On August 28, 2019, Jiangxi Province Center for Disease Control and Prevention (Jiangxi CDC) received a report of a suspected dengue case from Xin’gan County, and a second suspected case was reported from the same county a day later. The two cases were identified as dengue virus infections cases on August 29 and prompted the deployment of a team composed of an epidemiologist and a vector control specialist to Xin’gan County to provide health support, to investigate the infection source, to assess the effects of mosquito control, and especially to assess the risk of regional transmission.
On August 29, 2019, the first local suspected dengue case in Xin’gan County was laboratory-confirmed in a patient with symptoms including fever, dizziness, headache, fatigue, and rash. The first case occurred in a 55-year-old male patient without recent travel history. The real-time RT-PCR test result suggested his blood sample was positive for DENV, and additional sample testing identified DENV-1.
During investigation two suspected dengue cases were found in brothers aged 13 and 14 and were likely the initial cases. They presented dengue fever-like symptoms on August 10 and 11 including fever, headache, and muscle or body aches. In late July, they followed their grandfather to visit their parents for summer vacation in Shiling Town, Huadu District, Guangzhou City. They returned to Xin’gan County on August 7 due to a local dengue fever outbreak in Shiling Town. On August 30 and 31, their grandparents presented dengue fever-like symptoms and tested positive for DENV on September 2.
The time distribution indicated that the date of onset of the first case was August 23, and the last case reported was September 8. The incidence was bimodal with the first peak from August 31 to September 2 (26 cases) and second peak from September 4 to September 5 (15 cases). The number of cases decreased rapidly after September 6 (Figure 1).Figure 1.
The onset time curve among the outbreak cases of dengue fever in Xin’gan County, Jiangxi Province, 2019. The date of onset of the first case was August 23, and the last case reported was September 8. After implementation of emergency mosquito control and clearing of breeding places led by the local government, the number of cases decreased rapidly after September 6.
The outbreak occurred in Jinchuan Town, Xin’gan County, including Huachengmen Village, Mexiang, Zhongshan, Binyang, Chengnan, Hejiashan, Chengbei, Shanzheng, and Wenchang communities. Overall, 70 cases (86.4%) occurred in Mexiang, Zhongshan, Binyang, and Chengnan communities and were surrounded by Binjiang Avenue, Yaocai Street, Shangye Road, and Chuannan Road covering an area of about 1.2 km2.
Most of the patients experienced atypical symptoms mainly including influenza-like symptoms such as fever, body ache, chills, headache, fatigue, etc. (Table 1).
Symptoms No. of cases Percentage/% Fever(≥38 ℃) 63 77.8 Body aches 35 43.2 Chills 31 38.3 Headache 25 30.8 Fatigue 24 29.3 Muscle pain 12 14.8 Arthralgia 9 11.1 Bleeding point of skin 4 4.9 Rash 2 2.5
Table 1. The main clinical symptoms among the outbreak cases of dengue fever in Xin’gan County, Jiangxi Province, 2019.
The median interval of time from onset to diagnosis was 2 days (1–8 days), and 23 (28.4%) cases were diagnosed at 4 days or longer after onset of symptoms. Doctors had a significantly improved capacity to diagnose dengue fever after training as the median onset-to-diagnosis interval of patients that were referred was found to be significantly longer than that of those who were not referred (Table 2). SPSS software (version 24.0, IBM Corporation, New York, USA) was used to carry out statistical analysis on data, non-parametric rank sum test were used for comparing population means among different groups if the normality assumption was violated, and the significance test level is α=0.05.
Characteristics The median interval (days) p value Hospital type Hospital 3(1–7) 0.130 Clinic 3(1–8) Referral status No 1(1–7) 0.002 Referral 3(1–8) Training Before 4(2–7) 0.004 After 2(1–8)
Table 2. The median onset-to-diagnosis time interval among the outbreak cases of dengue fever in Xin’gan County, Jiangxi Province, 2019.
The dynamic density of the Aedes vector was investigated daily by the Breteau index (BI), which investigates the number of positive containers per 100 houses, in the epidemic area to begin the adoption of control measures. At the beginning, the mean BI in the epidemic area was 24.45 with the highest BI being 48.00 in Meixiang Community and the lowest BI being 14.01 in Binyang Community. An elevated BI is consistent with an increased risk for DENV transmission and demonstrates that Xin’gan County was at risk for local DENV transmission. The BI quickly declined below the safety thresholds 2.55 days after implementation of emergency mosquito control and clearing of breeding places led by the local government.
All cases presenting dengue fever-like symptoms were tested with a dengue rapid diagnostic test (RDT) for DENV nonstructural protein 1 (NS1). Specimens that were RDT positive were further tested by real-time RT-PCR. From August 29 to September 10, 2019, a total of 510 samples were tested for evidence of DENV infection (all by RDT-NS1 alone, and 35 by both methods), 81 (15.9%) of whom tested positive for dengue. By September 10, 2019, 35 cases were laboratory-tested confirmed as being infected with DENV-1. Therefore, 35 out of 81 patients were confirmed by real-time RT-PCR tested results and the remaining 46 patients were clinically diagnosed.
The E genes of some strains were sequenced to construct a phylogenetic tree with the length of the amplified fragment being 1,782 base pairs (bp). The analysis result indicated that about 99.8% to 100% of the 35 strains of DENV-1 isolated from the patients in Xin’gan County were highly homologous. Compared to some strains isolated domestically and overseas, the virus strains from Xin’gan county in 2019 were closest to the strains isolated in Singapore (MF033254|25657|Singapore|2016) and Henan (MK905537|Henan201903|China: Henan Province|2019), and the strains isolated in cases imported from Cambodia to Jiangxi Province in 2019 had a homology of 99.4%, 99.6%, and 99.6%–100%, respectively.