Epidemiologic Investigation of the 2014 Chikungunya Outbreak in the U.S. Virgin Islands

BACKGROUND:  The chikungunya virus (CHIKV) epidemic in the Americas is of public health importance due to the lack of effective control and prevention strategies, the severe acute morbidity of the disease in a fully susceptible population, and the potential for persistent arthralgia to lead to long-term impaired physical functionality of infected individuals. 

METHODS:  Using surveillance data from the U.S. Virgin Islands (USVI) Department of Health (DOH), we describe the epidemiology and clinical manifestations of the outbreak on the USVI, as well as population-level risk factors associated with CHIKV disease. 

RESULTS:  A total of 1,929 suspected cases of CHIKV were reported to the USVI DOH between January 1 2014 and April 6 2015. Due to limited healthcare capacity, 1,039 (53.86%) of the suspected cases had blood specimens tested for CHIKV.  Of all suspected cases with a tested blood specimen, 277 (26.67%) were laboratory-negative and 762 (73.33%) were laboratory-positive. Median age of laboratory-positive cases was 47.9 years, whereas the median age of laboratory-negative individuals was 41.5 years. Individuals aged 65 and older were most affected by the outbreak (12.30 cases per 1,000 residents) compared to individuals aged 0 to 14 and 15 to 24 who were least affected by the outbreak (4.57 and 4.85 cases per 1,000 respectively). A higher percentage of laboratory-positive cases were female (61.58%) compared to male (38.42%) and overall, females had a higher proportion of reported CHIKV disease (7.98 cases per 1,000 residents) compared to males (5.71 cases per 1,000 residents). Laboratory-positive cases were 1.41 (95% confidence interval (CI): 1.13-1.76) times more likely to have myalgia compared to laboratory-negative individuals, 1.13 (95% CI: 1.02-1.24) times more likely to have chills/rigors and 1.13 (95% CI: 1.04-1.23) times more likely to be unable to walk. CHIKV cases were 25% (95% CI: 63%-88%) less likely to have diarrhea compared to laboratory-negative individuals. An increase in population density by 500 persons per square mile increased the risk of being a laboratory-positive CHIKV case by 7.81% (95% CI: 6.15-9.50%) and an increase in housing unit density by 300 houses per square mile increased the risk of being a confirmed case by 10.79% (95% CI: 8.86-12.76%). 

CONCLUSIONS: This analysis identifies important clinical manifestations of CHIKV disease, other than fever >38°C and arthralgia/arthritis, and suggests both individual and population-level risk factors for CHIKV disease and transmission in the USVI, which are crucial aspects to enhancing surveillance systems and mitigating future CHIKV outbreaks globally.