Scrub typhus is known to be an endemic disease in China.8The number of cases has been reported in many parts of China[6,8,10–13].
To identify the risk factors associated with ST in the district, we conducted a case-control study. In this case-control study, we found that most of the ST infected cases, 90 (77.6%) and 187 (80.6%) controls were farmers, which suggested that the disease endemicity is associated with agricultural activities and raising animals and livestock. Further, outdoor activities and less use of protection measures are significant factors associated with ST infection.
Factors that may contribute to the increase of ST included outdoor activities, changes in ecological environment, living in an endemic area might be the reason for increasing the chances of being bitten by chigger mites[14].These findings indicated significant differences between the two groups living in the environment. This is consistent with other findings[9].The results suggest that age above 40 years is closely associated with risk of ST infection[9]. Similarly, primary school level education is the most risk groups; which is in line with other studies[15].
Further, the results showed the details of the living environment of ST such as residential area (living in the edge of the village and living in the center of the village), raising animals and livestock. Others such as piling weeds in the yard and around the house. In addition to the presence of mouse activities, and living in houses near to grassland and vegetable fields (within 20 meters). If the house builds in the edge of the village and near to grassland, the vegetable field is more likely of growing of chigger mite and makes the disease transmission easy during the endemic period. These findings were supported by other previously published studies[9].
The infection of ST among farmers, those living in the center of the village and completed their primary school level education are exceptionally high, mainly due to their frequent contact with disturbed habitats in their surroundings. Another element is that ST infection in Luhe district is more common in female than male. It may be because most women generally spend more time on the farm and engaged in outdoor activities[11].A study conducted in Korea reported that females are more likely to be at risk of ST infection than males[16]. Moreover, the rise of farming activities has traditionally been associated with ST infection. Previous studies conducted in Taiwan and Korea point out that farmers are at a greater risk of ST due to their occupation status. Further, the individual-level of the risk factors associated with ST and outdoors activities showed protective associations of ST infection[14,16].
The daily morning exercise in the park, fishing in the rivers, drying clothes in the grasses, working as labourers especially in the vegetable field are the main factors associated with the risk of exposure; since the mites are widely spread in different types of vegetation, grasses, and scrubland. Factors such as raising of small animal and livestock (dog, pig, cattle, goat, cat, sheep, and rabbit) have been significantly associated with the risk of ST infectious. Similarly, keeping domestic animals in the yard and rodents at home has also been found to be a significant factor associated with the risk of ST in Luhe. Thus, findings similar to this study was reported in Darjeeling city, which is located in northeast India, but not so in Beijing, China[9,17–18]. The data subjected to multivariate logistic regression specified that those having a morning exercise in the park and those working in the vegetable field were significantly associated with the occurrence of scrub typhus.
The presence study outlined the following findings. First, this is the first case-control study in the area to identify the risk factors associated with ST infection in Luhe district for the first time. Second, owing to the growing public health concern for China CDC in the prevention and control of endemic ST infection, this results may assist the health authorities in Nanjing Municipal Center for Disease Prevention and Control for a development programme for health prevention and control ST infection in future.
There are several limitations to this study. First, the sample size of confirmed cases by serological and PCR are small. Possibly there exists some potential for recall bias because of using ST cases that had the infection during the seasonal outbreak 2015 to 2017, which notified by Nanjing Municipal Center for Disease Prevention and Control and it involved a long time from the onset of diseases to be interviewed. Secondly, the study represents only an endemic area in the northern part of the Nanjing city, which only consider an endemic area of ST by Nanjing CDC and they not provide information about others endemic area close to Nanjing.