey in parallel, were inspected for the physical and chemical nature of the aquatic environment. Water samples were collected four times during 2013, representing four seasons (i.e., ten habitats per season). The physico-chemical variables and mosquito breeding were statistically analysed to find their correlation with immature density of An. stephensi and also co-inhabitation with other mosquito species. Results: Anopheles stephensi prefer water with low nitrite content and high phosphate content. Parameters such as total dissolved solids, electrical conductivity, total hardness, chloride, fluoride and sulfate had a positive correlation in habitats with any mosquito species breeding (p < 0.05) and also in habitats with An. stephensi alone breeding. Fluoride was observed to have a strong positive correlation with immature density of An. stephensi in both overhead tanks and wells. Conclusion: Knowledge of larval ecology of vector mosquitoes is a key factor in risk assessment and for implementing appropriate and sustainable vector control operations. The presence of fluoride in potential breeding habitats and a strong positive correlation with An. stephensi immature density is useful information, as fluoride can be considered an indicator/predictor of vector breeding. Effective larval source management can be focussed on specified habitats in vulnerable areas to reduce vector abundance and malaria transmission. Keywords: Physico-chemical factors, Breeding habitats, Anopheles stephensi, Fluoride Background Mosquitoes exploit almost all types of lentic aquatic environments for oviposition [1]. Immature Anopheles thrive in a variety of aquatic ecosystems, such as fresh, brackish *Correspondence: † Shalu Thomas and Sangamithra Ravishankaran contributed equally to this work 1 IDVC Field Unit, National Institute of Malaria Research (ICMR), NIE Campus, 2nd Main Road, TNHB, Ayapakkam, Chennai 600 077, India Full list of author information is available at the end of the article waters found in rural, coastal and urban areas. Water quality of breeding habitat is an important determinant of whether or not the female mosquitoes will lay their eggs and the resulting immature stages will successfully complete their development to adults [2]. Characteristics of aquatic habitats are reported to influence the preferences of oviposition, possibility of hatching, immature development, pupation, and adult emergence, thus specifying the niche of a mosquito species [3]. Consequently breeding habitat characteristics could influence adult productivity © The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Thomas et al. Malar J (2016) 15:549 and if the mosquito is of epidemiological importance, it will have a strong impact on the disease transmission [4]. Selection of an appropriate aquatic medium for oviposition could be an innate behavioural characteristic of a mosquito species. What drives a mosquito to select its ovipositional site from an array of aquatic habitats is yet to be fully ascertained and this information would be of paramount importance for focussing intervention operations to target potential vector breeding habitats, thereby reducing manpower and expenditure. Information on the physico-chemical properties of the breeding habitat, a potential key element for larval surveillance could help in the implementation of better vector control programmes. Effective larval source management (LSM) requires a thorough knowledge of the breeding ecology of the mosquito species, its ovipositional preference, spatial and temporal distribution of the breeding sites besides, physical, chemical and biological characteristics of the habitats. Physico-chemical characteristics of the mosquito breeding habitat such as pH, optimum temperature, total suspended solids, total dissolved solids, electrical conductivity, have an impact on larval development and survival [5]. Furthermore, temperature, salinity, carbonates and nitrates have been shown to correlate with the presence or development of quality of Anopheles larvae in pools [6]. Physico-chemical parameters of Anopheles breeding habitats in Iran indicated that there was a significant relationship between water temperature, conductivity, total alkalinity, sulfate, chloride, and Anopheles species distribution and abundance [7]. Anopheles stephensi, the vector responsible for urban malaria in Chennai, India, breeds mainly in clean/clear water, such as overhead tanks, wells, cisterns, roof gutters, curing pits in construction sites, fountains and ornamental tanks. Besides other man-made habitats, such as barrels or drums, sumps or underground tanks, and plastic pots/containers also contributes to enhanced mosquito/vector breeding [8]. Quantifying water quality in Anopheles breeding habitats may give more insight into its breeding profile, particularly in urban settings. It was observed that a larger proportion of OHTs support breeding of An. stephensi, compared to wells and other breeding habitats. Nevertheless, it was unclear whether this difference was because of differences in the abiotic factor, such as water quality, or biotic, such as co-inhabitation of other mosquito species with the urban vector [8]. Since relatively little information is available on habitat selection and physico-chemical factors determining oviposition behaviour of An. stephensi in field settings, the present study aimed to find the relationship between the physico-chemical factors of breeding hab (...truncated)