Scrub typhus is a rickettsial disease caused by Orientia tsutsugamushi/Orientia chuto. It is a major public health concern in the Asia-Pacific region.1 Among the various clinical manifestations, eschar is a typical cutaneous feature that results from the bites of infected chigger.23 It serves as a pathognomonic marker for the diagnosis of scrub typhus owing to its early appearance during the disease course.345 Additionally, the absence of eschar has been linked to poor outcomes under these conditions.6 The prevalence of eschars varies by region, being the highest in East Asia (78.7%) and lowest in South Asia (32.8%).2478 Dark skin color potentially contributes to the low detection rate in South Asia. In Korea, the detection rate of eschar is approximately 70–80%, with the most common location being the anterior body (including the armpit) (46.2%), followed by the lower extremities (19.0%), buttocks (13.4%), and back (8.8%).1 Eschar distribution may be influenced by chigger behavior, host characteristics, and regional genotypic differences in O. tsutsugamushi.

Eschar is typically described as a black necrotic crust that covers a wound. However, the clinical appearance of eschar varies considerably in morphology depending on its anatomical location and disease progression, potentially complicating diagnoses.2478 This variability can lead to underrecognition or misinterpretation of the lesion, particularly in non-endemic regions or among healthcare providers unfamiliar with its diverse presentations.4

Despite their clinical importance, the morphological characteristics of eschars have not yet been systematically evaluated. In this study, we aimed to categorize eschar types and analyze their distribution to improve the early recognition and diagnosis of scrub typhus.

This retrospective study was conducted at Jeonbuk National University Hospital, and confirmed cases of scrub typhus reported between January 2018 and December 2020 were reviewed.

The diagnosis of scrub typhus was established based on one of the following criteria: 1) an indirect immunofluorescence assay (IFA) total antibody (IgM/G) titer ≥ 1:80 against O. tsutsugamushi, 2) a ≥ 4-fold increase in IFA titer in paired sera, or 3) a positive nested PCR result targeting the 56-kDa type-specific antigen gene of O. tsutsugamushi.9 Dermoscopic examinations were performed using a polarized handheld dermatoscope (DermLite DL3 and DL4; 3Gen LLC, San Juan Capistrano, CA, USA) at 10× magnification. Eschars were categorized into 5 distinct morphological types: erythematous macules, vesicular, erosive/ulcerative, crusted, and sclerotic. The body was divided into six anatomical regions: head/neck, anterior, back, upper extremities, lower extremities, and the inguinal area.3 The head/neck region included the head, face, and neck, and extends down to the clavicle. The anterior region included the front of the upper body, including the axilla and flank. The back encompassed the posterior upper body, waist, and lumbar regions. The upper extremities included the shoulders, whereas the lower extremities refer to the areas below the pelvis. The inguinal region was defined as the area extending from below the umbilicus to above the pelvis, including the anogenital region and the buttocks.

Statistical analyses were conducted using χ² or Fisher’s exact tests (SPSS version 29.0; IBM Corp., Armonk, NY, USA). Statistical significance was set at P < 0.05.

Among the 88 confirmed cases of scrub typhus, 81 had eschar information. The mean age of the patients was 64.6 ± 14.7 years, and 50 (61.7%) were women.

The eschar types were classified as follows: erythematous macules (n = 2; 2.5%), vesicular (n = 4; 4.9%), erosive/ulcerative (n = 11; 13.6%), crusted (n = 60; 74.1%), and sclerotic (n = 4; 4.9) (Fig. 1). The most common eschar sites were the anterior region (44.4%), inguinal area (18.5%), upper extremities (12.3%), lower extremities (11.1%), head/neck (9.9%), and back (3.7%) (Fig. 2, Table 1). No statistically significant difference was found in eschar type according to anatomical site. However, when grouped into exposed (head/neck, upper/lower extremities) and covered (anterior torso, back, and inguinal) areas, eschar type distribution differed significantly (P = 0.038). The covered areas had a higher prevalence of erythematous macules (3.7% vs. 0.0%), vesicular lesions (5.6% vs. 3.7%), and erosive/ulcerative lesions (16.7% vs. 7.4%) but no sclerotic eschars (0.0% vs. 14.8%).

Notably, 21 (25.9%) patients presented with atypical eschars lacking a black crust (types 1, 2, 3, and 5). These were most commonly located in the anterior region (n = 9; 42.9%), followed by the head/neck (n = 4; 19.0%), upper extremities (n = 3; 14.3%), lower extremities (n = 2; 9.5%), and back (n = 1; 4.8%). Each atypical eschar type showed a preference for specific body sites (e.g., sclerosis in the head/neck, erosion or ulceration in the inguinal region, erythematous macules in the anterior region, and vesicular lesions in the extremities). No significant differences were found between typical (crusted) and non-typical eschars based on sex, age (≥ 60 vs. < 60 years), or body site.

The proper detection of eschars in scrub typhus is crucial for patient outcomes because delayed recognition can lead to inappropriate treatment and severe complications. In multivariate analysis, the absence of eschar was identified as an independent prognostic factor for fatal outcomes, likely due to delayed diagnosis and treatment.6 Therefore, increased clinical awareness and improved diagnostic strategies based on a comprehensive understanding of eschar characteristics are essential for timely intervention and optimal prognosis.

This cross-sectional study highlighted the diverse morphological presentations and widespread distribution of eschars, underscoring the complexity of clinical diagnoses. Consistent with previous studies in Korea, the anterior body (chest, abdomen, and flank) was the most common eschar site. Notably, 66.7% of the eschars were found in the covered areas (anterior torso, axilla, back, and inguinal region), emphasizing the need for a thorough head-to-toe skin examination. Additionally, 25.9% of the cases presented with atypical eschars lacking black crust, raising concerns about potential diagnostic errors if clinicians rely solely on classical presentations. Given the importance of early and accurate diagnosis for effective disease management, the recognition of these atypical forms of eschar is critical.10

Our findings also suggested an association between eschar morphology and body site, particularly between covered and exposed areas. Erythematous, vesicular, and erosive/ulcerative eschars were more prevalent in the covered regions, whereas sclerotic eschars were exclusively found in the exposed areas, particularly in the head and neck. This site-specific variation may have been influenced by the physiological characteristics of different skin regions or the timing of eschar detection. The predominance of erosive/ulcerative eschars in intertriginous areas such as the axilla and groin may be due to increased humidity and friction, which could delay recognition and lead to misdiagnosis. Eschars usually evolve over time, progressing from an initial macule or papule at the chigger bite site to vesicular, ulcerative, or crusted lesions, eventually healing with a sclerotic patch.11 While typical crusted eschars are easily identifiable, early stage lesions (erythematous macules) and late-stage sclerotic eschars may be overlooked, particularly in unusual sites, such as the head and neck, with lower clinical suspicion. However, the factors influencing the eschar morphology remain unclear. Previous exposure to O. tsutsugamushi strains or the use of antibiotics has been associated with the size and morphology of the eschar.4101213141516 Further studies are required to elucidate whether various factors, including skin type, bite location, causative species, host immune response, and treatment influence the morphology and distribution of eschars in scrub typhus.

This retrospective study, conducted in a single institution located in the southwestern region of Korea, has inherent limitations. First, our findings may not be generalizable to other populations. Second, this study focused primarily on eschar morphology without evaluating its causal association with host immunity, treatment, and outcomes. Third, the small sample size and diverse eschar presentations restricted statistical analysis.

Despite these limitations, this study systematically classified eschar morphologies in scrub typhus to improve the early diagnostic accuracy and highlight the challenges posed by atypical forms. Given the scarcity of large-scale studies focusing on eschar characteristics, this study provides valuable insights into clinical practice by enhancing diagnostic accuracy. Future multicenter studies with larger cohorts are required to validate these findings and further explore the clinical significance of eschar characteristics in scrub typhus.