Seasonality, annual trends, and characteristics of dengue among ill returned travelers, 1997-2006

Abstract

We examined seasonality and annual trends for dengue cases among 522 returned travelers reported to the international GeoSentinel Surveillance Network. Dengue cases showed region-specific peaks for Southeast Asia (June, September), South Central Asia (October), South America (March), and the Caribbean (August, October). Travel-related dengue exhibited annual oscillations with several epidemics occurring during the study period. In Southeast Asia, annual proportionate morbidity increased from 50 dengue cases per 1,000 ill returned travelers in nonepidemic years to an average of 159 cases per 1,000 travelers during epidemic years. Dengue can thus be added to the list of diseases for which pretravel advice should include information on relative risk according to season. Also, dengue cases detected at atypical times in sentinel travelers may inform the international community of the onset of epidemic activity in specific areas.

Figure 1

Dengue fever in returned travelers as a proportion of monthly morbidity in all ill returned travelers to all regions of the world. Cumulative proportionate morbidity of 21 per 1,000 ill returned travelers (522 dengue reports among 24,920 ill travelers from October 1997 through February 2006) is shown by the horizontal line. *Proportionate morbidity is expressed as number of dengue cases per 1,000 ill returned travelers.

Figure 2

Changes in dengue morbidity during regional epidemics. Heavy red and blue lines show dengue in returned travelers as a proportion of monthly morbidity in all ill returned travelers to Southeast Asia (A) and Thailand (B) during the epidemic years of 1998 and 2002 (red lines) and during all other nonepidemic years (blue lines). Black horizontal dashed lines represent mean proportionate morbidity over all months for that area during the cumulative 1997–2006 period in travelers; red horizontal dashed lines represent mean proportionate morbidity over all months during the 2 outbreak years (1998 and 2002) in travelers. Each gray line in panel B tracks month-by-month reports to the World Health Organization (WHO) of the total number of dengue cases in the endemic Thai population for a single year from 1998–2005. *Proportionate morbidity is expressed as number of dengue cases per 1,000 ill returned travelers.

Figure 3

Seasonality of dengue in returned travelers by region. Dengue in returned travelers is shown as a proportion of monthly morbidity in all ill returned travelers to each region. Horizontal dashed lines represent the mean proportionate morbidity over all months for that region during the cumulative 1997–2006 period in travelers. Data for Southeast Asia exclude the outbreak years of 1998 and 2002. *Proportionate morbidity is expressed as number of dengue cases per 1,000 ill returned travelers.