Entomological risk factors for the high incidence of dengue in the forest fringe areas of Kerala, India

Abstract

Incidence of dengue in Kottayam district, Kerala commences with cases reported from forest fringe areas of Kanjirappally every year. An entomological investigation was carried out to study the relative abundance of dengue vector and to delineate their breeding habitats in the forest fringe areas of Kanjirappally taluk. Four villages (Erumeli, Mundakayam, Kootikal, Koruthode) located in the forest fringe areas of Kanjirappally taluk have been selected for the study. In each study area all the breeding sources in and around 24 households covering an area of about 0.5 Sq Km were surveyed and checked for vector breeding at fortnightly intervals from February 2021 to October 2021. Immatures were collected from positive breeding site and were brought to the laboratory for emergence and species identification. Larval indices were calculated using WHO methods. Ae albopictus was found to be the most prevalent and widely distributed vector species and the primary vector Ae. aegypti was not found during the household survey. The maximum Container index and Breteau index were found to be 71.45 and 51.90 during the month of June and July which shows a linear positive correlation with rainfall and significantly higher pupal index was found during the month of May (2.96). Peridomestic water storage containers and rubber plantation associated containers were the main breeding habitat observed which accounts 64.08% and 80.29% of pupal production during the dry and wet seasons respectively. Sylvan environment of rubber plantation together with favourable climatic conditions are the key factors for the prevalence of dengue vector in the forest fringe areas of Kerala. 1. Introduction Dengue is caused by virus belonging to family flaviviridae and having four serotypes DENV-1, DENV-2, DENV-3 and DENV-4. Dengue infection ranged from simple fever to much more severe and sometimes fatal dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) [1]. According to WHO about 40% of the world's population are now at risk of dengue and among all the WHO regions, the Southeast Asia and the Western Pacific regions contribute nearly 75% of the global burden of dengue [2]. Aedes aegypti and Aedes albopictus are the major vectors for dengue and dengue hemorrhagic fever. Before 1990, dengue was not widespread in India. The first case was reported in 1996 in the rural areas of Haryana state followed by several outbreaks in different parts of the country [3]. In recent years the number of dengue cases has increased tremendously all over India and almost all states are endemic for dengue [4, 5]. A study done by Oxford university on global distribution and burden of dengue estimates that India alone contribute 34% (33 million infections) of global total and 100 million asymptomatic cases occurring annually [6]. With an area of only about 1.5% of the country and about 2.8% of the population, Kerala state reported more than 13.1% of dengue cases in India for the last seven years. Dengue virus was detected in wild caught Aedes albopictus around Kozhikode airport of Malappuram District, Kerala [7]. Also the detection of virus from field collected adult male and female Ae. albopictus [8] suggest evidence for vertical transmission of dengue virus in Kerala. All four serotypes of dengue virus (DENV), DENV-1, DENV-2, DENV-3 and DENV-4, were found to be prevalent in the state [9]. For the past ten years dengue cases with varying degree of severity have been