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S. invicta are more active from spring to autumn [20], and most fire ant sting events in China occur in the same period [14] since that is when people are more likely to encounter them. Thus, we conducted our experiment during the warmer months of the year. We were invited to present educational seminars on the biology and management of S. invicta to residents in communities, farmers in suburbs and workers at gardening and landscaping companies in S. invicta-infested regions in Guangdong province several times, as we perform S. invicta investigation and management projects in Guangdong province. After the seminars, attendees were chosen at random to fill out an intervention health questionnaire. Meanwhile, participants were also recruited randomly through street investigations in Guangzhou city. Investigators were familiar with the symptoms of S. invicta stings and the biological characteristics of S. invicta. An intervention health questionnaire was considered acceptable if it was completed by the participant in its entirety. All of the surveys were reviewed by experienced physicians and entomologists in March and September of 2015. Data were obtained through a questionnaire (Supplementary material, S1 Table). Symptoms of depression and anxiety were evaluated using the Brief Patient Health Questionnaire Mood Scale (PHQ-9) [21] and the Generalized Anxiety Disorder Screener 7-item (GAD-7) scale [22], which are based on the fifth editions of the Diagnostic and Statistical Manual of Mental Disorders (DSM-V) and the DSM-V-TR criteria [23], respectively. The Beck Anxiety Inventory (BAI) was also used [24]. Sleep disturbances were evaluated using questions 1–8 of the Pittsburgh Sleep Quality Index, 5th subscale (PSQI(5)) [25]. The Impact of Event Scale-Revised (IES-R) which includes intrusion, avoidance, and hyperarousal subscales [26], and a post-traumatic stress disorder (PTSD) checklist of symptoms based on criteria from the DSM-V were also used to survey the psychological conditions of people who had been stung by S. invicta. Other characteristics of participants were also collected including demographic features, histories of chronic medical and psychiatric conditions, exposure to other animals, and experiences with particularly stressful events within the previous year. S. invicta sting statuses were initially determined by self-reports. Details related to S. invicta stings were recorded, such as dermatological lesions and ant samples (Fig 1). Participants who self-reported attacks by S. invicta were asked to identify the culprit insect on an identification tool containing pictures of S. invicta and other aggressive insects such as wasps, bees, and other ants. Meanwhile, investigators also confiremed whether participants experienced symptoms associated with S. invicta stings such as dermatological evidence or allergic reactions after attacks by the insects (Fig 1).
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0199424.g001 Algorithm for the attribution of a diagnosis of a Solenopsis invicta sting to the presence of characteristic lesions and clear evidence. Individuals who were stung by S. invicta within 30 or fewer days were placed in the stung group. Individuals who had not experienced an S. invicta stung or who had been stung by S. invicta in the past but not within the previous 30 days were placed in the un-stung group. Although fire ant stings are painful and the venom can cause skin redness, swelling, pustules, urticaria, edema and allergic shock, the symptoms do not last for 30 days, and nearly disappear 2–3 weeks after the sting (personal observation). Meanwhile, we also referred to the research of Susser et al. [6], who evaluated whether or not bed bug infestations can cause mental health effects. In their study, 30 days was used as the time period for determining if subjects were assigned to the exposed or unexposed group [6]. Unlike bed bugs, S. invicta is not a hematophagous insect, and its stings are also easier to avoid than bites from bed bugs. Taken together, we believe that the use of 30 days as a cut off for classifying individuals as being stung or unstung is justified. Since “symptoms last for more than 1 month” is a required criterion in the DSM-V criteria for PTSD [23], we compared the intrusion, avoidance, and hyperarousal subscales of the PTSD checklist between the stung and unstung groups to evaluate whether fire ant stings can cause PTSD. A total of 96 valid questionnaires were obtained, out of which 37 participants were classified as stung and 59 participants were classified as unstung (Fig 1). We further analyzed the mental health effects of S. invicta attacks in mainland China based on these data.
This study was reviewed and approved by The Medical Ethics Committee of Guangzhou Huiai Hospital (Supplementary material, S1 Fig and S1 File). All of the participants in this investigation agreed to allow the data they provided to be used for research purposes (Supplementary material, S2 File).
A χ2 analysis was used to compare the different characteristics between the stung and unstung groups. Scores for the PHQ-9, GAD-7 scale, BAI and PSQI(5) were classified as “present” or “absent”. Symptoms were considered “present” when scores reached 10 or higher out of 27 possible points on the PHQ-9, five or higher out of 21 on the GAD screen, 45 or higher of 74 on the BAI, and 10 or higher of 24 on the PSQI(5) [6, 24]. Scores on the BAI were corrected using the formula y = int(x) before the test, where y represents the scores we used in the test and x represents the original scores in the investigation [24]. The IES-R was only used to evaluate the psychiatric conditions of people who were stung by S. invicta. There were two groups in this test: 30-day group (individuals who were stung by S. invicta within in the previous 30 days, i.e., the stung group) and the over 30-day group (individuals from the unstung group who were stung by S. invicta in the past but not within the previous 30 days). Scores of 35 above were suggestive of PTSD [27]. The IES-R has three parts; each part was scored and the total scores were calculated. All of the statistical data from the IES-R were tested for normal distribution using the Shapiro-Wilk test and for the homogeneity of variances using Levene’s test. The independent t-test was used to test the difference between those stung by S. invicta in the 30-day group and the over 30-day group. The internal consistency and reliability of the psychometric tools (PHQ-9, GAD-7, PSQI(5), BAI, and IES-R) were determined by computing Cronbach’s alpha values [28]. In general, items with an alpha correlation of 0.7 are considered to have adequate internal consistency [29]. Analyses were performed using SPSS 18.0 (SPSS Inc., Chicago, IL, United States).
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