Worldwide, vector-borne diseases (VDBs) spread by mosquitoes have resulted in a global societal deception that has killed lives and forced significant financial outlays to maintain social order. Several species of mosquitoes, which are the most frequent carriers of diseases, can cause dengue, chikungunya, malaria, and other illnesses (1). Among them, malaria is a potentially fatal infectious disease that severely affects vulnerable communities in tropical and subtropical locations where the environment is conducive to transmission. Although malaria transmission appears to be declining worldwide due to vector-borne control interventions, the 2021 estimation indicates that there are 168 million cases and 427,854 malaria deaths globally (2). In the last decade, India has faced a significant decline in malaria cases and deaths, with 1018 deaths in 2010, steeply decreased to 90 in 2021 (2). WHO Global Technical Strategy for Malaria (GTS) fixed its target to eliminate malaria globally by 2030 Asia-Pacific countries, including India, have pledged to eliminate malaria by 2030 and reducing 50% mortality rate is a mandatory goal at the global scale (2). WHO Global Technical Strategy (GTS), the Asia Pacific Leaders Malaria Alliance (APLMA), Malaria Elimination Roadmap, and the National Framework for Malaria Elimination (NFME) 2016–2030 have been developed together with partners and key stakeholders in a vision to eliminate malaria throughout the country by 2030 (NVBDCP et al., 2016). And also a target to reduce the Annual Parasite Index (API) of less than 1 by 2024 and contribute to improved health, quality of life and alleviating poverty (3). To sustain zero indigenous morbidity and mortality, newer intervention tools were implemented with the Early case Detection and Prompt Treatment (EPDT) strategy, providing Insecticide-treated bed nets (ITN) and Long-Lasting Insecticidal Nets (LLINs) to the residents for vector control, early diagnosis and prompt treatment with Artemisinin based Combination Therapy (ACT), using Indoor Residual Sprays (IRS) to protect at-risk population under Integrated Vector Management (IVM) process (4).
Indian states such as Madhya Pradesh, Andhra Pradesh, Maharashtra, Bihar, West Bengal, Odisha and North East regions are highly prone to malaria endemic, contributing around 97% of total malaria cases (4). In the last decade, India has made tremendous progress in reducing malaria mortality and morbidity. Despite the steep decrease in malaria incidence across India, there are few endemic pockets where malaria remains a significant public health challenge to pose a stiff challenge to India’s malaria elimination efforts. In order to eliminate the parasite and prevent its recurrence, finding these final pockets of transmission is essential (5). In the Indian state of West Bengal, malaria is predominantly transmitted by P. vivax and is also co-endemic with P. falciparum, considered the deadliest form of malaria, varied across due to different physiographic zones, political border (interstate and international) with high-endemic malaria region. Under the NFME, West Bengal is situated in the pre-elimination phase of Category 2 with an API of less than one and one or more districts reporting an API of more than one. In 2018, the Health and Family Welfare Department of the Government of West Bengal officially declared malaria as a notifiable disease to entail on-time diagnosis and reporting by all government and private hospitals/laboratories, including non-governmental organization (NGO) run hospitals, as well as individual medical practitioners to strengthen capturing of surveillance data which is a matter concern (6).
Space-time disease mapping of vector outbreaks is an informative tool for public health interventions that provide information like rate of transmission, cyclical pattern, intensity and risk of diffusion to the new location, persistent nature, etc. (7, 8). Malaria cluster identification can aid in the demarcation of problem regions and the deployment of focused programme interventions suited for the eradication phase (5). Focused interventions in malaria-risk regions are expected to be more cost-effective than uniform resource allocation, especially in resource-constrained settings for long-term eradication programmes (9–11). Furthermore, knowing the seasonal pattern of malaria transmission and obtaining information of seasonal behaviour can assist in estimating the period for malaria transmission in order to initiate appropriate and effective control measures (12–14).
Scan Statistics is one of the most common statistical methods used to identify the clusters of cases spatially and temporally (15). Whereas Kulldorff’s univariate (STSS) identifies space-time clusters of single diseases, multivariate STSS can evaluate clusters of multiple diseases that co-occurred − (16, 17). Univariate space-time statistics have been used to identify the outbreaks and space time clusters of diseases, such as malaria (5, 18, 19), Dengue and Chikungunya (20), COVID 19 (21), Lyme disease (22), Chikungunya (23) and other public health problems like crime (24), deaths of despairs (25) etc. Space time scan statistics were also used to identify the cluster pattern of P. vivax and Falciparum individually in Ahmedabad City (26), Karnataka (27), Bhutan (5) etc. Whereas multivariate space time scan statistics can examine space time clusters of the simultaneous co-occurrence or co-existence of multiple diseases at one point of time used to identify outbreaks of dengue and chikungunya in Colombia (20), deaths of despairs in the US (25), etc.
It is crucial to comprehend the spatio-temporal distribution of malaria vectors at the block level for developing intervention strategies since West Bengal has a porous international border with Bangladesh and a national border with high endemic states. However, no large-scale studies have examined geographic patterns of both P. vivax and P. falciparum malaria in West Bengal. This study is the first attempt to evaluate seasonal variability and the retrospective space–time distribution of individual and co-occurring - P. vivax and P. falciparum malaria across all 341 blocks in West Bengal, to the best of our knowledge. The purpose of the present study is to fill this gap in the understanding of seasonal patterns and the spatial and spatiotemporal distribution of two vectors of malaria and to estimate the relative risk of each high space time cluster at block level in West Bengal during 2011–2021.
The remainder of this paper is organized as follows: Section 2 describes the geographical location of the study area and the data available for P. vivax and P. falciparum in West Bengal, and the selected methods to identify seasonal variability; significant space time clusters and evaluation of relative risk. Section 3 compares the result of the study, including exploratory analysis and seasonal pattern of malaria vectors, and compares the size and duration of space time clusters for both of the malaria vectors with their co-occurring. Finally, section 4 concludes by summarizing the main findings with a discussion and highlighting the strengths and limitations of the study and some suggested directions for further research.