Standardisation of Units for Assessment of Adult Disease Vector Density under Vector Control Programmes in India

Abstract

Vector density is one of the most frequently used monitoring parameters of entomological surveillance under any vector control programme. Vector control applications are guided by the density of vectors or their abundance in different seasons and settings. The vectors of different common vector-borne diseases viz. malaria, filaria, kala-azar, dengue, chikungunya, Zika and Japanese Encephalitis (JE) have different bionomics. Scientists, researchers, and public health entomologists of various research institutes and programmes are engaged in studying vector bionomics through vector surveillance activities. The most common parameter used to estimate the density of vector and non-vector species of both mosquitoes and flies is the collection of species in a given unit of time. In the malaria control programme, it started as a collection of resting vector mosquitoes at a specified time of dawn and dusk. These are expressed in a number of forms viz. ‘per man hour’, ‘per ten man hour’ and ‘ten man hour’ to ascertain the level of vector population and its increasing or decreasing trend with climatic factors which may be correlated with the active transmission of the disease. The minimum level of density at which active transmission was evidenced has been termed as ‘critical density’. Various vector species have different critical densities. Many other parameters are used to estimate vector or non-vector populations but such different units may often lead to confusion among the field functionaries. This article describes the significance of ‘per man-hour density’, the methodology which has been in practice for ages and the statistical method for its calculation. To avoid misconception, it should be understood that the density expressed for a particular species is the ‘differential density’ and not the absolute density.