SFTSV poses serious public health concerns globally, because it causes tick-borne haemorrhagic fever with a high case fatality rate (12–50%)[14–15]. Currently, the pathogenic mechanism and transmission route of SFTSV are still under study. SFTS is a viral infectious disease. As a result, there is neither an effective therapeutic drug nor an effective vaccine to prevent the disease. Therefore, a retrospective analysis of the epidemiological characteristics, clinical symptoms and laboratory indicators of SFTS is of great significance for early detection, early diagnosis, early treatment and improvement of the prognosis.
According to the results of this study, SFTS incidence was concentrated in April-June. The peak of tick-to-person transmission was in May, accounting for 37.83% (14/37) of cases, and the high incidence of person-to-person transmission was in April, accounting for 58.33% (7/12) cases. Seasonal characteristics may be related to the decrease in the number of ticks in different regions, as may climate change, the living habits of local residents and their awareness of SFTSV. A majority of middle-aged people were found in the person-to-person transmission infection group, because health care workers are mostly middle-aged. The main route of infection was contact with the blood/respiratory secretions of a patient. Prior studies have identified direct contact with patient blood without personal protective equipment as a major risk factor for contracting SFTS[16–17]. Previous studies have suggested that non-blood contact was also associated with developing the illness[18–19]. Exposure to urine and faeces was associated with SFTSV transmission , and in a study by Jeong et al. SFTSV RNA was detected in tracheal and gastric aspirates. Huang et al. demonstrated the potential existence of infective virus particles in the mucosa and secretions of patients who may cause asymptomatic infection. The majority of the tick-to-person transmission infection group were elderly farmers, a finding similar to many other studies. This may be due to the local elderly engaging in tea picking, mowing and other outdoor activities (resulting in greater exposure), their own immunity status and combined with underlying diseases and so on. There was a history of tick bites in the tick-to-person transmission infection group within 2 weeks of the onset of the disease and living in mountainous and hilly areas, suggesting that SFTS has a significant regional effect.
SFTS clinical symptoms and laboratory tests are basically consistent with the relevant literature reports[21–22]. The clinical manifestations of the patients were mostly accompanied by fatigue, vomiting, abdominal pain, diarrhoea, muscle soreness, cough and other symptoms. Some patients had central nervous system symptoms, such as disturbance of consciousness, apathy and slow responses. In this study, the proportion of consciousness disorders in the tick-to-person transmission infection group was significantly higher than that in the person-to-person transmission infection group. Patients with disturbances of consciousness may develop encephalitis. A previous study reported the occurrence of encephalitis in 19.1% of patients hospitalized for SFTS, with fatal outcomes in 44.7% of these patients, based on a large sample collected over a long period. For most SFTS patients, the PLT and leucocyte counts were lower than the lower limit of the normal value, with liver and kidney function damage, myocardial damage and abnormal coagulation function. This study compared the clinical laboratory indexes of the person-to-person transmission infection group to those of the tick-to-person transmission infection group and found that the differences in WBC, LYM, and PLT counts, as well as in ALB, ALT, AST, BUN, CyS-C, GLU, CK, CK-MB, LDH, α-HBDH, TropT, AMY, LIP, PCT, APTT, TT and D-D levels, were statistically significant. The results showed that the leucocyte, LYM and PLT counts decreased more obviously in the tick-to-person transmission infection group, and injuries to the myocardium, pancreas and coagulation system were more serious. LDH and CK levels and decreased LYM percentages were associated with death[24–27]. Longer APTT (> 62.6 s) has been suggested to predict a fatal outcome. Moreover, the GLU level of the tick-to-person transmission infection group was higher than that of the person-to-person transmission infection group, which suggested that GLU level control in the tick-to-person infection group was poor. The reasons were as follows: (1) some patients in the tick-to-person transmission infection group had a history of diabetes mellitus; (2) the liver function of the tick-to-person transmission infection group was seriously damaged and the ability to regulate blood sugar was decreased; (3) drugs affecting blood sugar, such as glucocorticoids, were used in the tick-to-person transmission infection group. Reduced consciousness and abnormal laboratory parameters had high predictive values in identifying patients at greater risk for death. In clinical practice, these indicators could be used to reduce the fatality rate. This study also found that the number of critically ill patients in the person-to-person transmission infection group was 16.67% (2/12), which was significantly lower than that in the tick-to-person transmission infection group (91.89%, 34/37). The number of deaths in the tick-to-person transmission infection group was 29.73% (11/37), but there were no deaths in the person-to-person transmission infection group. These results suggest that tick-to-person transmission is more serious and that the risk of poor prognosis is higher for this infection group. The reason may be that most of the infected people are elderly with low immunity or more basic diseases. On the other hand, it may be caused by tick bites, which spread more viruses and/or bacteria simultaneously. Studies have shown that serum viral load levels are closely related to disease severity.
The investigation found that the average time from onset to treatment was 2.5 days in the person-to-person transmission infection group and 5.0 days in the tick-to-person transmission infection group, indicating that the treatment of the tick-to-person transmission infection group is not timely enough, which delays the treatment time and leads to poor prognosis. The awareness of SFTSV and ticks in the high-incidence areas of Anhui Province is seriously insufficient, and the government should strengthen the propaganda and education on SFTS and ticks in high-incidence areas. Studies have shown that the interval between the onset and diagnosis is associated with mortality. It is suggested that the diagnostic ability of primary medical institutions for the disease needs to be further improved, and training and education should be strengthened. For patients with a relevant epidemiological history and clinical manifestations, health care workers should avoid missed diagnosis.