Transmission and mortality risk assessment of severe fever with thrombocytopenia syndrome in China: results from 11-years’ study – Infectious Diseases…

Temporal and spatial distribution of SFTS clusters in China

Between 2011 and 2021, 35 SFTS clusters were reported in China, which involved 118 patients, of which 26 died (CFR=22.0%). The CFR was higher among female patients (31.4%, 16/51) than among male patients (14.9%, 10/67). Moreover, the CFR was higher among patients aged60years (35.3%, 24/68) than among patients aged<60years (4.0%, 2/50).

There was an annual increase in the incidence of SFTS clusters, which was the highest in 2020 (n=9), followed by 2018 and 2021 (n=6). The incidence rates of clusters 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), which indicated an epidemic seasonality during summer and autumn.

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

The SFTS clusters were reported in the provinces of Anhui (n=16), Shandong (n=8), Jiangsu (n=4), Zhejiang (n=3), Hubei (n=2), and Hunan (n=2). The number of individuals involved in each cluster ranged from two to twelve persons, with the median number being two. The sex ratio (male/female) of the included patients was 1.311 (67/51). The age range and mean age of the patients were 1884 years and 59.014.2 years, respectively.

The infection routes of the index patients in 14 and 16 clusters were tick bites and suspected tick bites, respectively, with those of the remaining five clusters being unknown. The index patients were exposed to the ticks by picking tea leaves in the tea garden (10.0%, 3/30); farming in the field (10.0%,3/30); weeding and raising livestock in yards or their surroundings (30.0%, 9/30); laboring in the hills (27.0%, 8/30), including hunting, cutting wood, digging trees, picking fruits, and looking for medical herbs; and contact with the blood of a dog infected by tick bites (3.3%, 1/30) or both laboring in the hills and weeding and raising livestock in yards or their surroundings (20.0%, 6/30).

There were 17 clusters that resulted in secondary patients through the index patients via human-to-human transmission. Among them, four occurred in hospitals, three occurred in homes, and the other ten occurred in both hospitals and patients homes. The secondary patients included the primary cases family members, relatives, doctors and nurses, and even fellow villagers. The exposure routes comprised blood contact (i.e. contact with blood or bloody fluids and secretions from the patients) and non-blood contact (i.e. contact with patients fluids or secretions other than blood or inhalation of Brucella-containing aerosol) while providing care for the index patients, transferring dying patients with hemorrhagic clinical manifestation, or during burial preparations. Nosocomial infection occurred in two clusters, which involved one doctor and one nurse in each cluster. The doctor was exposed while performing a sputum suction operation without a closed sputum suction tube and/or touching the patients blood without personal equipment protection (PEP). The nurse was infected while changing sheets contaminated with fresh blood from the same patient; however, she wore gloves without wearing mask, indicating possible infection by aerosol inhalation. Another doctor and nurse were infected through non-blood contact while providing medical care without any PEP to another patient. The transmission routes of two clusters that involved eleven and seven secondary patients with nosocomial infection are illustrated in Fig.2A and B, respectively.

A Transmission routes for one SFTS cluster in Anji County, Zhejiang Province, 2014. B Transmission routes for one 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 BPatient L); among them, nine patients were infected by blood contact while the other two patients were infected through inhalation of Brucella-containing aerosol in a confined mourning room, without direct contact with the patient or other possible exposure. All the secondary patients did not wear personal protection equipment during the exposure. The index patient had been exposed to a tick bite while picking tea leaves on the tea garden. The serum positive detection rates of SFTSV IgG 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 male farmer who was 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. SFTS severe fever with thrombocytopenia syndrome, SFTSV severe fever with thrombocytopenia syndrome virus

Among 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.

The median numbers of infected individuals among the clusters with and without secondary human-to-human transmission were 2.0 (2.02.0) and 3.0 (2.06.0), respectively (U=71.00, P=0.003). The transmission model of SFTS clusters with and without secondary human-to-human transmission are summarized in Fig.3.

Transmission model and risk of different human-to-human transmission modes among SFTS in China. Note: The left picture describes the 30 index patients exposure ways to SFTSV. All were exposed during their routine laboring related with agriculture. There are six index patients exposed to confirmed or suspected tick bites during both laboring in the hills and weeding and raising livestock in yards or their surroundings. SFTS severe fever with thrombocytopenia syndrome, SFTSV Severe fever with thrombocytopenia syndrome virus, SAR the secondary attack rate

Infection through blood contact showed a higher SAR than infection through non-blood contact [50.6% vs 3.0%, RR=16.61, 95% confidence interval (CI): 10.2326.67, P<0.05]. Infection through contact with a bleeding corpse showed a higher SAR than infection through blood contact during hospital care (i.e., contact with a living patients blood, bodily fluids, or secretions) (66.7% vs 34.5%, RR=1.93, 95% CI: 1.113.37, P<0.05), as shown in Table 2 and Fig.3.

Univariate analysis of risk factors revealed that longer time interval between onset and diagnosis (U=796; P<0.05), higher sex ratio (male/female) (2=4.56; P<0.05), and older age (t=6.09, P<0.05) were observed in the group with dead patients than in that with cured patients. There was a significant between-group difference in the infection routes (2=11.51, P<0.05) but not in occupation (2=0.04, P>0.05). Further details are provided in Table 3.

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: 1.0831.722, P=0.009] and old age (OR=1.095; 95% CI: 1.0311.163, P=0.003) were mortality risk factors in these clusters. Specifically, the interval from onset to diagnosis and age were positively correlated with the mortality risk (Table 4).

Continue reading here:
Transmission and mortality risk assessment of severe fever with thrombocytopenia syndrome in China: results from 11-years' study - Infectious Diseases...