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Full Research Article

Infestation and Distribution of Ascoschoengastia indica (a potential vector of scrub typhus) on Small Mammals in Yunnan Province, southwest China (2001-2022)

H
Hai-Bo Wang1
Y
Yan Lv1
P
Peng-Wu Yin1
W
Wen-Yu Song1
P
Pei-Ying Peng2
T
Ti-Jun Qian1
X
Xian-Guo Guo1,*
1Institute of Pathogens and Vectors, Key Laboratory of Ectoparasite Systematics and Evolution of Yunnan Provincial Education Department, Yunnan Provincial Key Laboratory for Zoonosis Control and Prevention, Dali University, Dali, Yunnan, 671000, China.
2Institute of Microbiology, Qujing Health Medical College, Qujing, Yunnan, 655100, China.

ABSTRACT

Background: This study aimed to reveal the infestation and distribution patterns of the chigger mite Ascoschoengastia indica (a potential vector of scrub typhus) on small mammals in Yunnan Province of southwest China, a focus of scrub typhus.

Methods: Using field survey data from 34 investigation sites in Yunnan Province between 2001 and 2022, a retrospective analysis was conducted. Infestation indices, including constituent ratio (Cr), prevalence (PM), mean abundance (MA) and mean intensity (MI), were calculated. Levins’ index (B'i), Pianka’s index (Oij) and the association coefficient (V) were utilized to measure host niche breadth, host niche overlap and interspecific relationships.

Result: A total of 4,848 A. indica were collected from 165 small mammals across 13 of the 34 sites. Overall infestation indices were PM = 3.17%, MA = 0.93 and MI = 29.38. The mites were predominantly concentrated in southern Yunnan, with Rattus tanezumi being the principal host (Cr = 92.49%). The abundance of A. indica exhibited biases associated with host sex and age and varied in different geographical landscapes, habitats and altitude gradients, with a distinct preference for low-altitudes. Ascoschoengastia indica had a substantial geographical and host overlap with another vector, Leptotrombidium deliense.

INTRODUCTION

Chigger mites serve as transmission vectors for scrub typhus and certain species can also transmit other zoonotic diseases, such as hemorrhagic fever with renal syndrome (HFRS) (Phasomkusolsil et al., 2009; Yu and Tesh, 2014). Chigger mites are highly diverse, with over 3,000 known species worldwide and more than 500 species recorded in China (Nielsen et al., 2021; Zhou and Ren, 2023). Despite this high diversity, not all species can effectively transmit the causative agent of scrub typhus, Orientia tsutsugamushi (Ot). Among the species reported from China, six are recognized as primary vectors for scrub typhus, while over ten others, including Ascoschoengastia indica (Hirst, 1915), are regarded as potential vectors (Moniuszko et al., 2022; Zhou and Ren, 2023). Yunnan Province in southwest China, is an important endemic area for scrub typhus in the country (Li et al., 2024; Qin et al., 2025). Furthermore, Yunnan is a renowned tourist destination bordering Vietnam, Myanmar and Laos. With the continuous expansion of tourism and border trade, frequent population mobility-including cross-border movement-has exacerbated the epidemic spread and transboundary transmission of scrub typhus. Scrub typhus has shown a marked upward trend in Yunnan in recent years, marking it as the province with the fastest-growing incidence rate in China. The reported incidence increased from 0.65 cases per 100,000 population in 2006 to 23.94 per 100,000 in 2022, with the primary epidemic zones progressively shifting toward southern and western Yunnan (Li et al., 2024). Ascoschoengastia indica (Hirst, 1915) is an independent species of chigger mite, belonging to the genus Ascoschoengastia within the family Trombiculidae (Hirst, 1915; Wharton, 1946) and it is considered one of potential vectors of scrub typhus, for O. tsutsugamushi (Ot) was once detected from the mite (Binh et al., 2020). Although A. indica is one of important chigger species with medical importance specific studies focusing on this chigger mite remain scarce. Using statistical and ecological methods, this study conducted a retrospective analysis of field survey data collected from 34 investigation sites in Yunnan Province between 2001 and 2022. The objective is to reveal the distribution and host selection patterns of A. indica, thereby enriching the understanding of this chigger mite species and providing scientific information for the prediction, early warning and prevention and control of the mites in Yunnan Province.

MATERIALS AND METHODS

Field surveys and chigger mite identification
 
The raw data originated from field surveys conducted at 34 sites throughout Yunnan Province between 2001 and 2022. Sampling investigations were carried out in the indoor and outdoor habitats of each survey site. The indoor habitats covered human dwellings, stables, barns and their surrounding areas. The outdoor habitats included farmlands, shrublands and adjacent woodlands. Host animals (rodents and other sympatric small mammals) were live-trapped with cage traps (18 × 12 × 9 cm; Guixi Mousetrap Apparatus Factory, Guixi, Jiangxi, China) baited with peanuts, corn, or other alternative food baits. The cage traps (mousetraps) were set in the afternoon or evening and inspected the following morning. The placement of cage traps was flexibly adjusted according to habitats. In open habitats, 25 cage traps were set for each group in a straight line, with adjacent traps spaced 5 meters apart and 20 meters between two lines. In paddy fields, the traps were placed along the bank of the fields. In the indoor habitats, the traps were placed along the base of walls with approximately one trap per 15 m2. To ensure the comparability of sampling among different survey sites, approximately 200 traps were deployed at each site per day for 15 consecutive days, resulting in a total of about 3,000 traps deployed at each survey site (Liu et al., 2025). Chigger mites were routinely collected from the body surface of each host and subsequently preserved in 70% or 75% ethanol (Liu et al., 2025; Lv et al., 2025a). After being rinsed in distilled water or clean water, each collected chiggers was mounted onto a glass slides with Hoyer’s medium (Jacinavicius et al., 2013). After dehydration, clearing and drying, each mounted specimen was examined under a microscope (Olympus Corporation, Tokyo, Japan) and identified to species according to relevant taxonomic literature and identification keys (Huang et al., 1995; Li et al., 1997; Wilson et al., 2017; Nielsen et al., 2021). Following the completion of taxonomic identification for all collected chigger mites, A. indica was selected as the target species for this study.
 
Statistical analysis
 
Using statistical and ecological methods, conventional infestation indices for A. indica were calculated, including the constituent ratio (Cr), prevalence (PM), mean abundance (MA) and mean intensity (MI) (Mandla et al., 2021; Liu et al., 2025). Levins’ index (B'i) was employed to measure the host niche breadth of the mites, while Pianka’s index (Oij) was utilized to determine the host niche overlap between A. indica and other dominant chigger species in the same region (Lv et al., 2025b). Furthermore,  interspecific associations of A. indica with other dominant sympatric chigger mites were assessed using the association coefficient (V) (Liu et al., 2025). Analyses were conducted with R 4.5.3 software using the “stats” and “rstatix” packages. The chi-square test was used to test the statistical significance of prevalence (PM). The statistical significance of mean abundance (MA) and mean intensity (MI) was tested using Mann-Whitney U or Kruskal-Wallis H tests. For multiple comparisons, P values were corrected with Benjamini–Hochberg adjustment. Results with P < 0.05 were considered statistically significant. The relevant results were visualized using Sankey diagrams, rose diagrams, bubble charts and boxplots. All the chigger identification and statistical analyses were completed at Dali University, Dali, Yunnan, China. The following equations are the relevant formulas for calculating the constituent ratio (Cr), prevalence (PM), mean abundance (MA), mean intensity (MI), Levins’ index (B'i), Pianka’s index (Oij) and association coefficient (V) (Mandla et al., 2021; Liu et al., 2025; Lv et al., 2025b).













 
Ni = Number of A. indica collected.
N = Total number of all chigger species identified.
Hm = Number of hosts harboring A. indica.
H = Total hosts examined.
Bi'= Levins’ index for reflecting the host niche breadth of chigger species i.
Oij = Pianka’s index for reflecting the host niche overlap between chigger species i and j.
S = Number of host species (host resource categories);
Pih and Pjh = The respective proportions of chigger species i and j collected from host species h.
V = The association coefficient between A. indica and other sympatric dominant species of chigger mite (L. deliense in the present study).
a = The number of hosts harboring both species of L. deliense and A. indica.
b = The number of hosts harboring L. deliense only.
c = The number of hosts harboring A. indica only.
d = The number of hosts harboring neither species.
n = The total number of hosts examined.

RESULTS AND DISCUSSION

Distribution and overall infestation of A. indica
 
From 13 positive survey sites, 5,197 hosts (rodents and other small mammals) were captured and examined. A total of 4,848 A. indica were identified from 165 hosts at 13 survey sites. The majority of A. indica were collected from Jinghong in southern Yunnan (the constituent ratio Cr = 71.39%, 3461/4848), followed by Yuanjiang (Cr = 22.90%, 1110/4848), with these two locations accounting for a combined Cr of 94.29% (Fig 1). The 4,848 A. indica identified across the 13 locations (survey sites) were distributed among 10 host species from 5 genera, 4 families and 2 orders (Rodentia and Scandentia). The vast majority of these chigger mites were found on the rat Rattus tanezumi of the order Rodentia (Cr = 92.49%, 4484/4848). The constituent ratios (Cr) of A. indica across different host taxonomic levels were visualized in Fig 2. The overall prevalence (PM) of A. indica across all host animals was 3.17% (165/5197), with a mean abundance (MA) of 0.93 mites/host and a mean intensity (MI) of 29.38 mites/host. On its primary host, R. tanezumi, the specific infestation indices were PM = 5.97%, MA = 2.11 mites/host and MI = 35.31 mites/host.

Fig 1: Map of Yunnan Province, southwest China and the distribution of the 34 survey sites (2001-2022).



Fig 2: The Sankey diagram visualization of the quantitative distribution of A. indica across different hosts taxonomic levels in Yunnan Province, southwest China (2001-2022).


       
The results indicate that A. indica is predominantly distributed in southern Yunnan, accounting for a high constituent ratio (Cr) of 94.29% (Fig 1), which indicates that A. indica has a preference for specific geographical areas. Although A. indica was found on 10 host species, the majority of the mites (Cr = 92.49%) were found on the rat R. tanezumi (Fig 2). The result indicates that A. indica also has a preference for specific hosts. Besides Yunnan Province, A. indica has also been recorded in Vietnam, Myanmar and Laos, all of which border southern Yunnan (Nadchatram et al., 1980; Binh et al., 2020; Elliott et al., 2022). Ascoschoengastia indica is also distributed in Thailand with much higher prevalence than in Yunnan (Chaisiri et al., 2019). Situated in low-latitude zones, the warm and moist climate in southern Yunnan is quite similar to that in its neighboring countries, Vietnam, Myanmar, Laos and Thailand. The warm and moist climate is beneficial to the survival, development and reproduction of A. indica (Zhou and Ren, 2023; Li et al., 2024), which may lead to its distribution in tropical and subtropical regions. At the 13 positive survey sites where A. indica was found, a total of 2,126 R. tanezumi were captured, which provide abundant host sources for A. indica.
 
Distribution and infestation of A. indica in different geographical and ecological environments
 
Within the 13 locations where A. indica was collected, a comparative analysis was conducted on its distribution and infestation status on host animals across different geographical landscapes, habitats and altitudinal gradients. The results showed that the prevalence (PM = 3.75%) of A. indica in the flatland areas exceeded that recorded in the mountainous areas (PM = 2.73%, P < 0.05). However, both the mean intensity (MI = 41.38) and mean abundance (MA = 1.13) of A. indica in the mountainous areas exceeded the corresponding values in flatland areas (MI = 18.09, MA = 0.68, P < 0.05) (Table 1). The host animal composition varied across different habitats. In the residential habitats, the host composition was simple, with A. indica detected only on R. tanezumi and R. norvegicus, with R. tanezumi being the predominant host. In the agricultural and non-agricultural habitats, the host composition was more complex, but the highest abundance of A. indica was still collected from R. tanezumi. Across all three habitats, the primary host of A. indica was consistently R. tanezumi, which harbored the highest abundance of A. indica mites (Fig 3A and C). Among the five different altitudinal gradients, A. indica was primarily concentrated in the two low-altitude gradients of “< 500 m” and “500-1000 m”. The primary host animal remained to be R. tanezumi and the abundance of A. indica was highest at these low-altitude gradients (Fig 3B and D).

Table 1: Infestation indices of Ascoschoengastia indica in different geographical landscapes and on different sexes and ages of hosts in Yunnan Province, southwest China (2001-2022).



Fig 3: Comparison of the distribution of A. indica across different altitudes and habitats in Yunnan Province, southwest China (2001-2022).


       
The distribution and infestation of A. indica varied in different geographical and ecological environments, which reflects the environmental heterogeneity of chigger mite infestation. Prevalence was higher in flatland areas, whereas mean abundance and mean intensity were higher in mountainous areas. These differences suggest roles for host composition, opportunities for contact with chiggers and local microhabitat conditions (Chaisiri et al., 2019; Wu et al., 2021).The predominance of R. tanezumi across habitats is consistent with its broad habitat use and availability as a host (Wu et al., 2021). The concentration of A. indica at lower elevations may reflect the combined effects of temperature, vegetation and primary host distribution (Elliott et al., 2019).
 
Effects of host sex and age on A. indica infestation
 
According to the survey results, a total of 165 host animals belonging to 10 species were found to be infested by A. indica. Regarding host sexes, the mean intensity (MI = 38.28) and mean abundance (MA = 1.17) of A. indica on female hosts exceeded the corresponding values on male hosts (MI = 21.21, MA = 0.71). However, the prevalence (PM = 3.05%) of A. indica on female hosts was slightly lower than that on male ones (PM = 3.34%). The differences of all these infestation indices between different host sexes were not statistically significant (P > 0.05). With respect to host ages, the mean intensity (MI = 66.83) and mean abundance (MA = 1.14) of A. indica on juvenile hosts were higher than the corresponding values recorded for adult hosts (MI = 23.01, MA = 0.86), with the difference of mean abundance between the two age groups being statistically significant (P < 0.01). Conversely, the overall prevalence of A. indica on adult hosts (PM = 3.74%) was significantly higher than that on juvenile hosts (PM = 1.71%) and this difference was also statistically significant (P < 0.01) (Table 1).
       
Previous studies have proposed differences in host behavior, hormone levels and immunity as explanations for sex-related variation in ectoparasite infestation, alongside effects of parasite traits, host species and environmental conditions (Debnath et al., 2020; Jayalakshmi et al., 2023; Lv et al., 2025a). In the present study, the infestation indices (PM, MA, MI) of A. indica varied on different sexes and ages of hosts, which may reflect the sex and age biases of hosts in chigger infestation (Liu et al., 2025; Lv et al., 2025a). The MI and MA of A. indica on female hosts exceeded those on male hosts, but female hosts showed a lower PM than male ones. The variation pattern of PM was inconsistent with that of MI and MA between different sexes of hosts. Since the differences of these indices between male and female hosts were of no statistical significance (P > 0.05), it cannot be determined from the available results which sex of hosts is more susceptible to the chigger infestation and further research is still required. Regarding host ages, adult hosts showed higher PM than juvenile hosts (P < 0.01), but juvenile hosts had higher MA than adult ones (P < 0.01). From the present results, it is also difficult to explain the inconsistent variation patterns of PM and MA between different ages of hosts and more research still remains to be made in the future.
 
Comparative analysis of A. indica and L. deliense
 
Across the 13 sampling sites where A. indica was collected, a high abundance of L. deliense (5,352 individuals) was found alongside a large number of A. indica (4,848 individuals). Both of these vector chigger mites primarily parasitized R. tanezumi. At these 13 locations, a total of 2,126 R. tanezumi were captured, from whose body surfaces 16,209 chigger mites were obtained. Ascoschoengastia indica was the most abundant species (Cr = 27.66%, 4,484/16,209 individuals), closely followed by L. deliense (Cr = 27.13%, 4,398/16,209 individuals). Regarding host selection, the niche breadth of L. deliense (B'i= 1.47) was significantly broader than that of A. indica (B'i= 1.17). The niche overlap index between A. indica and L. deliense was exceptionally high, approaching 1 (Oij = 0.99), with a certain degree of positive association (V = 0.11, V > 0) in their host selection.
       
From the 13 positive survey sites where A. indica was collected, a lot of L. deliense was also found. Leptotrombidium deliense is the major vector of scrub typhus in China (Zhou and Ren, 2023). The coexistence of abundant A. indica and L. deliense at the 13 sites may increase the potential risk of the transmission and focus persistence of scrub typhus in the survey areas. Ascoschoengastia indica and L. deliense are two independent chigger species in different genera. Although these two chigger species had a very high niche overlap (Oij = 0.99) with the same principal host (R. tanezumi) in host selection, L. deliense showed higher niche breadth (B'i) than A. indica. A high niche breadth (B'i) indicates a broad host range and low host specificity and vice versa (Lv et al., 2025b). The lower niche breadth (B'i) of A. indica indicates that A. indica has a narrower range of hosts with higher host specificity than L. deliense though they have similar host selection for their principal host, R. tanezumi. The low host specificity of chigger mites is beneficial to their transmitting pathogens among different hosts (Lv et al., 2025b). The lower niche breadth (B'i) of A. indica further suggests that A. indica has less potential risk of transmitting O. tsutsugamushi (the pathogen of scrub typhus) than L. deliense from ecological perspective.
       
The present study retrospectively reports the infestation and distribution of A. indica on small mammals in Yunnan Province of southwest China for the first time. The results of the study enrich the understanding of this chigger mite species and provide scientific information for the prevention and control of the mites in Yunnan Province. The present study, however, only focuses on the infestation and distribution patterns of A. indica without the detection of O. tsutsugamushi and other relevant pathogens. From the present results, we still cannot determine the actual role of A. indica in the transmission and focus persistence of scrub typhus in the survey areas and more research (including the molecular detection of relevant pathogens) remains to be done in the future.

CONCLUSION

Ascoschoengastia indica is primarily distributed in the low-altitude regions of southern Yunnan, with R. tanezumi being its principal host. Its infestation exhibits pronounced environmental heterogeneity with the sex and age biases of hosts. Its co-existence with abundant L. deliense may increase the potential risk of the transmission and focus persistence of scrub typhus in the survey areas.

ACKNOWLEDGEMENT

We sincerely thank Cheng-Fu Zhao, Rong Fan, Zhi-Wei Zhang, Ya-Fei Zhao, some other colleagues and college students for their contributions to field investigations and laboratory work. This study was financially supported by the Prevention and Control of Emerging and Major Infectious Diseases-National Science and Technology Major Project (No. 2025ZD01900106) and the Research and Development Fund of Dali University (Nos. KY2319101340, KY2519103340).
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.
 
Informed consent
 
The capture and use of animals were approved by the local wildlife affairs authority and the Ethics Committee of Dali University.

CONFLICT OF INTEREST

The authors declare no conflicts of interest.

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    Full Research Article

    Infestation and Distribution of Ascoschoengastia indica (a potential vector of scrub typhus) on Small Mammals in Yunnan Province, southwest China (2001-2022)

    H
    Hai-Bo Wang1
    Y
    Yan Lv1
    P
    Peng-Wu Yin1
    W
    Wen-Yu Song1
    P
    Pei-Ying Peng2
    T
    Ti-Jun Qian1
    X
    Xian-Guo Guo1,*
    1Institute of Pathogens and Vectors, Key Laboratory of Ectoparasite Systematics and Evolution of Yunnan Provincial Education Department, Yunnan Provincial Key Laboratory for Zoonosis Control and Prevention, Dali University, Dali, Yunnan, 671000, China.
    2Institute of Microbiology, Qujing Health Medical College, Qujing, Yunnan, 655100, China.

    ABSTRACT

    Background: This study aimed to reveal the infestation and distribution patterns of the chigger mite Ascoschoengastia indica (a potential vector of scrub typhus) on small mammals in Yunnan Province of southwest China, a focus of scrub typhus.

    Methods: Using field survey data from 34 investigation sites in Yunnan Province between 2001 and 2022, a retrospective analysis was conducted. Infestation indices, including constituent ratio (Cr), prevalence (PM), mean abundance (MA) and mean intensity (MI), were calculated. Levins’ index (B'i), Pianka’s index (Oij) and the association coefficient (V) were utilized to measure host niche breadth, host niche overlap and interspecific relationships.

    Result: A total of 4,848 A. indica were collected from 165 small mammals across 13 of the 34 sites. Overall infestation indices were PM = 3.17%, MA = 0.93 and MI = 29.38. The mites were predominantly concentrated in southern Yunnan, with Rattus tanezumi being the principal host (Cr = 92.49%). The abundance of A. indica exhibited biases associated with host sex and age and varied in different geographical landscapes, habitats and altitude gradients, with a distinct preference for low-altitudes. Ascoschoengastia indica had a substantial geographical and host overlap with another vector, Leptotrombidium deliense.

    INTRODUCTION

    Chigger mites serve as transmission vectors for scrub typhus and certain species can also transmit other zoonotic diseases, such as hemorrhagic fever with renal syndrome (HFRS) (Phasomkusolsil et al., 2009; Yu and Tesh, 2014). Chigger mites are highly diverse, with over 3,000 known species worldwide and more than 500 species recorded in China (Nielsen et al., 2021; Zhou and Ren, 2023). Despite this high diversity, not all species can effectively transmit the causative agent of scrub typhus, Orientia tsutsugamushi (Ot). Among the species reported from China, six are recognized as primary vectors for scrub typhus, while over ten others, including Ascoschoengastia indica (Hirst, 1915), are regarded as potential vectors (Moniuszko et al., 2022; Zhou and Ren, 2023). Yunnan Province in southwest China, is an important endemic area for scrub typhus in the country (Li et al., 2024; Qin et al., 2025). Furthermore, Yunnan is a renowned tourist destination bordering Vietnam, Myanmar and Laos. With the continuous expansion of tourism and border trade, frequent population mobility-including cross-border movement-has exacerbated the epidemic spread and transboundary transmission of scrub typhus. Scrub typhus has shown a marked upward trend in Yunnan in recent years, marking it as the province with the fastest-growing incidence rate in China. The reported incidence increased from 0.65 cases per 100,000 population in 2006 to 23.94 per 100,000 in 2022, with the primary epidemic zones progressively shifting toward southern and western Yunnan (Li et al., 2024). Ascoschoengastia indica (Hirst, 1915) is an independent species of chigger mite, belonging to the genus Ascoschoengastia within the family Trombiculidae (Hirst, 1915; Wharton, 1946) and it is considered one of potential vectors of scrub typhus, for O. tsutsugamushi (Ot) was once detected from the mite (Binh et al., 2020). Although A. indica is one of important chigger species with medical importance specific studies focusing on this chigger mite remain scarce. Using statistical and ecological methods, this study conducted a retrospective analysis of field survey data collected from 34 investigation sites in Yunnan Province between 2001 and 2022. The objective is to reveal the distribution and host selection patterns of A. indica, thereby enriching the understanding of this chigger mite species and providing scientific information for the prediction, early warning and prevention and control of the mites in Yunnan Province.

    MATERIALS AND METHODS

    Field surveys and chigger mite identification
     
    The raw data originated from field surveys conducted at 34 sites throughout Yunnan Province between 2001 and 2022. Sampling investigations were carried out in the indoor and outdoor habitats of each survey site. The indoor habitats covered human dwellings, stables, barns and their surrounding areas. The outdoor habitats included farmlands, shrublands and adjacent woodlands. Host animals (rodents and other sympatric small mammals) were live-trapped with cage traps (18 × 12 × 9 cm; Guixi Mousetrap Apparatus Factory, Guixi, Jiangxi, China) baited with peanuts, corn, or other alternative food baits. The cage traps (mousetraps) were set in the afternoon or evening and inspected the following morning. The placement of cage traps was flexibly adjusted according to habitats. In open habitats, 25 cage traps were set for each group in a straight line, with adjacent traps spaced 5 meters apart and 20 meters between two lines. In paddy fields, the traps were placed along the bank of the fields. In the indoor habitats, the traps were placed along the base of walls with approximately one trap per 15 m2. To ensure the comparability of sampling among different survey sites, approximately 200 traps were deployed at each site per day for 15 consecutive days, resulting in a total of about 3,000 traps deployed at each survey site (Liu et al., 2025). Chigger mites were routinely collected from the body surface of each host and subsequently preserved in 70% or 75% ethanol (Liu et al., 2025; Lv et al., 2025a). After being rinsed in distilled water or clean water, each collected chiggers was mounted onto a glass slides with Hoyer’s medium (Jacinavicius et al., 2013). After dehydration, clearing and drying, each mounted specimen was examined under a microscope (Olympus Corporation, Tokyo, Japan) and identified to species according to relevant taxonomic literature and identification keys (Huang et al., 1995; Li et al., 1997; Wilson et al., 2017; Nielsen et al., 2021). Following the completion of taxonomic identification for all collected chigger mites, A. indica was selected as the target species for this study.
     
    Statistical analysis
     
    Using statistical and ecological methods, conventional infestation indices for A. indica were calculated, including the constituent ratio (Cr), prevalence (PM), mean abundance (MA) and mean intensity (MI) (Mandla et al., 2021; Liu et al., 2025). Levins’ index (B'i) was employed to measure the host niche breadth of the mites, while Pianka’s index (Oij) was utilized to determine the host niche overlap between A. indica and other dominant chigger species in the same region (Lv et al., 2025b). Furthermore,  interspecific associations of A. indica with other dominant sympatric chigger mites were assessed using the association coefficient (V) (Liu et al., 2025). Analyses were conducted with R 4.5.3 software using the “stats” and “rstatix” packages. The chi-square test was used to test the statistical significance of prevalence (PM). The statistical significance of mean abundance (MA) and mean intensity (MI) was tested using Mann-Whitney U or Kruskal-Wallis H tests. For multiple comparisons, P values were corrected with Benjamini–Hochberg adjustment. Results with P < 0.05 were considered statistically significant. The relevant results were visualized using Sankey diagrams, rose diagrams, bubble charts and boxplots. All the chigger identification and statistical analyses were completed at Dali University, Dali, Yunnan, China. The following equations are the relevant formulas for calculating the constituent ratio (Cr), prevalence (PM), mean abundance (MA), mean intensity (MI), Levins’ index (B'i), Pianka’s index (Oij) and association coefficient (V) (Mandla et al., 2021; Liu et al., 2025; Lv et al., 2025b).













     
    Ni = Number of A. indica collected.
    N = Total number of all chigger species identified.
    Hm = Number of hosts harboring A. indica.
    H = Total hosts examined.
    Bi'= Levins’ index for reflecting the host niche breadth of chigger species i.
    Oij = Pianka’s index for reflecting the host niche overlap between chigger species i and j.
    S = Number of host species (host resource categories);
    Pih and Pjh = The respective proportions of chigger species i and j collected from host species h.
    V = The association coefficient between A. indica and other sympatric dominant species of chigger mite (L. deliense in the present study).
    a = The number of hosts harboring both species of L. deliense and A. indica.
    b = The number of hosts harboring L. deliense only.
    c = The number of hosts harboring A. indica only.
    d = The number of hosts harboring neither species.
    n = The total number of hosts examined.

    RESULTS AND DISCUSSION

    Distribution and overall infestation of A. indica
     
    From 13 positive survey sites, 5,197 hosts (rodents and other small mammals) were captured and examined. A total of 4,848 A. indica were identified from 165 hosts at 13 survey sites. The majority of A. indica were collected from Jinghong in southern Yunnan (the constituent ratio Cr = 71.39%, 3461/4848), followed by Yuanjiang (Cr = 22.90%, 1110/4848), with these two locations accounting for a combined Cr of 94.29% (Fig 1). The 4,848 A. indica identified across the 13 locations (survey sites) were distributed among 10 host species from 5 genera, 4 families and 2 orders (Rodentia and Scandentia). The vast majority of these chigger mites were found on the rat Rattus tanezumi of the order Rodentia (Cr = 92.49%, 4484/4848). The constituent ratios (Cr) of A. indica across different host taxonomic levels were visualized in Fig 2. The overall prevalence (PM) of A. indica across all host animals was 3.17% (165/5197), with a mean abundance (MA) of 0.93 mites/host and a mean intensity (MI) of 29.38 mites/host. On its primary host, R. tanezumi, the specific infestation indices were PM = 5.97%, MA = 2.11 mites/host and MI = 35.31 mites/host.

    Fig 1: Map of Yunnan Province, southwest China and the distribution of the 34 survey sites (2001-2022).



    Fig 2: The Sankey diagram visualization of the quantitative distribution of A. indica across different hosts taxonomic levels in Yunnan Province, southwest China (2001-2022).


           
    The results indicate that A. indica is predominantly distributed in southern Yunnan, accounting for a high constituent ratio (Cr) of 94.29% (Fig 1), which indicates that A. indica has a preference for specific geographical areas. Although A. indica was found on 10 host species, the majority of the mites (Cr = 92.49%) were found on the rat R. tanezumi (Fig 2). The result indicates that A. indica also has a preference for specific hosts. Besides Yunnan Province, A. indica has also been recorded in Vietnam, Myanmar and Laos, all of which border southern Yunnan (Nadchatram et al., 1980; Binh et al., 2020; Elliott et al., 2022). Ascoschoengastia indica is also distributed in Thailand with much higher prevalence than in Yunnan (Chaisiri et al., 2019). Situated in low-latitude zones, the warm and moist climate in southern Yunnan is quite similar to that in its neighboring countries, Vietnam, Myanmar, Laos and Thailand. The warm and moist climate is beneficial to the survival, development and reproduction of A. indica (Zhou and Ren, 2023; Li et al., 2024), which may lead to its distribution in tropical and subtropical regions. At the 13 positive survey sites where A. indica was found, a total of 2,126 R. tanezumi were captured, which provide abundant host sources for A. indica.
     
    Distribution and infestation of A. indica in different geographical and ecological environments
     
    Within the 13 locations where A. indica was collected, a comparative analysis was conducted on its distribution and infestation status on host animals across different geographical landscapes, habitats and altitudinal gradients. The results showed that the prevalence (PM = 3.75%) of A. indica in the flatland areas exceeded that recorded in the mountainous areas (PM = 2.73%, P < 0.05). However, both the mean intensity (MI = 41.38) and mean abundance (MA = 1.13) of A. indica in the mountainous areas exceeded the corresponding values in flatland areas (MI = 18.09, MA = 0.68, P < 0.05) (Table 1). The host animal composition varied across different habitats. In the residential habitats, the host composition was simple, with A. indica detected only on R. tanezumi and R. norvegicus, with R. tanezumi being the predominant host. In the agricultural and non-agricultural habitats, the host composition was more complex, but the highest abundance of A. indica was still collected from R. tanezumi. Across all three habitats, the primary host of A. indica was consistently R. tanezumi, which harbored the highest abundance of A. indica mites (Fig 3A and C). Among the five different altitudinal gradients, A. indica was primarily concentrated in the two low-altitude gradients of “< 500 m” and “500-1000 m”. The primary host animal remained to be R. tanezumi and the abundance of A. indica was highest at these low-altitude gradients (Fig 3B and D).

    Table 1: Infestation indices of Ascoschoengastia indica in different geographical landscapes and on different sexes and ages of hosts in Yunnan Province, southwest China (2001-2022).



    Fig 3: Comparison of the distribution of A. indica across different altitudes and habitats in Yunnan Province, southwest China (2001-2022).


           
    The distribution and infestation of A. indica varied in different geographical and ecological environments, which reflects the environmental heterogeneity of chigger mite infestation. Prevalence was higher in flatland areas, whereas mean abundance and mean intensity were higher in mountainous areas. These differences suggest roles for host composition, opportunities for contact with chiggers and local microhabitat conditions (Chaisiri et al., 2019; Wu et al., 2021).The predominance of R. tanezumi across habitats is consistent with its broad habitat use and availability as a host (Wu et al., 2021). The concentration of A. indica at lower elevations may reflect the combined effects of temperature, vegetation and primary host distribution (Elliott et al., 2019).
     
    Effects of host sex and age on A. indica infestation
     
    According to the survey results, a total of 165 host animals belonging to 10 species were found to be infested by A. indica. Regarding host sexes, the mean intensity (MI = 38.28) and mean abundance (MA = 1.17) of A. indica on female hosts exceeded the corresponding values on male hosts (MI = 21.21, MA = 0.71). However, the prevalence (PM = 3.05%) of A. indica on female hosts was slightly lower than that on male ones (PM = 3.34%). The differences of all these infestation indices between different host sexes were not statistically significant (P > 0.05). With respect to host ages, the mean intensity (MI = 66.83) and mean abundance (MA = 1.14) of A. indica on juvenile hosts were higher than the corresponding values recorded for adult hosts (MI = 23.01, MA = 0.86), with the difference of mean abundance between the two age groups being statistically significant (P < 0.01). Conversely, the overall prevalence of A. indica on adult hosts (PM = 3.74%) was significantly higher than that on juvenile hosts (PM = 1.71%) and this difference was also statistically significant (P < 0.01) (Table 1).
           
    Previous studies have proposed differences in host behavior, hormone levels and immunity as explanations for sex-related variation in ectoparasite infestation, alongside effects of parasite traits, host species and environmental conditions (Debnath et al., 2020; Jayalakshmi et al., 2023; Lv et al., 2025a). In the present study, the infestation indices (PM, MA, MI) of A. indica varied on different sexes and ages of hosts, which may reflect the sex and age biases of hosts in chigger infestation (Liu et al., 2025; Lv et al., 2025a). The MI and MA of A. indica on female hosts exceeded those on male hosts, but female hosts showed a lower PM than male ones. The variation pattern of PM was inconsistent with that of MI and MA between different sexes of hosts. Since the differences of these indices between male and female hosts were of no statistical significance (P > 0.05), it cannot be determined from the available results which sex of hosts is more susceptible to the chigger infestation and further research is still required. Regarding host ages, adult hosts showed higher PM than juvenile hosts (P < 0.01), but juvenile hosts had higher MA than adult ones (P < 0.01). From the present results, it is also difficult to explain the inconsistent variation patterns of PM and MA between different ages of hosts and more research still remains to be made in the future.
     
    Comparative analysis of A. indica and L. deliense
     
    Across the 13 sampling sites where A. indica was collected, a high abundance of L. deliense (5,352 individuals) was found alongside a large number of A. indica (4,848 individuals). Both of these vector chigger mites primarily parasitized R. tanezumi. At these 13 locations, a total of 2,126 R. tanezumi were captured, from whose body surfaces 16,209 chigger mites were obtained. Ascoschoengastia indica was the most abundant species (Cr = 27.66%, 4,484/16,209 individuals), closely followed by L. deliense (Cr = 27.13%, 4,398/16,209 individuals). Regarding host selection, the niche breadth of L. deliense (B'i= 1.47) was significantly broader than that of A. indica (B'i= 1.17). The niche overlap index between A. indica and L. deliense was exceptionally high, approaching 1 (Oij = 0.99), with a certain degree of positive association (V = 0.11, V > 0) in their host selection.
           
    From the 13 positive survey sites where A. indica was collected, a lot of L. deliense was also found. Leptotrombidium deliense is the major vector of scrub typhus in China (Zhou and Ren, 2023). The coexistence of abundant A. indica and L. deliense at the 13 sites may increase the potential risk of the transmission and focus persistence of scrub typhus in the survey areas. Ascoschoengastia indica and L. deliense are two independent chigger species in different genera. Although these two chigger species had a very high niche overlap (Oij = 0.99) with the same principal host (R. tanezumi) in host selection, L. deliense showed higher niche breadth (B'i) than A. indica. A high niche breadth (B'i) indicates a broad host range and low host specificity and vice versa (Lv et al., 2025b). The lower niche breadth (B'i) of A. indica indicates that A. indica has a narrower range of hosts with higher host specificity than L. deliense though they have similar host selection for their principal host, R. tanezumi. The low host specificity of chigger mites is beneficial to their transmitting pathogens among different hosts (Lv et al., 2025b). The lower niche breadth (B'i) of A. indica further suggests that A. indica has less potential risk of transmitting O. tsutsugamushi (the pathogen of scrub typhus) than L. deliense from ecological perspective.
           
    The present study retrospectively reports the infestation and distribution of A. indica on small mammals in Yunnan Province of southwest China for the first time. The results of the study enrich the understanding of this chigger mite species and provide scientific information for the prevention and control of the mites in Yunnan Province. The present study, however, only focuses on the infestation and distribution patterns of A. indica without the detection of O. tsutsugamushi and other relevant pathogens. From the present results, we still cannot determine the actual role of A. indica in the transmission and focus persistence of scrub typhus in the survey areas and more research (including the molecular detection of relevant pathogens) remains to be done in the future.

    CONCLUSION

    Ascoschoengastia indica is primarily distributed in the low-altitude regions of southern Yunnan, with R. tanezumi being its principal host. Its infestation exhibits pronounced environmental heterogeneity with the sex and age biases of hosts. Its co-existence with abundant L. deliense may increase the potential risk of the transmission and focus persistence of scrub typhus in the survey areas.

    ACKNOWLEDGEMENT

    We sincerely thank Cheng-Fu Zhao, Rong Fan, Zhi-Wei Zhang, Ya-Fei Zhao, some other colleagues and college students for their contributions to field investigations and laboratory work. This study was financially supported by the Prevention and Control of Emerging and Major Infectious Diseases-National Science and Technology Major Project (No. 2025ZD01900106) and the Research and Development Fund of Dali University (Nos. KY2319101340, KY2519103340).
     
    Disclaimers
     
    The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.
     
    Informed consent
     
    The capture and use of animals were approved by the local wildlife affairs authority and the Ethics Committee of Dali University.

    CONFLICT OF INTEREST

    The authors declare no conflicts of interest.

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