Numerous research studies have been conducted on SFTS since it was first reported; however, the pathological mechanism is still unclear. The 14.3% mortality rate in our data is much higher than the 5.2% surveillance rate from China[12], consistent with Li Hao's study(16.2%)[13]. Mortality rates vary considerably and may be related to differences in the number of cases, geographical distribution, level of medical care, and statistical calibre. Patients who ultimately die from STFS may have some features suggestive of a poor prognosis in the early stages of the disease. Therefore, assessing the prognostic factors during patient admission is crucial, enabling clinicians to make optimal treatment decisions promptly.
In recent years, this study found a fluctuating upward trend of SFTS cases in the surrounding areas of Nanjing, China. The occurrence of SFTS is seasonal, primarily occurring between April and October. The incidence of the disease is predominantly among middle-aged and elderly farmers. Previous studies have shown that all age groups are susceptible to SFTS infection[14]. However, similar to previous studies[8, 15, 16], this work found that the D group was older than the S group, and there was a statistically significant difference between the two groups. Multifactorial analysis also revealed that age was an independent risk factor for the outcome of patient death. Regarding underlying diseases, cardio-cerebrovascular and neuropsychiatric diseases were statistically significant in univariate analyses but excluded in multivariate analyses.
This study shows that 97.13% of the patients were observed to have fever, and other relatively common clinical manifestations included diarrhea, vomiting, abdominal pain, cough, etc. However, no significant differences were found between the two groups in these clinical manifestations, which suggests that it is difficult to assess the prognosis of the patients only by the common clinical manifestations. Among the clinical features we studied, HS and NS were the two mortality risk factors. HS manifests as skin petechiae, gingival bleeding, and epistaxis in patients with mild symptoms and gastrointestinal bleeding and pulmonary bleeding in patients with severe symptoms. NS includes transient disorders of consciousness, such as drowsiness, some of which resolve or disappear as the disease progresses, and in severe cases, disorientation, delirium agitation, disorientation, recurrent grand mal seizures, and even coma. Patients with HS and NS had 3.39 and 4.89 times higher increased risk of death compared to those without these symptoms, respectively. This is similar to previous studies[17–19]. HS is associated with many factors, such as thrombocytopenia, abnormalities of the coagulation system, damage to vascular endothelial cells caused by infection[20], and platelet phagocytosis by macrophages[21]. NS is attributed to STFS-related encephalopathy/encephalitis]22].
SFTS patients have lower platelet counts, impaired liver, kidney, and cardiac function, and coagulation problems in their serum. Previous studies showed that platelets, AST, AST, lactate dehydrogenase (LDH), Creatine kinase (CK), APTT, BUN, and creatinine were risk factors associated with the death of SFTS patients[18, 23, 24]. The present study showed statistical differences between the two groups in univariate analysis in platelets, albumin, PT, APTT, creatinine, BUN, and AST. However, in multivariate analysis, statistical differences existed only in platelets, PT, and APTT, suggesting that these three laboratory markers are independent risk factors for the outcome of SFTS death. In this study, the D and S groups showed different levels of thrombocytopenia, PT, and APTT prolongation, with more remarkable changes in the D group. A survey of dynamic monitoring of laboratory markers in patients with STFS found that PT, APTT, and platelet recovery gradually in the surviving group, but patients in the death group had further worsened PT, APTT, and platelet levels with the deterioration of the disease[25]. Based on an in vitro study, the SFTS virus attaches itself to mouse platelets and encourages primary mouse macrophages to phagocytose the platelets[26]. Additionally, platelets are crucial for maintaining the integrity of the endothelium cell barrier[27]. Vascular endothelial dysfunction in patients with STFS activates endogenous coagulation, causing abnormal coagulation and consuming large amounts of platelets, leading to thrombocytopenia and disseminated intravascular coagulation(DIC) [28, 29].
SFTS viral replication is reflected in viral load, one of the laboratory diagnostic markers of STFS. This study demonstrated that patients in the D group had higher viral loads, and SFTS viral load ≥ 107 copies/ml was an independent risk factor for death. Similarly, previous studies have shown that patients with high viral load are more likely to develop serious infections [30–32]. According to another prospective study, dynamic changes in viral load positively correlated with disease severity and serum PLT, WBC, LDH, AST, and CK levels[33]. Continuous monitoring of changes in viral load may help determine the prognosis of SFTS. Serum viral load in SFTS patients is associated with a host cytokine storm induced by IL-1RA, IL-6, IL-10, G-CSF, IP-10, and MCP-134, and the exact mechanism needs to be further investigated[34]. Previous studies have reported that some cytokines[24, 35–37], as well as lymphocyte subsets[31, 36]and pancreatic enzymes[24, 38] are risk factors. This study did not consider these factors because of the lack of data.
Nomogram provides a comprehensive, at-a-glance, and easy-to-use data visualization tool that accurately calculates the probability of an individual patient's specific outcome. In the present study, we constructed a novel nomogram model on the poor prognosis of STFS patients with seven high-risk screened factors. We validated the model using ROC, calibration, and decision curves, suggesting that the nomogram model has satisfactory predictive ability. This model identifies high-risk STfFS patients at admission, allowing for active interventions and better patient outcomes and survival.
This study has several limitations. First, our nomogram model was based on retrospective data collection from a single center. Secondly, there were relatively few samples in Group D, so we could not divide the data into validation and training sets to verify the internal validity of the model. Thirdly, this study incorporated viral load into the model, but nucleic acid tests cannot be performed in primary hospitals.