Evaluation of the risk of transmission and mortality of severe fever with thrombocytopenia syndrome in China: results of an 11-year study | Infectious diseases of poverty

Temporal and spatial distribution of SFTS clusters in China

Between 2011 and 2021, 35 SFTS clusters were reported in China, involving 118 patients, of whom 26 died (CFR=22.0%). The CFR was higher in women (31.4%, 16/51) than in men (14.9%, 10/67). In addition, the CFR was higher in patients aged ≥ 60 years (35.3%, 24/68) than in patients aged

There has been a yearly increase in the incidence of SFTS clusters, which was highest in 2020 (not= 9), followed by 2018 and 2021 (not= 6). The cluster incidence rates in April, May, June, July, August and September were 17.4%, 22.9%, 20.0%, 17.1%, 8.6% and 11.4% , respectively (Fig. 1), indicating epidemic seasonality. during summer and fall.

Fig. 1
Figure 1

The seasonality of SFTS clusters in China from 2011 to 2021. SFTS, severe fever with thrombocytopenia syndrome

SFTS clusters have been reported in Anhui provinces (not = 16), Shandong (not = 8), Jiangsu (not = 4), Zhejiang (not = 3), Hubei (not = 2), and Hunan (not= 2). The number of individuals involved in each cluster ranged from two to twelve people, with the median number being two. The sex ratio (male/female) of the patients included was 1.311 (67/51). The age range and the average age of the patients were 18 to 84 years and 59.0 ± 14.2 years, respectively.

Infection routes and location of SFTS clusters in China

The routes of infection of index patients in 14 and 16 clusters were tick bites and suspect tick bites, respectively, with those of the other five clusters unknown. Index patients were exposed to ticks while picking tea leaves in the tea garden (10.0%, 3/30); field agriculture (10.0%, 3/30); weeding and raising livestock in or around yards (30.0%, 9/30); working in the hills (27.0%, 8/30), including hunting, chopping wood, digging trees, picking fruit and searching for medicinal herbs; and contact with the blood of a dog infected by tick bites (3.3%, 1/30) or both working in the hills and weeding and raising livestock in or around yards (20.0% , 6/30).

There were 17 clusters that resulted in secondary patients via index patients via human-to-human transmission. Of these, four occurred in hospital, three at home, and the remaining ten occurred both in hospital and at patients’ homes. Secondary patients included family members of primary cases, relatives, doctors and nurses, and even other villagers. Routes of exposure included blood contact (i.e. contact with blood or bloody fluids and patient secretions) and non-blood contact (i.e. contact with patient fluids or secretions other than blood or inhalation of Tweezer-containing an aerosol) when caring for index patients, transferring dying patients with clinical hemorrhagic manifestations or during funeral preparations. Nosocomial infection occurred in two groups, which involved one physician and one nurse in each group. The physician was exposed while performing a sputum suction operation without a closed sputum suction tube and/or touching the patient’s blood without personal protective equipment (PPE). The nurse was infected while changing sheets contaminated with fresh blood from the same patient; however, she wore gloves without wearing a mask, indicating possible infection from aerosol inhalation. Another doctor and a nurse were infected through non-blood contact while providing medical care without PEP to another patient. The routes of transmission of two groups that involved eleven and seven secondary patients with nosocomial infection are shown in Figures 2A and B, respectively.

Figure 2
Figure 2

A Transmission routes for an SFTS cluster in Anji County, Zhejiang Province, 2014. B Transmission routes for an SFTS cluster in Hanshan County, Anhui Province, 2020. A Patient A was the index patient and died of massive bleeding while being transferred from hospital to home. The patient had infected 11 secondary patients (Patient B–Patient L); among them, nine patients were infected through blood contact while the other two patients were infected through inhalation of Tweezer– containing an aerosol in a confined bereavement room, without direct contact with the patient or other possible exposure. Not all secondary patients wore personal protective equipment during exposure. The index patient had been exposed to a tick bite while picking tea leaves in the tea garden. Positive serum levels of SFTSV IgG detection were 1.6% and 2.0% in healthy people and ducks, respectively, living in the village where the index patient lived. B The index patient (A) was a 51-year-old farmer who became infected through contact with the blood of a dead dog that had been bitten by ticks. He had infected seven secondary cases. Specifically, five family members and relatives were infected through blood contact while a nurse and a doctor were infected through non-blood contact. SFTSsevere fever with thrombocytopenia syndrome, SFTSVsevere fever with thrombocytopenia syndrome virus

Of the remaining 18 clusters that caused no human-to-human transmission, eleven, six and one occurred in the village living environment, fields and tea garden respectively. Further details are provided in Table 1.

Table 1 Characteristics of SFTS clusters in China, 2011-2021

The median number of infected individuals among clusters with and without secondary human-to-human transmission was 2.0 (2.0–2.0) and 3.0 (2.0–6.0), respectively (you= 71.00, P= 0.003). The transmission pattern of SFTS clusters with and without secondary human-to-human transmission is summarized in Fig. 3.

Figure 3
picture 3

Transmission pattern and risk of different modes of human-to-human transmission among SFTS in China. Note: The image on the left depicts the exposure pathways of the 30 index patients to SFTSV. All were exposed during their routine work related to agriculture. There are six index patients exposed to confirmed or suspected tick bites while working in the hills and weeding and herding livestock in or around yards. SFTSsevere fever with thrombocytopenia syndrome, SFTSVSevere fever with thrombocytopenia syndrome virus, SARsecondary attack rate

Risk assessment of different modes of transmission among clusters that caused human-to-human transmission

Blood contact infection showed higher DAS than nonblood contact infection [50.6% vs 3.0%, RR = 16.61, 95% confidence interval (CI): 10.23–26.67, P < 0.05]. Infection by contact with a bleeding corpse showed a higher SAR than infection by blood contact during hospital care (i.e. RR= 1.93, 95% CI: 1.11–3.37, P

Table 2 Relative risk between different routes of transmission among SFTS clusters

Mortality risk factors among clusters

Univariate analysis of risk factors revealed that a longer time interval from onset to diagnosis (you= 796; Pχ2= 4.56; Pyou= 6.09, Pχ2= 11.51, Pχ2= 0.04, P> 0.05). Further details are provided in Table 3.

Table 3 Univariate analysis of risk factors for death in SFTS clusters*

Statistically significant variables in the univariate analysis were included in the binary logistic regression model as independent variables. This model showed that the time interval from onset to diagnosis [odds ratio (OR) = 1.385; 95% CI: 1.083–1.722, P = 0.009] and old age (WHERE= 1.095; 95% CI: 1.031–1.163, P= 0.003) were risk factors for mortality in these clusters. Specifically, the interval from onset to diagnosis and age were positively correlated with mortality risk (Table 4).

Table 4 Logistic regression analysis of risk factors for death in SFTS clusters

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