- Statistical analysis of indigenous and imported cases of dengue-fever in Kaohsiung between 2013 and September 2018
1-1. Statistics for total imported dengue-fever cases in Kaohsiung city
A total of 239 imported dengue cases (32, 44, 61, 37, 34 and 31 cases for 2013, 2014, 2015, 2016, 2017 and 2018, respectively) were identified in Kaohsiung during 2013–2018 (Table 1), including 80 women (33.5%) and 159 men (66.5%). The majority of cases involved individuals arriving from Southeast Asian countries; 48 from the Philippines (20.0%), 47 from Indonesia (19.6%), 33 from Vietnam (13.8%), 30 from Malaysia (12.6%), 24 from Thailand (10.0%) and 13 from Singapore (5.4%). Moreover, cases were also imported from the Indian subcontinent (India, Bangladesh, Sri Lanka, and Maldives) and the South Pacific region (Tuvalu).
Further analysis of the 239 imported dengue cases found that the 142 of the infected individuals were Taiwanese nationals who had traveled abroad (59.4%). However, foreign workers and tourists entering the country accounted for a further 15.5% and 8.8% respectively (Table 2). Analysis of patient occupation is included in Table 3, with home management, business and studying the most commonly cited occupations (in total accounting for 46.4% of cases). Figure 1 shows fluctuations in the monthly reporting rates of imported dengue cases in Kaohsiung City (Fig. 1).
1-2. Predominant serotypes of annual indigenous dengue-fever cases in Kaohsiung city
Between 2013 and September 2018 there were 35,148 indigenous dengue-fever incidences in Kaohsiung city, with 70 cases in 2013, 14,999 cases in 2014, 19,723 cases in 2015, 342 cases in 2016, 3 cases in 2017, and 11 cases up until the end of September in 2018 respectively. Primary dengue-fever incidences were concentrated in the 24 consecutive months between May 2014 to April 2016 (Fig. 2). During this period, there were 35,062 indigenous dengue-fever cases in Kaohsiung (99.8%). The major epidemic dengue-fever serotype in 2014 was serotype 1, while the major epidemic dengue-fever serotype from 2015 to April 2016 was serotype 2.
1-3. Concealment of cases for more than 3 days
Further analysis of the 239 confirmed imported cases revealed that 37 infected individuals did not stay within local communities after becoming infected with dengue; this includes 5 of the 32 cases which occurred in 2013 (15.6%); 3 of the 44 cases in 2014 (6.8%); 1 of the 61 cases in 2015 (1.7%); 10 of the 37 cases in 2016 (27.0%); 12 of the 34 cases in 2017 (35.3%); and 6 of the 31 cases in 2018 (19.4%) (Table 4).
We also identified a significant number of cases whose concealment period was greater than 3 days (139/239, 58.16%). Of these, we found 24 such cases in 2013 (75.0% of cases that year), 32 in 2014 (72.7%), 39 in 2015 (63.9%), 22 in 2016 (59.5%), 14 in 2017 (41.2%) and 8 in 2018 (19.4%) (Fig. 3).
- Statistical analysis of the number indigenous and imported cases of dengue-fever in Kaohsiung city between 2013 and September 2018.
The 35,148 confirmed diagnosed indigenous dengue-fever cases collected in Kaohsiung between 2013 and September of 2018, along with the 239 imported cases, were sorted by month of identification (n=69). A significant positive correlation (Paired sample correlation coefficient, r = 0.407, p = 0.001) was found between the numbers of imported and indigenous cases reported each month (Table 5).
However, actual clinical surveillance reports were delayed by around 1 to 2 months during local dengue fever epidemics; furthermore, a concealment period of at least 2 days was common. Instead of investigating correlations between reported cases in the same month, we therefore tested for relationships between the number of imported cases in a single month and the number of indigenous cases reported two months later, after also accounting for differences in the concealment period. We found significant positive correlations between the number of imported cases reported and the number of indigenous cases reported two months later when accounting for concealment periods of more than 2 days (Pearson correlation coefficient, r = 0.459, p = 0.0001), more than 3 days (r = 0.394, p = 0.001) and more than 4 days (r = 0.387, p = 0.001) (Table 6).