Overhead tank is the potential breeding habitat of Anopheles stephensi in an urban transmission setting of Chennai, India

Abstract

BackgroundWells and overhead tanks (OHT) are the major breeding sources of the local malaria vector, Anopheles stephensi in the Indian city of Chennai; they play a significant role in vector breeding, and transmission of urban malaria. Many other man-made breeding habitats, such as cemented cisterns/containers, barrels or drums, sumps or underground tanks, and plastic pots/containers are maintained to supplement water needs, temporarily resulting in enhanced mosquito/vector breeding. Correlating breeding habitats with immature vector abundance is important in effective planning to strengthen operational execution of vector control measures.MethodsA year-long, weekly study was conducted in Chennai to inspect available clear/clean water mosquito breeding habitats. Different breeding features, such as instar-wise, immature density and co-inhabitation with other mosquito species, were analysed. The characteristics of breeding habitats, i.e., type of habitat, water temperature and presence of aquatic organisms, organic matter and green algal remnants on the water surface at the time of inspection, were also studied. Immature density of vector was correlated with presence of other mosquito species, malaria prevalence, habitat characteristics and monthly/seasonal fluctuations. All the data collected from field observations were analysed using standard statistical tools.ResultsWhen the immature density of breeding habitats was analysed, using one-way ANOVA, it was observed that the density did not change in a significant way either across seasons or months. OHTs contributed significantly to the immature population when compared to wells and other breeding habitats of the study site. The habitat positivity of wells and OHTs was significantly associated with the presence of aquatic organisms, organic matter and algal remnants. Significant correlations of malaria prevalence with monthly immature density, as well as number of breeding habitats with immature vector mosquitoes, were also observed.ConclusionsThe findings that OHTs showed fairly high and consistent immature density of An. stephensi irrespective of seasons indicates the potentiality of the breeding habitat in contributing to vector density. The correlation between vector breeding habitats, immature density and malaria prevalence indicates the proximity of these habitats to malaria cases, proving its role in vector abundance and local malaria transmission. The preference of An. stephensi to breed in OHTs calls for intensified, appropriate and sustained intervention measures to curtail vector breeding and propagation to shrink malaria to pre-elimination level and beyond.

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

“…In the present study, potential An. stephensi habitats, OHTs and wells were randomly selected from each season of our yearlong weekly study of 20 sentinel and 20 random sites conducted in parallel in the same area [8], to evaluate physico-chemical properties of the habitat and its relation to occurrence and abundance of mosquito breeding (Fig. 1).…”

“…A total of 40 habitats i.e., ten habitats (five OHTs and five wells) were randomly selected from each season of a longitudinal study carried out in parallel to check the breeding pattern of vector mosquitoes [8]. Water samples were collected four times during April 2013 to March 2014, representing four seasons: April (summer), July (pre-monsoon), December (winter), and February (post-monsoon).…”

“…The habitats were also observed for the presence or absence of direct sunlight or shade, colour, and temperature of water at the time of inspection. Habitat particulars such as whether they were perennial (water stored throughout the year), temporary (occasionally filled/retained depending on need), open (with gaps allowing entry of mosquitoes or completely mosquito-proofed leaving no gaps), were considered from a longitudinal study carried out in parallel [8]. Immature collection in wells were done using ‘well nets’ (conical drop nets 20 cm in diameter and 10 cm deep, that were lowered into the well on a string), while ladles with a volume of 250 mL were used for OHTs, performing four dips per sampling occasion [10–12].…”

“…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.…”

“…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.…”