Identifying clustering of cholera cases using geospatial analysis in Kolkata and surrounding districts: data from patients at tertiary care referral hospitals

Background: Cholera cases have increased globally across the Eastern Mediterranean, Africa, Southeast Asia, and parts of Europe since early 2024. This study aims to identify cholera hotspots and understand the spatial distribution of cholera in Kolkata and surrounding regions, a key cholera reservoir. Additionally, we examine sociodemographic factors and aspects related to water, sanitation, and hygiene (WASH).

Methods: Cholera clusters were detected using kernel density estimation and spatial autocorrelation through Global Moran's-I statistics, with local cluster patterns examined using Local Moran's-I statistics. Cholera cases from August 2021 to December 2023, treated at two tertiary care facilities in Kolkata: Infectious Diseases and Beleghata General Hospital and Dr. B C Roy Post Graduate Institute of Paediatric Sciences Hospital were included. Additionally, through a case-control study, 196 culture-confirmed cholera cases and 764 age/sex-matched neighborhood controls were enrolled, to investigate cholera risk factors.

Findings: Spatial analysis revealed a concentration of 196 cholera cases in Kolkata and its surrounding regions of Howrah, Hooghly, and North and South 24 Parganas. Hotspot analysis showed significant clustering in several Kolkata wards (31, 33, 56, 46, 57, 58, 59, 61, 66, 71, and 107), particularly in the northern, central, and east Kolkata wetlands areas (Global Moran's I statistic = 0.14, p < 0.001). These clusters had proximity between cases, with a median distance of 187.7 m, and 25.5% of cases as close as 73.9 m apart, suggesting localized transmission. Hotspots were identified with an average distance of 1600 m between them. Local Moran's I analysis found dense "high-high" clusters in these areas (p < 0.01), with a mean Moran's I index of 0.3, (range 0.1-4.6). The case-control study revealed that males were more likely to contract cholera, with an adjusted odds ratio of 2.4 (p < 0.01). There was no significant association found between cholera infection and sociodemographic factors or various WASH practices.

Interpretation: The findings emphasize the importance of targeted interventions, especially in identified hotspots, to mitigate cholera transmission. Addressing Socio-economic, and environmental factors especially improvement in WASH practices may further enhance prevention effects.

Funding: The author KK, received funding from the program of the Japan Initiative for Global Research Network on Infectious Diseases, (grant id: JP23wm0125004), from the Ministry of Education, Culture, Sports, Science and Technology in Japan, and Japan Agency for Medical Research and Development.