Epidemiology and risk factors in a scrub typhus cluster in Odisha, India, 2023: Investigating with a One Health approach

Introduction

Scrub Typhus accounts for approximately one-fourth of all acute undifferentiated febrile illness cases in India. In September 2023, Odisha, an eastern Indian state, reported several scrub typhus deaths. We investigated a scrub typhus cluster in Block-A, Odisha, to describe the epidemiology and assess risk factors.

Methodology

We conducted an unmatched (1:1) case-control investigation. Line-list of scrub typhus IgM-ELISA tested individuals was collected from the district laboratory. Cases were defined as scrub typhus IgM-ELISA positive residents of Block-A between July1 and October30, 2023 and controls were test-negative block-A residents. We interviewed cases and controls from list of tested individuals for socio-demographic, clinical and exposure histories. We present adjusted-odds ratio and 95% confidence intervals. We conducted an entomological survey in Block-A, isolated mites to detect pathogen and strain (47kDa Real Time PCR) and report entomological indices.

Results

Total 151 cases were reported in 2023 in Block-A, with case-fatality rate of 2.6%, median age 30 years, and inter-quartile range 15–46 years. Among the 61 interviewed cases, 46% were hospitalised, 13% admitted in ICU, 16% visited health-facility in second week of symptom onset and 30% were tested in second week of symptom-onset. We enrolled 61 cases and 61 controls, among whom recent farm/forest visit was significantly associated [adjusted OR = 6.1, 95%CI: 2.0-18.4] with illness on multivariate analysis. Chigger-index for block-A was 18.94(322/17); 36%(4/11) of tested mite-pools were positive for Orientia tsutsugamushi and sequences were identified to be Karp-like.

Conclusions

Our investigation confirmed a lab-verified scrub typhus cluster in Block-A, identifying farm/forest visits without protective clothing as the most significant factor for acquiring infection. Detection of infected vector, hosts, and pathogen completed the epidemiological triad of scrub typhus, establishing local transmission in Block-A. We sensitised frontline-healthcare workers on early identification and timely referral, recommended community education on limiting mite exposure and seeking timely medical attention.

Key Messages

• We aimed to describe the epidemiology of scrub typhus and identify associated risk factors.

• We identified recent farm and forest visits as a significant risk factor for scrub typhus infection, and detected infected vectors, maintenance and incidental hosts, along with the pathogen, therefore completing the epidemiological triad and confirming local transmission in this northwestern region of Odisha.

• Scrub typhus, a neglected and re-emerging zoonosis affecting large parts of India, including northwestern Odisha, has limited epidemiological evidence in literature; therefore, our one-health investigation, providing comprehensive evidence of local transmission and presence of significant risk factors, addresses this critical gap in the literature, emphasising the urgent need for targeted interventions in these endemic areas.

Introduction

Scrub Typhus, a vector-borne rickettsial zoonotic infection, caused by Orientia tsutsugamushi, has been in transmission in the Asia-Pacific region known as the tsutsugamushi triangle since before the twentieth century [1]. With an estimated global annual incidence of one million, scrub typhus is one of the most underdiagnosed and underreported diseases, often requiring hospitalisation [2]. The seroprevalence of scrub typhus, estimated in a global systematic review was 23% in febrile patients and 10% in healthy individuals, predominantly among Asian countries like China, South Korea, Thailand and India [3]. In India, although national figures are not known, various systematic reviews and isolated studies have estimated scrub typhus to account for approximately one-fourth of acute undifferentiated febrile illness cases [4, 5]. In India, scrub typhus is reported under the national disease surveillance programme known as Integrated Disease Surveillance Programme (IDSP) since 2021, with a recent increase in testing capacity and reporting.

Odisha is an eastern state in India, comprising 30 districts with a total population of 41.97 million. This state has a tropical climate with monsoons, and vegetation ranges from tropical evergreen forests to deciduous forests, grasslands, and coastal mangroves [6]. In September 2023, the Integrated Disease Surveillance Program (IDSP) received media alerts regarding deaths due to scrub typhus in multiple districts of Odisha, with Sundargarh district reporting the highest number of cases in the state. Sundargarh district has 17 sub-districts, known as “Blocks” with an average population of 130,000 per block. The cases in the district were scattered and reported from across all blocks. In response, a team was deputed from the National Centre for Disease Control (NCDC), New Delhi, including two Epidemic Intelligence Service (EIS) officers. We conducted an epidemiological investigation of a Scrub Typhus cluster in the most affected block (Block-A) of Sundargarh district to describe the epidemiology of scrub typhus cases and assess potential risk factors.

Methodology

Descriptive epidemiology

We collected data on individuals tested for scrub typhus IgM-ELISA in Block-A, from the District Public Health Laboratory (DPHL) of Sundargarh District for 2023. A case was defined as scrub typhus IgM-ELISA positive resident of Block-A between July 1 and October 30, 2023. We described the sociodemographic, clinical, and exposure history of cases. We assessed the healthcare infrastructure (diagnostic facilities, intensive care units, and drug availability) and scrub typhus outbreak preparedness at the district through interviews with district health staff and field observations. We assessed the knowledge of frontline-healthcare workers (FHWs) on scrub typhus using a structured questionnaire on disease transmission and presentation (six questions); screening and diagnosis (five questions); and environmental risk factors (five questions).

Case control investigation

To assess the risk factors for acquiring scrub typhus infection, we conducted a 1:1 unmatched case-control investigation. Based on literature review and data gathered from descriptive epidemiology, a hypothesis was generated that “rodent presence within the house or adjacent veranda (semi-open area) is associated with illness”.

Case and control definition and inclusion

We defined a case as scrub typhus IgM-ELISA positive resident of Block-A from July 1 to October 30, 2023, and controls were defined as IgM-ELISA test-negative residents of Block-A in the same time frame. All cases included in the descriptive analysis were also enrolled in the case-control investigation, and controls were enrolled from the DPHL list of test-negative patients. Both cases and controls were individuals who presented to the outpatient clinic at the district hospital with acute febrile symptoms (fever, myalgia) and were tested in DPHL for malaria, dengue and scrub typhus. Scrub typhus tests were conducted by IgM ELISA (InBios kit: sensitivity 93%, specificity 91%) [7].

Sample size calculation

We calculated a sample size of 122 (61 cases, 61 controls) using stat calc (Epi Info V7.2.5.0) with 90% confidence interval, 75% power using ‘presence of rodents in and around house’ as exposure, with a ratio of 1:1 for cases to controls, assuming exposure among controls to be 50%, and keeping exposure among cases as 71%, as noted from our descriptive epidemiology results [8]. we enrolled all the cases that were available and consented to interviews. A 90% confidence level and 75% power was chosen to obtain the sample size within the constraints of the outbreak setting.

Data collection tool

We interviewed the cases and controls after obtaining informed consent, using a structured questionnaire developed in English and translated into the local language (Odiya), for socio-demographic, clinical (symptoms, treatment-seeking, hospital admission, disease progression, complications), and possible exposure history. Exposures were collected for environmental conditions, household factors, and behavioural practices indicating contact with chigger-mite habitats and rodent-associated environments. We present here the results for significant exposures. Exposure history was taken for one month prior to symptom onset, considering scrub typhus incubation period of 5–21 days (rounding off to one month for ease of asking during interview) [9, 10]. We also extracted clinical data from the medical records of cases admitted to the district hospital.

Entomology survey

To gather evidence for the presence of potentially infected maintenance hosts (rodents) and vectors (chigger-mites) in the environment surrounding the reported cases in Block-A, and to inform the district health authorities on preventive measures, we conducted an entomological survey with the district entomologists.

Rodent trapping, ectoparasite collection and processing

Rodents were trapped by placing rodent traps in and around the houses (domestic and peridomestic area- verandah 10 m around the house) of reported cases. To place the traps, case houses were selected purposely, from the villages and localities with the highest number of cases in the block. We collected ectoparasites on the captured rodents by reverse combing the fur with a fine-toothed comb, and collected rodent sera. The rodent and isolated ectoparasite samples were tested at the National Centre for Disease Control (NCDC), Delhi. Chigger-mites were isolated and morphologically identified under a stereomicroscope using standard taxonomic keys [11]. Weil–Felix test (WFT) was performed to detect rickettsial antigen and assess infection.

Strain identification and phylogenetic testing

Chigger-mites collected from the same rodent were pooled together and subjected to real-time PCR targeting the 47 kDa gene to confirm the presence of Orientia tsutsugamushi in each pool at Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh [12]. Phylogenetic analysis was performed using conventional nested-PCR targeting the 56 kDa gene, followed by Sanger sequencing, to identify the specific strain of Orientia tsutsugamushi [13].

Data analysis

Based on the descriptive analysis of cases, we created composite variables by combining variables that reflect similar underlying risk factors for exposure to chigger-mite habitats and rodent-infested environments. We calculated medians and proportions and described the cases by time, place, and person. For the case-control analysis, we compared the socio-demographic characteristics of cases and controls using the chi-squared test and considering a P-value < 0.05 as significant. We conducted a bivariate analysis to assess the association between potential risk factors and scrub typhus. We calculated the crude odds ratio (OR) with 95% confidence intervals (CI), and exposures with a significant crude OR were further analysed using a multivariate model to calculate adjusted ORs with 95% CI.

We calculated entomological indices using the data collected on rodents and chigger-mites. The sequences obtained on phylogenetic analysis were analysed using Basic Local Alignment Search Tool (BLAST), and a phylogenetic tree was constructed using Molecular Evolutionary Genetics Analysis (MEGA11) [14].

Ethical statement

This epidemiological investigation was conducted as part of a public health response to an outbreak to identify risk factors and control the disease. Strict data protection protocols, reviewed by the National Centre for Disease Control (NCDC), were followed when collecting information from cases and controls. Informed consent was obtained before interviewing the cases and controls. All records were de-identified and anonymized before analysis. All statutory permissions were obtained from the NCDC, Central Surveillance Unit, IDSP, New Delhi and IDSP, Sundargarh District, Odisha, according to the state and central government laws for investigation during disease outbreaks (Epidemic Diseases Act no.3, 1897). Rodent and mite samples were collected by trained district entomologists with permission from the District Surveillance Unit (DSU), Sundargarh District, Odisha. The investigation was conducted in accordance with the 1964 Declaration of Helsinki and its later amendments and the International Council for Laboratory Animal Science (ICLAS) guidelines.

Results

Descriptive epidemiology: sociodemographic, clinical, and exposure factors

In Block-A, 151 scrub typhus positive cases were reported in 2023, including four deaths (case fatality rate of 2.6%). The incidence was 5.6 per 10,000 population, which was the highest among all the blocks within Sundargarh district. Cases in Block-A started rising in July 2023, peaked at the end of September 2023, and then started declining in November (Fig. 1). Among 151 scrub typhus cases, the median age was 30 years with an interquartile range (IQR) of 15–46 years, 83 (55%) males, and 83 (55%) residing in a semi-urban area. Here, a semi-urban area is defined as a town or municipality with a population of 10,000 to 99,000 [15]. Paediatric population (< 15 years) and young adults [16–30] each constituted 25% of the total cases.

Fig. 1.

Distribution of scrub typhus cases by week of testing, Block-A, Sundargarh district, Odisha, 2023 (N = 151)

Among the 151 cases reported in Block-A in 2023, we interviewed 61 cases that fit our case definition for descriptive analysis and the case-control investigation. Among the 61 cases, median age was 25 years with an IQR of 12–45 years. Among these cases, 51% were males, 30% belonged to paediatric age group (age < 15 years), and 31 (51%) lived in semi-urban areas. Among the 43 adults, 13 (30%) worked in soil-related environments (farmers, labourers, gardeners, and forest workers). Most common symptoms noted other than fever were headache (64%), cough (39%), and myalgia (34%), with eschar in 8 (13%) cases (Table 1).

Table 1.

Sociodemographic and clinical profile scrub typhus cases, Block-A, 2023 (N = 61)

Socio-demographic factors			n		(%)
Age group
< 15			18		(30)
15–30			16		(26)
31–45			12		(20)
46–60			10		(16)
> 60			05		(08)
Semi-Urban residence			31		(51)
Received some formal education (n = 47)a			43		(91)
Soil related occupationb (n = 43)			13		(30)
Tribal			25		(37)
Symptoms
Fever			61		(100)
Headache			39		(64)
Cough			24		(39)
Myalgia			21		(34)
Coryza			17		(28)
Eschar			08		(13)
Disease progression
Hospital admission			28		(46)
Systemic involvement			12		(20)
ICU admission			06		(13)
Median interval (IQR)
Symptom onset to accessing healthcare			3 (0–13)
Healthcare facility visit to testing			1 (0–16)

^a Cases under 7 years were excluded, and data was not available for a few cases; ^b Including farmers, forest workers, gardeners

Among cases, 46% were hospitalised, 20% developed multiple-organ involvement, and 13% were admitted to the ICU (Table 1). Median interval from symptom onset to accessing healthcare was 3 days, ranging from 0 to 13 days, and 10 (16%) cases visited a health facility in the second week of symptom onset. The median interval from healthcare facility visit to testing was 1 day, ranging from 0 to 16 days and 18 (30%) were tested in the second week of symptom onset (Table 1). Among 43 cases tested for IgG-ELISA, 72% were positive.

All four death cases (median age = 43 years, range = 28–65 years) presented with fever and systemic symptoms (neurological and gastroenteritis), had co-morbidities (diabetes mellitus, hypertension, anaemia, chronic liver disease) and developed multi-organ involvement with complications (diabetic ketoacidosis, seizures, stroke, pneumonia, acute respiratory distress). Interval from symptom-onset to visiting health-facility ranged from 3 to 12 days. Three of the four death cases were referred to a tertiary hospital 90 km away, but none reached the referral centre —one died at home, one died in transit, and one died after admission in a private hospital; and the fourth was admitted to and died in the district hospital ICU. At the district hospital, drugs specific to scrub typhus treatment were available in adequate quantities and limited (five-bedded) ICU facilities were available. Critical patients were referred to a tertiary-care referral centre (90 km away) in the neighbouring district.

Knowledge assessment

We assessed the knowledge of scrub typhus among all available frontline-healthcare workers (n = 151) in Block-A, including community health workers (64%), nurse midwives (31%), and medical officers (5%). The healthcare workers scored an average of 62% on awareness about scrub typhus transmission and clinical presentation, 72% regarding the disease diagnosis and complications, and 55% regarding environmental risk factors and preventive measures.

Case control investigation

The socio-demographic characteristics of the 61 cases and 61 controls were comparable (P values > 0.05) (Table 2). On bivariate analysis, recent (past one month) forest/farm visit [OR = 4.27, 95%CI: 1.9–9.1]; house proximity (< 100 m) to vegetations (bushes, forest and farms) [OR = 3.0, 95%CI: 1.0-9.1]; and open drainage outside the house [OR = 2.23, 95% CI: 1.0-4.7] were significantly associated with illness. On multivariate analysis, after adjusting for age, sex, education, and place of residence, only recent farm/ forest visit was significantly associated [adjusted OR = 6.1, 95% CI: 2.0-18.4] with illness (Table 2).

Table 2.

Sociodemographic and risk factors among cases (N = 61) and controls (N = 61), Block-A, Sundargarh, Odisha, 2023

Socio-demographic factors	Cases	Controls
	n	n	p value
Median age (IQR) in years	25 (12–45)	33 (21–39)	0.47
Female	30	31	0.85
High school and above	34	30	0.87
Soil-related occupationa	14	6	0.08
Kutcha housing	35	30	0.34
Tribe	16	15	0.60
Risk factors (last 1 month)	Odds Ratio (95% CI)
	Cases n (%)	Controls n (%)	OR		aOR b
Sitting in grassy or bushy areac	33 (54)	12 (20)	4.8	(2.1–10.7)	2.8	(1.1–7.5)
Forest/ farm recent (1 month) visit	38 (62)	17 (28)	4.2	(1.9–9.1)	2.1	(0.8–5.4)
House proximity to vegetation (< 100 m)	56 (92)	48 (79)	3.0	(1.0–9.1)	1.5	(0.4–5.3)
Soil-related occupationa	14 (23)	06 (10)	2.7	(1.0–7.6)	2.5	(0.7–8.4)
Dampness of walls	26 (43)	13 (21)	2.7	(1.2–6.0)	1.7	(0.5–5.7)
Firewood storage	30 (49)	17 (28)	2.5	(1.1–5.3)	0.7	(0.2–2.4)
Open drainage	27 (44)	16 (26)	2.2	(1.0–4.7)	2.0	(0.6–6.2)
Domestic animals/ pet	22 (59)	15 (41)	1.7	(0.7–3.7)
Cattle-related activities (cleaning/grazing/milking)	22 (58)	16 (42)	1.5	(0.7–3.4)
Uncemented floors	35 (57)	30 (49)	1.2	(0.7–6.4)
Recent rodent sighting	41 (49)	42 (51)	0.9	(0.4 – 1.9)
Not changing clothes after work	29 (50)	29 (50)	0.9	(0.4–2.0)
Not wearing protective clothing in field [n (cases) = 52, n (controls) = 55]	51 (98)	53 (96)	0.5	(0.0–5.9)

^aIncluding farmers, forest workers, gardeners; ^bAdjusted for age and sex, ^chistory of sitting or playing (children) in grassy/bushy area

Entomological survey

We laid 100 rodent traps in the rural (50) and semi-urban (50) parts of the block around the houses of the reported cases. Total 20 rodents were captured (17 were live), with a trap positivity rate of 20%. Of the 17 live rodents captured, 16 were infested with 330 ectoparasites (mites, ticks, fleas), including 322 chigger-mites (of species Leptotrombidium deliense). Rodent infestation rate was 94% and Chigger index was 18.94, and 33% rodent-sera samples were positive for Scrub Typhus antigen (OXK) (Table 3).

Table 3.

Entomology survey results, Block-A, Sundargarh district, Odisha 2023

Entomological indexa/ Lab parameter	Result	Numerator/ Denominator
Trap positivity ratea	20%	(20/100)
Rodent infestation rateb	94%	(16/17)b
Chigger Indexc	18.9	(322/17)
Rural area Chigger Index	22.3	(223/10)
Semi-Urban area Chigger Index	14.1	(99/7)
Seropositive rodents by WFTe	33%	(5/15)
Mite pools positive (Orientia)	36%	(4/11)
Phylogenetic analysis	Karp-like strain

Entomological indices: ^aTrap positivity rate: (number of rodents collected/ number of traps set)*100; ^brodent infestation rate: (number of rodents infested with ectoparasites/ number of rodents collected)*100; ^cChigger index (number of chigger mites/ number of rodents collected); ^d17 rodents found live, ^eCritical value= 0.69, ^eWeil-Felix test (OXK Ag for Scrub typhus)

Strain identification and phylogenetic analysis

A total of 11 mite-pools were subjected to DNA extraction and PCR testing to confirm the presence of the causative pathogen, out of which four (36%) were positive for Orientia tsutsugamushi (Table 3). Phylogenetic tree was constructed using Neighbour-joining tree based on base-sequence homologies of 56-kDa type-specific genes (Fig. 2). Bootstrap value was set to 1000. The sequences were identified as Karp-like, showing close similarity to sequences from chigger-mites in Chandigarh, and blood and eschar samples from Assam, Kerala, Maharashtra, Karnataka and Pondicherry (Fig. 2). The sequences were submitted to IBDC (Indian Biological Data Centre)/ INSDC (International Nucleotide Sequence Database Collection) and assigned the following accession numbers in European Nucleotide Archive (ENA): OZ116685.1, OZ116632.1, OZ116684.1 [16–18].

Fig. 2.

Phylogenetic tree of isolated O. tsutsugamushi sequences (shown at the top, with accession numbers from European Nucleotide Archive-ENA, marked in red triangle), were identified as Karp-like, with two reference Karp-like sequences and other control sequences shown alongside

Discussion

The scrub typhus case cluster in Block-A in the Sundargarh district was part of a state-wide cluster of scrub typhus cases, with Block-A having the highest incidence among all blocks within the Sundargarh district. More than half the cases were among semi-urban residents, with a quarter of cases among the pediatric and young adult populations each. Visit to a farm or forest (one month prior to symptom-onset) was significantly associated with illness. The entomological survey confirmed the presence of an infected maintenance host and vector, confirming the transmission of scrub typhus in Block-A, Sundargarh district, Odisha.

Age and sex predilection, occupation, symptoms and disease severity of our cases were consistent with reported literature [5, 19, 20]. Contrary to most literature findings reporting a higher incidence in rural areas, over half of our cases resided in semi-urban areas, which have a higher population density and urban-housing structures with surrounding environment and risk factors similar to rural areas [5]. Development and expansion of semi-urban areas has been reported to disrupt existing mite-habitats in rural, forest, and farmland areas, increasing human interaction with chigger-mites and rodents, leading to scrub typhus infection [21–24].

Living in proximity to forests, grassland, fields, and bushes (within 100 m) surrounding the house was a significant risk factor for infection in our investigation, with similar results reported in studies from India and China [25–27]. These environments are known to harbour chigger-mite habitats, increasing the likelihood of transmission [28, 29].

Our investigation identified storage of wooden logs, and open drainage around the house as potential risk factors for scrub typhus, consistent with studies in Darjeeling in India, Bhutan and Vietnam [30–32]. Wooden log storage inside the house has been reported to attract rodents. Open drainage around houses creates damp environments, favouring rodent and chigger-mite proliferation [24]. Although the odds for these environmental factors attracting rodents were significantly higher in our cases, the odds for household rodent presence in cases and controls were similar. This could be due to the uneven distribution of chigger-mites among rodents, influenced by several environmental factors not documented in our investigation.

One-third of our cases had soil-related occupations, which have been consistently identified as risk factors in studies across India, China, Bhutan, and South Korea [25, 27, 31, 33]. However, upon multivariate analysis this association was not statistically significant in our analysis, similar to a study from Vietnam [32]. This could be due to the limited statistical power of our case-control investigation or similar occupation patterns across the semi-urban and rural areas. Also, our questionnaire captured the primary occupation of our cases, while some cases with non-soil-related occupations were also intermittently engaged in soil-related work to supplement their income.

In our investigation, both cases and controls routinely engaged in behaviours that increase the likelihood of transmission, such as walking barefoot, not using protective measures, and not changing clothes after working in fields. However, more specific history of sitting or playing (children) in grass or bushy area was significantly associated with illness among cases, consistent with findings form previous study in Bhutan [31]. A history of recent farm or forest visit (within past month) was also significantly associated with the illness. This suggests that despite certain similar behaviours among cases and controls, proximity to mite habitats (exposure on visiting and sitting) and spatial distribution of chigger-mites in the environment can result in increased transmission.

To the best of our knowledge, this is the first vector survey conducted in this geographical area in northwest Odisha. Chigger-index observed in our survey was above the critical value of 0.69 per rodent, indicating the environment as highly conducive to scrub typhus transmission [34]. Although previous vector surveys in India document presence of scrub typhus vectors, our investigation links these findings to the incident scrub typhus cases within the same local context [8, 35, 36]. Our investigation confirmed local transmission in this geographical area by detecting infection (presence of Orientia tsutsugamushi, i.e., agent) in humans (incidental host), rodents (maintenance host), and chigger-mites (vector/reservoir), completing the epidemiological triad. Karp-like sequences have also been identified among chigger-mites in eastern Odisha, southeastern and southwestern India, Thailand, Taiwan, and China, and are associated with severe clinical outcomes compared to other genotypes, with higher rates of respiratory and cardiovascular complications [37–42]. In our investigation, the chigger-index was higher in rural areas, which is consistent with proximity to forests and farmlands, and the abundant presence of rodents that support chigger-mite proliferation. However, more cases were reported from the semi-urban area, which could be attributable to higher population density, differential health-seeking behaviour due to better access, and population movement between rural and semi-urban areas. Serological analysis in our investigation revealed IgG positivity in majority of tested human samples, this could indicate either high specificity to scrub typhus infection, or secondary infection and high endemicity in the area [43, 44]. This field investigation, conducted as part of outbreak response had certain limitations. Sample size was calculated using prevalence of exposure among cases from the preliminary descriptive analysis, however we enrolled maximum cases feasible within resource and time constraints and ensured adequate power through sensitivity analyses. The entomological survey was limited to villages with case clustering. The rodent sera testing using Weil-Felix (WFT) were conducted as per the national protocol in India, despite the known low specificity and results were interpreted with caution.

Conclusions

Our investigation confirmed the presence of infected vectors, maintenance and incidental hosts, and the causative pathogen, thus completing the epidemiological triad of scrub typhus and establishing the presence of local transmission in Block-A, with people engaging in agricultural work or visiting forests and farms without protective clothing being the most at-risk factor for acquiring infection. We conducted a training session addressing the knowledge gaps on scrub typhus among the healthcare workers.

We recommend community education on wearing full-sleeved clothing and footwear to limit mite exposure while walking through farms or forests during transmission season; improving housing-conditions and rodent-proofing their residences. We recommend community education on seeking timely medical attention and continued sensitisation of healthcare workers to have a high index of suspicion for scrub typhus in febrile cases to reduce delays in treatment-seeking and testing. Identification of Karp-like strain of Orientia tsutsugamushi in chigger-mite pools, reported fatalities, and current scrub typhus surveillance and preparedness indicate the need for strengthening routine scrub typhus surveillance, outbreak preparedness, including capacity to manage severe cases and a well-functioning referral mechanism. We conducted a one-time entomological survey. However, to identify the burden of infected vectors, track their distribution and density over time, and provide early warning signals, we recommend ongoing entomological surveillance in disease hotspots. In conclusion, we recommend adopting a One-Health approach by gathering inputs from both human and vector surveillance to guide public health responses during scrub typhus outbreaks.

Author contributions

AG, DLB, SKV, and SC conceptualised the study, developed the protocol and tools. AG and DLB conducted the field investigation and collected primary data. KCN supported the field investigation and translation of tools into the local language. AG, SKV, SC, and TD performed the data analysis. MB, TK, AM and MR performed the microbiological testing and analysis and drafted the phylogenetic strain testing component of the manuscript. AG created the original manuscript draft. TD is the senior author and guarantor of the scientific and ethical integrity of this manuscript. All authors contributed equally to the review and editing of the manuscript.

Funding

No financial support was received for the research, authorship, and/or publication of this article.

Data availability

Data collected and analysed in this investigation can be available upon formal request to the corresponding author, after approval from the National Centre for Disease Control (NCDC), Delhi, India. The data on phylogenetic strain testing are available on the European Nucleotide Archive (ENA Browser), on the following links:

https://www.ebi.ac.uk/ena/browser/view/OZ116685-OZ116685, https://www.ebi.ac.uk/ena/browser/view/OZ116632-OZ116632, https://www.ebi.ac.uk/ena/browser/view/OZ116684-OZ116684.

Declarations

Ethical approval

This epidemiological investigation was conducted as part of a public health response to an outbreak to identify risk factors and control the disease. Strict data protection protocols, reviewed by the National Centre for Disease Control (NCDC), were followed when collecting information from cases and controls. Informed consent was obtained before interviewing the cases and controls. All records were de-identified and anonymized before analysis. All statutory permissions were obtained from the NCDC, Central Surveillance Unit, IDSP, New Delhi and IDSP, Sundargarh District, Odisha, according to the state and central government laws for investigation during disease outbreaks (Epidemic Diseases Act no.3, 1897). Rodent and mite samples were collected by trained district entomologists with permission from the District Surveillance Unit (DSU), Sundargarh District, Odisha. The investigation was conducted in accordance with the 1964 Declaration of Helsinki and its later amendments and the International Council for Laboratory Animal Science (ICLAS) guidelines.

Competing interests

The authors declare no competing interests.