Epidemiologic changes of a longitudinal surveillance study spanning 51 years of scrub typhus in mainland China

Scrub typhus may be one of the world’s most prevalent, neglected and serious, but easily treatable, febrile diseases. It has become a significant potential threat to public health in China. In this study we used national disease surveillance data to analyze the incidence and spatial–temporal distribution of scrub typhus in mainland China during 1952–1989 and 2006–2018. Descriptive epidemiological methods and spatial–temporal epidemiological methods were used to investigate the epidemiological trends and identify high-risk regions of scrub typhus infection. Over the 51-year period, a total of 182,991 cases and 186 deaths were notified. The average annual incidence was 0.13 cases/100,000 population during 1952–1989. The incidence increased sharply from 0.09/100,000 population in 2006 to 1.93/100,000 population in 2018 and then exponentially increased after 2006. The incidence was significantly higher in females than males (χ2 = 426.32, P < 0.001). Farmers had a higher incidence of scrub typhus than non-farmers (χ2 = 684.58, P < 0.001). The majority of cases each year were reported between July and November with peak incidence occurring during October each year. The trend surface analysis showed that the incidence of scrub typhus increased gradually from north to south, and from east and west to the central area. The spatial autocorrelation analysis showed that a spatial positive correlation existed in the prevalence of scrub typhus on a national scale, which had the characteristic of aggregated distribution (I = 0.533, P < 0.05). LISA analysis showed hotspots (High–High) were primarily located in the southern and southwestern provinces of China with the geographical area expanding annually. These findings provide scientific evidence for the surveillance and control of scrub typhus which may contribute to targeted strategies and measures for the government.


Trend surface analysis
The trend surface analysis was employed to identify the geographic trends of the incidence (Fig. 6).A coordinate system was created (one axis for each direction with X for West-East and Y for South-North).The projections of incidence rates (Z axis) reflected the variation trend of West-East and South-North.The trend surface analysis showed that the incidence of scrub typhus increased gradually from north to south, and from east and west to the central area.

Spatial autocorrelation of scrub typhus in China
Moran's scatter plot and the significance assessment by permutation test of spatial autocorrelation for annualized average incidence of scrub typhus in China is presented in Fig. 7.The value of global Moran's I statistic (0.533) is shown in Fig. 7A, and the number of permutations (999) and Z scores (6.8056) are shown in Fig. 7B.The spatial autocorrelation analysis showed that a spatial positive correlation existed in the prevalence of scrub typhus on a national scale, which had the characteristic of aggregated distribution (I = 0.533, P < 0.05) (Fig. 7).The hotspots (High-High cluster area) and outliers of scrub typhus transmission in mainland China were identified through LISA analysis.LISA spatial cluster map shows the center of the cluster in color.High-High cluster indicates a statistically significant cluster of high scrub typhus incidence values.LISA analysis showed hotspots (High-High) were primarily located in the southern and southwestern provinces of China, such as, Yunnan,

Discussion
Since 1957, when scrub typhus was included in the Chinese notifiable disease monitoring system, the number of annually reported cases was approximately 1000 before 1986 in China, and then rose to > 2000 between 1986 and 1989 16 .Before 1986, scrub typhus was limited to south of the Yangtze River, from Zhejiang and Hunan in the north to Yunnan and Sichuan in the west, Hainan in the south, and Taiwan in the east.However, the regional distribution has expanded northward since then.Until 2014, all of the provinces in China except Qinghai reported scrub typhus cases 11 .There was a combined total of 182,991 scrub typhus cases during the time periods of 1952-1989 and 2006-2018, which were distributed in all of the provinces (31 provinces or municipalities) in mainland China.The amplitude and the magnitude of scrub typhus outbreaks in China remained rapid increase during the entire study period.Two main reasons may have contributed to this result.Firstly, as a naturalfocus disease, scrub typhus had been paid too little attention where it was previously unknown 10 .Following the discovery of the first confirmed cases of scrub typhus in China, enhanced awareness of scrub typhus directly resulted in increased surveillance, coupled with the improvement of the diagnostic tests, and resulted in more cases being diagnosed and reported 17 .Secondly, it's likely that rodents bearing infected mites may expand their ranges as climate changes, urbanization and globalization 18 .
In addition, the expansion of scrub typhus foci may be associated with secondary factors of human activities and environment change.For example, Deforestation, defined as land cover change from forest to non-forest regions, could be a human activities risk factor.The deforestation-induced secondary growth of scrub vegetation would provide a suitable environment for rodents, which are the natural hosts of vector mites, and may increase the density of mites 19,20 .Moreover, socio-economic factors, as a secondary cause, could have also served as important drivers for the transmission of scrub typhus in recent years.For example, the urbanization and change of land use may contribute to the spread of scrub typhus into urban areas by providing suitable habitats such as clearings, grasslands, and riverbanks for vectors and small rodents.
Scrub typhus is widespread in rural areas, which is similar to the patterns of geographic distribution in Southeast Asia 21,22 .The high endemicity in rural areas has been associated with people in rural areas who are exposed to environmental factors such as crops, bushes, animal pets and rodents 23 .Farmers constituted the majority of cases.Agricultural activities would increase the exposure to pathogen-carrying chigger mites 19,23,24 .For example, during the months of August to October, farmers are involved in harvesting activity in the fields, where they are exposed to the bites of larval mites.Notably, the disease was found to be prominent in elderly  www.nature.com/scientificreports/farmers, which may be associated with the rapidly changing demographics of rural areas in China.In modern China, young adults from rural areas often work as laborers in urban areas, leaving the elderly in their hometown taking care of grandchildren as well as farming 15,25 .
The seasonal characteristics of scrub typhus in this study were similar to results in previous studies abroad.For example, in Japan the overall seasonality by month of onset was bimodal; a larger peak occurred from October to December and a smaller peak occurred from April to June 26,27 .In Korea, the incidence rate starts to increase from September, hitting a peak in October and November and sharply decreasing in December 28,29 .However, in Bhutan, there were virtually no cases between December and June, followed by a sharp increase in July with a peak of incidence in September, and a sudden drop from October onwards 30 .The difference of peak period can be explained partially by the types of chigger mites and life-cycle of the vector mites 26 .The occurrence of scrub typhus in Japan in spring/summer is reportedly due to Leptotrombidium pallidum and in autumn/winter is mainly due to L. pallidum and L. scutellare 31 .L. scutellare is the major cause of scrub typhus in Korea 32 .A study conducted by Zhang et al. concluded that there were four different seasonal patterns for scrub typhus in China: summer, winter, autumn-winter, and annually 16 .The seasonality south of the Yangtze River had a summer pattern while it had an autumn-winter pattern north of the Yangtze river.L. deliense (Fig. 9) was regarded as the main vector of summer type scrub typhus and L. scutellare (Fig. 10) was regarded as the main vector of autumn-winter type scrub typhus in mainland China 33,34 .
There was spatial clustering with hot spots in the southern provinces (Yunnan, Guangxi, Guangdong, Hainan, etc.) in our study results.Chiggers might indicate the connection between climate change and cases of scrub typhus.Chiggers are mainly located in grassy fields, gardens, parks, forests, bush, and moist areas around lakes  or rivers, and their distribution is influenced by humidity, sunshine and temperature 35 .In the southern provinces of China, the climate is hot, humid and rainy, which is perfect for chigger mites.Moreover, the complicated topographic landform and high biodiversity of southern provinces may contribute to the extremely high species diversity of chigger mites in these provinces (especially Yunnan), which indirectly leads to a large number of scrub typhus cases 36 .The hot spots are largely concentrated in southern provinces, which may also be associated with societal changes.In recent years, the prosperity of ecotourism resulted in the development of southern regions in provinces such as Guangxi, Yunnan, and Guangdong, as well as a large increase of floating population in these provinces.This change in the population distribution could have increased the rate of human exposure to forests, riverbanks, and grassy regions that provide optimal environmental conditions for infected mites to thrive 37 .These hot spots should be targeted by policy-makers and local service providers for the establishment of refined disease control guidelines, including local vector control, health education, and promotion campaigns.In our results, Guangdong and Yunnan provinces have the highest number of scrub typhus cases (38,439 cases in Guangdong and 35,420 cases in Yunnan).In recent years, a rising trend was observed, which caused a huge disease burden in Guangdong province.Several reasons might contribute to the increase of scrub typhus cases, such as variation of genotypes of O. tsutsugamushi 38 , increased risk of exposure to vegetation with more and more parks in the city construction 39 , improvement of the surveillance system, and climate change.A previous study showed that climate factors played an important role in the spread of vector-borne diseases 40 .Yunnan, an inland province at a low latitude and high elevation in southwestern China, has a vast territory with diversified and unique nature resources.The altitude of Yunnan ranges from the lowest in Hekou town (76.4 m) to the highest at the summit of Meili Snow Mountain (6740 m) 41,42 .Yunnan is also a very wide mountainous region with complicated weather conditions.The mean annual temperature in some river valleys and flatlands with low altitude (less than 500 m) is very high, which can exceed 30℃.The mean annual temperature in high mountainous regions with high altitude (more than 3500 m), however, is very low and even no summer season all the year 43,44 .There are abundant species of plants and animals with a plenty of biologically diverse gene resources in Yunnan, which has been one of the hot places of biodiversity conservation in China 45,46 .In addition, in Yunnan there are three rivers (Jinshajiang River, Lancangjiang River, and Nujiang River) flowing from the northwest towards the southwest.The three rivers parallel to each other, forming the "Protected Area of Three Parallel Rivers", which is a famous "World Natural Heritage Area".It is considered as a hotspot of biodiversity with high species diversity in Asian continent 47,48 .The geographical location with unique landscape, complex topography, and diversified ecological environment of Yunnan provide a good place to the survival of rodents The three-dimensional trend surface analysis was done to frame the overall tendencies and to identify the outliers of the scrub typhus incidence in different geographical locations.In the present study, the LISA cluster maps revealed a higher incidence of scrub typhus infection in the southern provinces from 2006 to 2018, which agreed with the findings of the trend surface analysis.Some limitations must be acknowledged for this study: First, the bias could exist in this study because all the surveillance data used were extracted from a passive reporting system, which indicated that some cases of scrub typhus might go underreported because of the nonspecific clinical symptoms.However, the data used in present research were the most comprehensive and reliable data on scrub typhus available at national level in China.Second, the present surveillance system was the lack of data on the accurate distribution of chigger mite species, causing it impossible to explore the association among human cases, hosts, pathogens and vectors.

Ethics statement
This study was approved by the institutional review boards (IRB) of Qujing Medical College (approval number: QJMCLL2023-110).Written informed consent was obtained from all patients in the study.All the data analyzed in this study were de-identified to protect patient confidentiality.All methods were carried out in accordance with relevant guidelines and regulations.

Data collection and management
In The case definition for scrub typhus consists of an individual who has traveled to an endemic area or reported contact with chigger mites or rodents within 3 weeks before the onset of illness, along with clinical manifestations (such as high fever, skin rash, lymphadenopathy and eschars or ulcers), and at least one of the following laboratory criteria: an agglutination titer ≥ 1:160 in the Weil-Felix test using the OX K strain of Proteus mirabilis; a fourfold or more rise of antibody titer against O. tsutsugamushi using the indirect immunofluorescence antibody assay; detection of O. tsutsugamushi by PCR (polymerase chain reaction) in clinical specimens; or isolation of O. tsutsugamushi from clinical specimens 49

Spatial autocorrelation analysis
A global spatial autocorrelation analysis and a Local Indicators of Spatial Association (LISA) analysis were adopted to analyze the spatial patterns and the potential hotspots associated with scrub typhus incidence.Global Moran's I for global indication of spatial autocorrelation (GISA) reflects the similarity of attributes in spatially adjacent regions 52 .GISA was adopted to explore the global clustering characteristic of scrub typhus.Global Moran's I is calculated as follows: where n was the number of provinces (municipalities), x i and x j were the indicators of autocorrelations from unit index i and j, x was average and W i j was the matrix of spatial weights.If unit i was adjoined to unit j, W i j = 1; if not, W i j = 0. Global Moran's I Index, which ranged from − 1 to 1, reflects the similarity of attributes in spatially adjacent regions.Moran's I Index = 0 implied a random spatial distribution.Moran's I Index < 0 suggested a dispersing spatial distribution, and Moran's I Index > 0 implied a clustering spatial distribution 53 .Moran's I is calculated on the basis of Z-test.Z-test statistics can be expressed as: In the above formula, E (I) represents the mathematically expected value of Moran's I; Var (I) represents the variance of Moran's I. Z score (> 1.96 or < − 1.96) means P < 0.05, which indicated that scrub typhus incidence was not distributed randomly and cases were likely to cluster.
Local Moran's I for LISA is a measure of the similarity of difference between the attribute of the observation unit and those of surrounding units.It is calculated as follows: In the above formulae, n, x i , x j , W i j and x are the same as in the former formula.LISA was calculated to explore significant hot spots (High-High), cold spots (Low-Low), and outliers (High-Low and Low-High) 54 .
GISA analyses and LISA analyses can be realized by GeoDa0.9.5-i, a freely available spatial statistics software package (https:// geoda center.asu.edu/).Monte Carlo randomization was employed to assess the significance of Moran's I and the number of permutation tests was set to 999.Z score (≥ 1.96) indicated that scrub typhus incidence was not distributed randomly and cases were likely to cluster.

Figure 1 .
Figure 1.Annual average incidence rate and mortality of scrub typhus in China from 1952 to 1989 (phase I) and 2006-2018 (phase II).

Figure 2 .
Figure 2. The total number of annual cases (blue line with the triangle) fit with an exponential growth function (red broken line).

Figure 3 .
Figure 3. Age and sex distribution of reported scrub typhus cases in China, 2006-2018.

Figure 6 .
Figure 6.Three-dimensional trend surface analysis on incidences of scrub typhus in China, 2006-2018.This map was created by ArcGIS software (version 10.6, ESRI Inc.; Redlands, CA, USA).Homepage of ArcGIS software was https://www.esri.com/.Note The X-axis represents longitude; The Y-axis is latitude; The Z-axis shows incidence; the green line represents the incidence trend of the west-east direction, and the blue line represents the incidence trend of the north-south direction.

Figure 7 .
Figure 7. Global spatial autocorrelation analysis for annual average incidence of scrub typhus in China from 2006 to 2018.(A) Moran's scatter plot for annualized average incidence of scrub typhus.(B) Histogram for significance assessment of Moran's I.

Figure 8 .
Figure 8. Local indicators of spatial association (LISA) cluster maps for the average annual scrub typhus incidence from 2006 to 2018 in China.This map was created by ArcGIS software (version 10.6, ESRI Inc.; Redlands, CA, USA).Homepage of ArcGIS software was https://www.esri.com/.

Table 1 .
Demographic characteristics of reported scrub typhus cases in mainland China, 2006-2018.# P value calculated by use of χ 2 test.*Two columns (Rural and Urban) do not add up to equal the total because of missing data

Table 2 .
Reported cases and incidence of scrub typhus in the top 10 provinces, China, 2006-2018.

Total number of cases Annual average incidence rate/100,000
Vol.:(0123456789) Scientific Reports | (2024) 14:3138 | https://doi.org/10.1038/s41598-024-53800-y China, scrub typhus is a vector-borne notifiable disease and physicians are required by law to report cases to the China Center for Disease Control and Prevention through the China Information System for Disease Control and Prevention (CISDCP).Scrub typhus case reports include basic demographic and clinical data including gender, age, occupation, residential address, date of onset of symptoms, laboratory diagnosis, and clinical outcome for each case.In this study, all data for scrub typhus cases in 1952-1989 and 2006-2018 were extracted from CISDCP.Demographic information, such as gender, age, occupation, and address, was not included, and there was no information regarding the geographical distribution, in the surveillance data from 1952 to 1989.All of the surveillance data between 2006 and 2018 included basic demographic and clinical information reported by physicians.
. Patients with other established causes of fever were excluded.