Alarming increase of scrub typhus incidence in Yunnan province, southwest China, 2006--2017

Background: The bacteria Orientia tsutsugamushi is the causative agent of scrub typhus, mite-borne disease, which causes an acute febrile infectious illness in humans. An epidemiologic study was conducted to understand the characteristics of scrub typhus in Yunnan province and assist public health prevention and control measures. Methods: Based on the data on all cases reported in Yunnan from 2006 to 2017, we characterized the epidemiological features. Q-type cluster method of hierarchical cluster analysis was adopted to analyze the incidence of scrub typhus. Together with the results of clustering, geographical distribution of scrub typhus was described. Results: In total, 27,838 scrub typhus cases were reported in Yunnan during 2006-2017. Of these, 49.53% (13,787) were male and 50.47% (14,051) were female (P>0.05). Most patients were children aged 0-5 years (13.16%) (P<0.01) and farmers (68.41%) (P<0.05). An almost twenty-fold increase in the number of patients was observed in 2017 (6,337 cases) compared to 2006 (307 cases). Baoshan city and Lincang city had the most cases accounting for 41.94%, while Diqing prefecture had the lowest incidence (only 3 cases). The majority of conrmed cases occurred yearly in August with a high temperature and rains. Sixteen cities (prefectures) infected of Yunnan were classied into three groups numbered in sequence. Conclusion: The study results showed that the incidence of scrub typhus in Yunnan province is high and the annual incidence increased noticeably over time. Our results also indicate that surveillance and public education need to be focused on Baoshan City, Lincang City and Dehong Prefecture.


Background
Scrub typhus (Tsutsugamushi disease) is an ancient infectious disease primarily endemic in several Asia-Paci c nations [1,2]. It is an infection caused by the pathogen Orientia tsutsugamushi, which can be transmitted occasionally to humans after being bitten by infected Chigger mites (trombiculid mites) [3,4]. O. tsutsugamushi spreads throughout the body via the blood and lymphatic vessels, so that patients infected with this microorganism manifest a variety of clinical symptoms and signs such as myalgia and diffuse lymphadenopathy [5]. The larval stage is the only parasitic stage when the pathogen is transmitted to people and other vertebrates [6,7]. Rodents are important to maintenance of the disease in that they are known as incidental hosts for chigger mites [8]. It is estimated that more than half of the world's population live in tsutsugamushi disease endemic areas and that about 1 million cases occur annually [9]. Up to now, however, there is still no point-of-care diagnostics available and no reliable and effective human vaccine against tsutsugamushi disease, which will pose a signi cant threat to public health [2].
Scrub typhus is also widely distributed in China, including 16 cities or prefectures of Yunnan province [10,11]. In recent years, local outbreaks of scrub typhus have occurred in many areas of Yunnan, and recently identi ed endemic foci of scrub typhus are reported to be expanding [12][13][14]. The increased incidence of tsutsugamushi disease in Yunnan offers an opportunity to enhance our understanding of the epidemiological trends of this reemerging and infectious disease. The results from our study will provide an authentic and reliable evidence for local health-care authorities to design strategies to alleviate the health effects of scrub typhus.

Study area
Yunnan, an inland province at a low latitude and high elevation, lying between 21°8′ 29°15′N and 97°31′ 106°11′E in southwestern China, has a vast territory with diversi ed and unique nature resources. Located in the southwest of China, Yunnan harbors around 45.97 million residents in its 16 cities or prefectures with a total land area of nearly 394000 km 2 (http://stats.yn.gov.cn/tjsj/tjnj/201912/t20191202_908222.html). It mainly features a subtropical plateau monsoon climate type. Mean temperature for the hottest month (typically in July in Yunnan) is 19°C-22 °C, while mean temperature for the coldest month (typically in January) is 6 °C-8 °C. The annual average precipitations in most areas is more than 1000 mm (http://www.yn.gov.cn/yngk/).

Data collection and management
In China, scrub typhus is a noti able disease that must be reported to the China Center for Disease Control and Prevention. The case de nition for tsutsugamushi disease consists of an individual who has traveled to an endemic area or reported contact with chigger mites or rodents within three weeks before the onset of illness, along with clinical manifestations (such as high fever, lymphadenopathy, skin rash, 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 4-fold or more rise of antibody titer against O. tsutsugamushi using the indirect immuno uorescence antibody assay, detection of O. tsutsugamushi by PCR (polymerase chain reaction) in clinical specimens, or isolation of O. tsutsugamushi from clinical specimens [15]. Patients with other established causes of fever were excluded. A standard form was adopted by local physicians to collect individual information on each scrub typhus case, including epidemiological exposure histories, age, sex, address, date of onset, diagnosis, and laboratory test results. Routine case reporting is done by hospitals via the National Noti able Disease Report System within 24 hours [15]. To describe epidemic characteristics and explore the incidence clustering of con rmed cases of scrub typhus in Yunnan China at the city (prefecture) level, we used surveillance data reported con rmed scrub typhus cases from 2006 to 2017, routinely collected by the Yunnan Provincial Noti able Disease Report System. Population data at the city (prefecture) level were obtained from the National Bureau of Statistics of China, which is based on the Sixth National Census in 2010.

Statistical analysis
The differences in the incidence rates of scrub typhus cases by sex, age group, and occupation were analyzed using the chi-square test. P values <0.05 were considered statistically signi cant. Q type cluster method of hierarchical cluster analysis method together with Z-score standardization and squared Euclidean distance were adopted in the calculation clustering analysis under the statistical software package SPSS (v.25). The clustering result was presented in a dendrogram. The spatial distribution characteristics were showed in the form of a map of the incidence by ArcGIS 10.6.

Results
A total of 27,838 clinically diagnosed and laboratory con rmed cases were reported in Yunnan province during 2006-2017, among whom 11 patients died. The overall case fatality was 0.04%. The annual incidence ranged from 0.65/100,000 to 13.36/100,000, with a rapid uptrend observed year by year and with an average annual incidence rate of 4.87/100,000. Natural focus expansion was found. The number of con rmed scrub typhus cases peaked in 2017, when there were 20.64 times more reported cases than in 2006. Scrub typhus cases, the incidence rate (1/100,000) and the mortality rate (1/100,000) have been increasing rapidly from 2006 to 2017 in Yunnan province, southwest China (Table 1, Fig. 1 & Fig. 2).  (Fig. 3). Farmers were the primarily affected occupation, accounting for 68.41%, followed by students (2650 cases) and young children (4867 cases). The total cases of scrub typhus was 26 562, accounting for 95.42% in these three groups. While sex-speci c scrub typhus incidence from 2006 to 2017 did not change signi cantly over time, the ratio of male (49.53%) to female (50.47%) was 0.98:1 (P > 0.05) and most patients were children aged 0-5 years (13.16%) (P < 0.01).
The results of Hierarchical cluster analysis on the incidence of scrub typhus are described in Fig. 4. Sixteen cities or prefectures of Yunnan province are classi ed into three groups numbered in sequence. Baoshan City (1), Lincang City (2) and Dehong Prefecture (3) with highest incidence formed the rst group in the clustering dendrogram.
As we know, chigger mites are the only vector of tsutsugamushi disease [16,17]. The complicated topographic landform and high biodiversity of Yunnan may contribute the extremely high species diversity of chigger mites in this province [18], which indirectly leads to a large number of scrub typhus cases. There are The results of our study identi ed seasonal differences in the onset and duration of tsutsugamushi disease that suggest the existence of both summer and autumn transmission of scrub typhus (summer-autumn type) in Yunnan, which is not in accordance with scrub typhus type in other provinces of China [19][20][21]. In our study farmers and children aged less than 5 years old were the groups most at risk due to lower awareness of scrub typhus, poor protection during agriculture or eld recreational activities and reduced use of protective measures when playing in grassland or woods for children [22].
The results of our study indicated that from 2006 to 2017 scrub typhus occurred mainly in Baoshan City, Lincang City, Dehong Prefecture, Chuxiong Prefecture and Honghe Prefecture. Actually, the classi cation by hierarchical cluster analysis has produced results that correspond to the rate of incidence of scrub typhus in Yunnan province.
Our results also indicated that surveillance system, public education and public health efforts to control and prevent scrub typhus need to be focused on the areas with highest incidence. Baoshan City, Lincang City and Dehong Prefecture were the areas with the highest incidence of scrub typhus in Yunnan province. Located in southwest of the province, the three cities have high temperature, intense sunshine, abundant rainfall and relatively low atmospheric pressure, which made an ideal environment for living of chigger mites [23,24].

Conclusions
In our study, the incidence of tsutsugamushi disease in Yunnan province is on the rise, and the epidemic area is expanding. We should strengthen the ability of early diagnosis and detection, carry out health education, monitor the epidemic situation and risk factors of tsutsugamushi disease, and prevent the harm of tsutsugamushi disease. A set of preventive strategies including public health education and personal protection facilities should be promoted in high-risk populations (the elderly, farmers and children aged less than 5 years old) and cities (Baoshan City, Lincang City and Dehong Prefecture) in Yunnan province.

Declarations
Ethics approval and consent to participate The research protocol was approved by the human ethics committee of Yunnan Center for Disease Control and Prevention. All study-related data were anonymized before analyses.

Consent for publication
Not applicable.
Availability of data and materials The data of scrub typhus cases were routinely collected by the Yunnan Provincial Noti able Disease Report System. Epidemic features uctuation of scrub typhus cases along year in Yunnan Province, China [straight line represents the incidence rate (1/100,000); broken line stands for the mortality rate (1/100,000)]