[Epidemiological investigation on an outbreak of severe fever with thrombocytopenia syndrome in northwest Zhejiang province].
To investigate the source, transmission route and risk factors of an outbreak of severe fever with thrombocytopenia syndrome (SFTS). Case definition was made and suspected cases were searched. A standardized questionnaire was used to collect information on demographic features (age, gender, occupation, residential address), history of exposure, clinical signs and symptoms etc. Blood samples were collected from 12 suspected cases while index patient's blood samples were collected from walls of the residence. All samples were detected for SFTS virus using RT-PCR. Sero-prevalence rates of SFTS virus IgG were also conducted among healthy people, host and vectors. A total of 13 cases including 6 male and 7 female were identified during this outbreak in May 2014. Index patient developed illness onset on April 23 and died on May 1. Secondary patients would include 8 family members, 3 neighborhoods, 1 individual who lived in the same village, developing illness onset between May 10 and 16, with a peak on May 13. The incubation period was 9-15 days. Clinical signs and symptoms appeared as fever (100%), chill (92%), anergy (92%), body aches (92%), anorexia (92%), headache (77%), nausea (69%) etc. Neutropenia and thrombocytopenia also appeared. History of the index patient showed that she collected tea leaves in her hometown 1 month before the illness onset. After index patient died on May 1, 9 secondary patients had directly contacted the blood of the deceased. Data from the retrospective cohort study showed that 'direct contact with blood' was an important risk factor (RR = 43.36, 95% CI: 13.66-137.63, P = 0.000). Majority of the secondary patients of these clusters contracted the SFTS virus infection through exposure to the blood of the index patient. However, aerosol transmission could not be ruled out, suggesting that precaution should be taken for doctors, nurses and family members when looking after the patients with SFTS virus infection.