Identification of Rabies Hotspots and Risk Zones in South and Central Kerala (India)
Antony MU, Abraham SS and Jiji RS
Published on: 2024-02-09
Abstract
Risk-based regionalization can support rabies control programs in developing countries with a targeted approach instead of spreading the resources thinly across the entire region. The occurrence of rabies in India is neither clearly distributed nor complete due to a lack of authentic data. Kerala, the south most state of India, has a robust laboratory surveillance capacity for rabies. However, there is inadequately collated and published information on the areas within the state that are at risk of rabies. Reported case numbers are a useful and convenient tool for identifying areas at risk. In this preliminary study, we used geo-mapping to identify risk areas and hotspots of rabies in nine districts located in the south and central regions of Kerala based on laboratory-confirmed rabies cases over a two-year period. The districts were classified into different risk categories, and we compared the spatial distribution over time to understand the changing trend. The study found that Thiruvananthapuram, Trissur, and Kollam districts had the highest risk in terms of both the number of cases and hotspots. Idukki district was the least risky area, while other districts were classified as moderate to high-risk. Rabies cases were reported from all districts, indicating that there were no rabies-risk-free areas in the region. Additionally, all municipal corporation areas were identified as hotspots. The spatial trend did not appear to differ significantly over the period. This risk estimation can assist policymakers and program managers in implementing an evidence-based, targeted approach.
Keywords
Rabies; Hotspot; Risk zone; Spatial trend; Geo mappingIntroduction
Rabies is a major zoonotic disease worldwide, causing significant mortality in both humans and animals, particularly in developing countries. The density of reservoir populations, mainly domestic and stray dogs, plays a crucial role in the persistence of the disease [1]. Rabies in animals is underreported and underdiagnosed globally, and data on its prevalence in the animal population is limited. Currently available data only indicate the presence of the disease rather than its actual disease burden. To prioritize resources, there is an urgent need to assess risk areas as control programs scale up towards the elimination of dog-mediated rabies. In rabies control for resource-limited regions, risk-based compartmentalization is proposed to ensure judicious utilization of resources instead of spreading them thinly across the entire region. Reported case numbers, host population density, and reported animal bite cases are convenient means for identifying risk areas [1].
There have been relatively few studies published on the spatial epidemiology of rabies in Kerala, despite the state having an authentic data base from seven animal rabies laboratories distributed throughout the region. This preliminary study was conducted in nine districts of south and central Kerala and represents the first attempt to identify animal rabies risk areas in Kerala based on confirmed case reports. The study results, when considered within the context of limitations, can determine risk areas and hotspots for rabies. This information can assist policymakers and program managers in implementing an evidence-based, targeted approach.
Materials and Methods
The present study evaluated the density of animal rabies cases in southern and central Kerala as a preliminary attempt to identify risk areas and hot spots. This region was selected for the study due to its high dog population [2] and the concentration of rabies laboratories.
The study analyzed two years of laboratory passive surveillance data from the five rabies diagnostic laboratories located in this region. Animal rabies cases confirmed in the laboratories between January 2021 and December 2022 were spot-mapped by location using longitude and latitude. This was done to understand spatial distribution, case concentration, and hotspots. A location with three or more cases per year was considered a potential rabies hotspot [3]. The location unit was assigned to an administrative village. The state’s nine districts in this region have been categorized into different risk zones based on the total number of cases in a year. These zones were classified as very high (>50 cases), high (30-50 cases), moderate (10-30 cases), low (1-10 cases), and no risk (no cases). To confirm the fairness of the analysis, a time series pattern was also analyzed by comparing data from 2021 with that of 2022. The time increment was set to a year. The study did not use any mathematical or regression models for quantifying the location-based risk of animal rabies.
Results and Discussion
The study analyzed 686 cases of animal rabies confirmed in laboratories using the Direct Fluorescent Antibody Test (DFAT) between January 1, 2021, and December 31, 2022. Table 1 displays the district-level rabies risk and the number of potential hotspots in each district. Figures 1 and 2 depict the location and concentration of animal rabies cases, while figures 3 and 4 show the hotspots.
Table1: Rabies Risk- Based Regionalization of South and Central Kerala.
District (Region) |
Risk grade |
Number of hotspots |
||
|
2021 |
2022 |
2021 |
2022 |
Thiruvananthapuram (South) |
Very high |
Very high |
7 |
6 |
Kollam (South) |
High |
Very high |
2 |
2 |
Pathanamthitta (South) |
Moderate |
High |
1 |
1 |
Alapuzha (South) |
Moderate |
High |
2 |
2 |
Kottayam (South) |
Moderate |
High |
0 |
2 |
Idukki (Central) |
Low |
Low |
0 |
0 |
Ernakulam (Central) |
Moderate |
Moderate |
0 |
1 |
Trissur (Central) |
Very high |
Very high |
3 |
5 |
Palakkad (Central) |
Moderate |
High |
0 |
0 |
Thiruvananthapuram and Trissur districts were identified as being in the “very high” risk zone with a higher number of hotspots in both years. In the first year of analysis, Kollam district was classified as “high” risk but subsequently moved to “very high” risk in the following year. Idukki district was classified as “low risk” in both years. In 2021, the districts of Pathanamthitta, Alappuzha, Kottayam, Ernakulam, and Palakkad were categorized as having a “moderate” risk of rabies. However, in 2022, all these districts except Ernakulam moved to the "high-risk" category. Ernakulam district continued in the moderate-risk zone. Rabies cases were reported from all districts in the region in both years, meaning that none could enjoy the “no risk” status.
The districts categorized as ‘very high’ and ‘high risk’ with a greater number of hotspots had a higher dog population [2], demonstrating a positive correlation between them. This highlights the importance of prioritizing these areas in control programs. One reason for the relatively low number of rabies cases in Idukki district could be due to access limitations to diagnostic laboratories. All municipal corporation areas in the study region were identified as hotspots, possibly due to the dense human population and poor waste management, which led to an increase in the stray dog population.
The spatial trend of rabies with respect to risk zones, case concentration, and hotspots did not appear to differ much between the years, as shown in the figures and table. However, there was a drastic increase in the number of rabies cases in animals in 2022 in comparison with 2021. This could be due to poor vaccination coverage and weak population control activities in dogs in the preceding years, in the wake of COVID-19 restrictions [4].
Figure 1: Animal Rabies Case Distribution in South and Central Kerala: 2021.
Figure 2: Animal Rabies Case Distribution in South and Central Kerala: 2022.
Figure 3: Rabies Hotspots in 2021.
Figure 4: Rabies Hotspots in 2022.
There are many limitations to our study. One is that it was subjected to bias, such as potential underreporting from areas far from laboratories. The data used is passive surveillance data rather than the ideal active surveillance information. Moreover, the time series information deduced over a two-year period is insufficient to study the change in disease trend over a period of time. Laboratory-confirmed case data, although considered of high value due to the high accuracy of rabies tests, represent the tip of the iceberg in terms of true incidence.
On the brighter side, the area selected for the study had strong laboratory resources. The risk areas looked almost the same in both years of the study, and there was an apparent correlation between the identified rabies hotspots and the animal bite hotspots published by the Department of Animal Husbandry [6]. This may denote the strength of this analysis and the assumptions made. The risk areas identified here can be used for post-exposure risk assessment in humans as well. The control of zoonoses such as rabies requires a ‘One Health’ approach involving collaboration between the veterinary and medical communities [7]. The method exemplified here is simple and cheap and can be adopted to develop baseline data that can help to prioritize regions and resources in control programs.
Conclusion
The occurrence of rabies in India is neither clearly distributed nor complete, and the geographical pattern is therefore distorted. Although the state of Kerala has a robust laboratory surveillance capacity for rabies, there is inadequately collated published information on rabies risk areas within the state. This study identified rabies risk zones and hotspots in southern and central Kerala, which, translated within the context limitations, would be helpful for the formulation of region-specific control strategies and the prioritization of areas for control measures. We suggest further studies expanding analysis to more areas using a longer-term multiple data base and employing quantitative mathematical models along with ideal active surveillance to improve risk estimation.
Acknowledgements
The study is a part of the dissertation work of the first author for the award of post graduate diploma in ‘One Health’ by Kerala Veterinary and Animal Sciences University. Authors thank HOD, Pathology Department, Veterinary College, Trissur; Laboratory Directors, Animal Husbandry Department Laboratories (SIAD, Thiruvananthapuram, District Lab, Kollam, RDDL, Thiruvalla and Palakkad) for sharing data.
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