Dengue Sentinel Traveler Surveillance: Monthly and Yearly Notification Trends among Japanese Travelers, 2006-2014

Abstract

Background: Dengue is becoming an increasing threat to non-endemic countries. In Japan, the reported number of imported cases has been rising, and the first domestic dengue outbreak in nearly 70 years was confirmed in 2014, highlighting the need for greater situational awareness and better-informed risk assessment.

Methods: Using national disease surveillance data and publically available traveler statistics, we compared monthly and yearly trends in the destination country-specific dengue notification rate per 100,000 Japanese travelers with those of domestic dengue cases in the respective country visited during 2006-2014. Comparisons were made for countries accounting for the majority of importations; yearly comparisons were restricted to countries where respective national surveillance data were publicly available.

Results: There were 1007 imported Japanese dengue cases (Bali, Indonesia (n = 202), the Philippines (n = 230), Thailand (n = 160), and India (n = 152)). Consistent with historic local dengue seasonality, monthly notification rate among travelers peaked in August in Thailand, September in the Philippines, and in Bali during April with a smaller peak in August. While the number of travelers to Bali was greatest in August, the notification rate was highest in April. Annually, trends in the notification rate among travelers to the Philippines and Thailand also closely reflected local notification trends.

Conclusion: Travelers to dengue-endemic countries appear to serve as reliable "sentinels", with the trends in estimated risk of dengue infection among Japanese travelers closely reflecting local dengue trends, both seasonally and annually. Sentinel traveler surveillance can contribute to evidence-based pretravel advice, and help inform risk assessments and decision-making for importation and potentially for subsequent secondary transmission. As our approach takes advantage of traveler data that are readily available as a proxy denominator, sentinel traveler surveillance can be a practical surveillance tool that other countries could consider for implementation.

Fig 1. Distribution of reported imported dengue cases by sex and age, Japan, 2006–2014 (n = 1007).

Fig 2. Number of reported imported dengue cases by year and country visited, Japan, 2006–2014 (n = 1007).

Fig 3. Number of reported imported dengue cases aggregated over month by country visited, Japan, 2006–2014 (n = 1007).

Fig 4. Number of reported imported dengue cases per 100,000 travelers aggregated over month by country visited, Japan, 2006–2014.

For Indonesia, monthly data for Japanese travelers were only available for Bali province. Philippines data were restricted to those from 2006–2013 as monthly traveler data were not available for 2014. Monthly Japanese traveler data were not available for India.

Fig 5. Number of reported imported dengue cases per 100,000 travelers by year and country visited, Japan, 2006–2014.

Data for Japanese travelers visiting India and Indonesia were not available for 2014.

Fig 6

Number of domestic dengue cases reported from (A) the Philippines and (B) Thailand, and the annual number of imported dengue cases per 100,000 travelers visiting each country, 2006–2014.