Seroprevalence of scrub typhus among reported acute febrile illness

INTRODUCTION

Scrub typhus remains a major cause of acute febrile illness across the Asia–Pacific region, posing a substantial public health burden. It is estimated that nearly one billion individuals are at risk globally, with approximately one million cases occurring annually. Delay in diagnosis and treatment may result in serious complications, and the reported case fatality rate can be high in untreated cases.^[1,2] In India, there has been a notable resurgence of scrub typhus in recent years, with increasing reports from various regions.

The disease is caused by Orientia tsutsugamushi, an obligate intracellular Gram-negative organism transmitted to humans through the bite of infected larval trombiculid mites (chiggers). Among the vectors, Leptotrombidium deliense is recognized as an important species involved in transmission, particularly in tropical and subtropical areas.^[3,4] The persistence of the organism in nature is influenced by environmental factors such as temperature, humidity, and vegetation, which support the lifecycle of the vector. In addition, occupational exposure, especially among individuals involved in agriculture and outdoor activities, along with socioeconomic determinants, contributes to disease transmission.^[2,3]

Clinically, scrub typhus presents with a wide spectrum of non-specific manifestations, making early diagnosis difficult. Common symptoms include fever, headache, myalgia, rash, and lymphadenopathy. Although the presence of an eschar is considered characteristic, it is not consistently observed, particularly in Indian patients, thereby limiting its diagnostic utility. The clinical overlap with other endemic febrile illnesses such as malaria, dengue, and leptospirosis often leads to misdiagnosis or delay in initiating appropriate therapy. Hence, laboratory confirmation plays a crucial role in patient management and in preventing complications.^[5]

Several laboratory methods are available for the diagnosis of scrub typhus, including molecular assays and serological tests such as Weil–Felix test, indirect immunofluorescence assay (IFA), enzyme-linked immunosorbent assay (ELISA), and rapid immunochromatographic tests. Although isolation of O. tsutsugamushi in cell culture is possible, it requires specialized biosafety facilities and is not routinely feasible.^[6,7]

The Weil–Felix test, despite being widely used in resource-limited settings, has limited diagnostic accuracy due to low sensitivity and specificity.^[8] The IFA is considered the reference standard; however, its routine use is constrained by the need for specialized infrastructure, variability in antigen preparation, the requirement of paired sera, and subjective interpretation.^[6,9,10] In comparison, ELISA-based assays offer advantages such as ease of performance, objective interpretation, and suitability for large-scale screening.

In India, the Indian Council of Medical Research recommends IgM ELISA with an optical density (OD) cutoff value of 0.5 as a practical and reliable method for the diagnosis of scrub typhus.^[8] Considering the increasing burden of scrub typhus as a cause of acute febrile illness, the present study was undertaken to determine its seroprevalence among clinically suspected cases presenting to a tertiary care hospital in Southern Rajasthan.

MATERIAL AND METHODS

A hospital-based retrospective study was conducted in the Department of Microbiology at the American International Institute of Medical Sciences, Udaipur, over a period of 12 months from January 2025 to December 2025. Patients presenting with clinical features suggestive of scrub typhus and attending either the outpatient or inpatient departments were included in the study.

Serological testing for scrub typhus was performed by (Immunoglobulin M [IgM] Scrub Typhus Microlisa kit, J. Mitra and Co. Pvt. Ltd., New Delhi, India). The assay was carried out according to the manufacturer’s instructions. OD values were measured at a wavelength of 450 nm using an ELISA reader. An OD value >0.5 was considered positive for IgM antibodies against O. tsutsugamushi, as per the kit guidelines. Positive and negative controls were included in each run to ensure quality control and reliability of the results.

Clinical records of patients who tested positive for scrub typhus IgM antibodies were reviewed retrospectively. Relevant information, including demographic details, clinical presentation, laboratory parameters, and patient outcomes, was collected from case records and analyzed.

The study protocol was reviewed and approved by the Institutional Ethics Committee of the American International Institute of Medical Sciences, Udaipur (Approval No.: AIIMSUDR/2026/7588). The study was conducted in accordance with the ethical standards of the World Medical Association Declaration of Helsinki.

RESULTS

Out of 5,783 suspected cases tested for O. tsutsugamushi IgM antibodies, 661 (11.43%; 95% CI: 10.6–12.3) were positive for scrub typhus, O. tsutsugamushi IgM antibodies performed using the IgM Scrub Typhus Microlisa kit (J. Mitra and Co. Pvt. Ltd., New Delhi, India).

A higher proportion of seropositive cases was observed among males (405; 61.27%) compared to females (256; 38.73%), and this difference was statistically significant (χ² = 33.58, P < 0.001). The age of affected patients ranged from 7 to 89 years, indicating involvement across a wide age spectrum, including both pediatric and elderly populations.

A clear seasonal variation in scrub typhus cases was observed, with clustering during the monsoon and post-monsoon months. The highest positivity rate was recorded in August (193 cases; 23.91%), followed by September (128 cases; 14.72%) and October (85 cases; 13.86%). A gradual increase in seropositivity was noted from May (11 cases; 3.79%) to July (40 cases; 8.51%), coinciding with the onset of the monsoon season, followed by a peak in August. Positivity remained relatively high during September and October before declining toward the end of the year [Table 1 and Figure 1].

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Figure 1: Month-wise distribution of scrub typhus IgM ELISA-positive cases. IgM: Immunoglobulin M, ELISA: Enzyme-linked immunosorbent assay.

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The association between month of occurrence and scrub typhus seropositivity was statistically significant (χ² = 234.27, df = 11, p < 0.001), indicating a distinct seasonal trend.

Fever was the most common presenting symptom, observed in 646 patients (97.73%), followed by myalgia in 563 patients (85.17%) and rash in 432 patients (65.36%). Headache was reported in 256 cases (38.73%). The characteristic eschar [Figure 2] was identified in a small proportion of patients (36 cases; 5.45%). Lymphadenopathy was noted in 189 patients (28.59%), while gastrointestinal symptoms such as nausea, vomiting, or abdominal pain were present in 115 patients (17.40%) [Table 2].

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Figure 2: Arrow indicating characteristic eschar with central necrosis in scrub typhus.

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Laboratory findings showed thrombocytopenia in 453 patients (68.53%). Leukocytosis (total leukocyte count >11,000/mm³) was observed in 245 patients (37.06%). Elevated liver enzymes (aspartate aminotransferase and/or alanine aminotransferase) were seen in 467 cases (70.65%), while raised C-reactive protein levels were documented in 411 patients (62.18%).

The majority of patients were treated with doxycycline and showed clinical improvement before discharge.

DISCUSSION

Scrub typhus has re-emerged as an important cause of acute undifferentiated febrile illness in India and across the Asia–Pacific region. Its non-specific clinical presentation and limited availability of confirmatory diagnostic facilities often make early diagnosis challenging. In such settings, laboratory-based serological testing plays a crucial role in the timely identification and management of the disease.

In the present study, the seroprevalence of scrub typhus was 11.43% over a 12-month period. This finding is comparable to that reported by Thakur et al.,^[11] who documented a prevalence of 12%. In contrast, Gurung et al.^[12] reported a higher prevalence of 30.8% among patients with fever of unknown origin. The variation in prevalence across studies may be attributed to differences in study population, geographic distribution, seasonal factors, and diagnostic methods used.

A male predominance was observed in the present study, with males accounting for 61.27% of cases compared to 38.73% females. Similar findings have been reported by Gurung et al.,^[12] who also noted a higher proportion of affected males. This trend may be explained by increased occupational exposure of males to outdoor environments, particularly agricultural fields, which enhances contact with mite-infested vegetation.

A distinct seasonal trend was evident, with the majority of cases occurring during the monsoon and post-monsoon months, particularly from August to October. Comparable seasonal patterns have been described by Kaur et al.^[4] and Lakshmi et al.,^[1] who also reported a peak incidence during or following the monsoon season. This clustering is likely related to environmental conditions such as increased humidity, dense vegetation, and favorable temperature, which promote the proliferation and activity of vector mites.^[13]

Fever was the most common clinical manifestation in the present study, observed in 97.73% of patients. Similar observations have been reported by Mishra et al.^[14] and Singh et al.,^[3] who documented fever in nearly all cases, while Vanlalruati et al.^[15] reported a slightly lower frequency. Although fever is a predominant feature, its absence in a minority of patients may be attributed to early presentation, prior antibiotic use, or host-related factors.

Other frequently observed symptoms included myalgia, headache, and gastrointestinal manifestations such as nausea, vomiting, and abdominal pain.^[16] These findings are consistent with those reported by Sivakumar et al.^[17] The variability in clinical features across studies, as shown in Table 3, highlights the heterogeneous nature of scrub typhus and its overlap with other febrile illnesses.

The prevalence of eschar in the present study was low (5.4%), which is consistent with findings from other Indian studies. The detection of eschar is known to vary geographically and is often less frequently identified in darker-skinned individuals, including South Asian populations, compared to Caucasian or East Asian patients.^[1,16]

Laboratory abnormalities such as thrombocytopenia (68.53%), elevated liver enzymes (70.65%), and raised C-reactive protein levels (62.18%) were commonly observed, indicating systemic inflammation and hepatic involvement.^[18] Similar laboratory profiles have been described in other studies, supporting their role as useful adjuncts in the diagnosis of scrub typhus in patients with acute febrile illness.

Overall, the wide variability in clinical presentation underscores the diagnostic challenge posed by scrub typhus. The presence of nonspecific symptoms often delays clinical suspicion. Therefore, a high index of suspicion, particularly during the monsoon and post-monsoon seasons, along with timely serological testing, is essential for early diagnosis and prompt initiation of appropriate therapy in endemic regions.

This study provides region-specific data on the seroprevalence and seasonal trends of scrub typhus in Southern Rajasthan. It highlights the diagnostic utility of IgM ELISA in resource-limited settings and emphasizes the importance of early clinical suspicion during monsoon months to improve patient outcomes.

CONCLUSION

Scrub typhus is a significant cause of acute febrile illness in Southern Rajasthan, showing marked seasonal clustering during the monsoon months. IgM ELISA serves as an effective and feasible diagnostic modality, and associated laboratory abnormalities such as thrombocytopenia and transaminitis may aid in early clinical suspicion. Strengthening laboratory-based diagnostic capacity can facilitate timely diagnosis, prompt treatment, and improved patient outcomes. Early recognition and timely treatment can significantly reduce disease-related morbidity and mortality in endemic regions.