Norovirus Outbreak Aboard USNS Mercy during Pacific Partnership 2015

Abstract

Background. USNS Mercy is a US Navy hospital ship routinely deployed to Southeast Asia to provide humanitarian assistance and disaster response support. Norovirus (NoV) infection is characterized by acute onset of nausea, vomiting, abdominal cramping, and non-bloody diarrhea. Both hospitals and ships are prone to NoV outbreaks. Attack rates (alpha) in excess of 40% have been reported aboard military and civilian ships. A NoV outbreak was detected aboard USNS Mercy while in Roxas City, Philippines, in July 2015. This is the first known shipboard NoV outbreak where real-time molecular surveillance testing was used to facilitate detection and guide control measures. Methods. Syndromic diarrhea surveillance was performed during the Pacific Partnership mission from 17 May 2015 to 29 September 2015 and included all personnel that presented to sick call with the chief complaint of nausea, vomiting, or diarrhea. Multiplex PCR testing was used to identify infectious agents when increased rates of diarrhea were observed. Basic demographic and clinical data were collected prospectively. Early, aggressive control measures including isolation, hand washing campaigns, screening of food handlers, and decontamination of high traffic areas were implemented. Results. A NoV outbreak was detected from 17 July 2015 to 3 August 2015. The baseline incidence of GI-related illness was 0.4% (sigma +/- 0.3%). Patients who had diarrhea and nausea/vomiting were included as cases. A total of 72 cases were identified. The average census during this period was 1023 (alphao = 7.0%); 5/6 stool samples tested were positive for NoV. Of the cases, 85% of patients presented to medical attention within the first 48 hours after symptom onset. Also, 36% of cases were identified from a single berthing space (alphab = 25%), and 69% of all cases were from berthing spaces connected by a common ladder well. Conclusion. Hospital ships are particularly vulnerable to diarrheal outbreaks during humanitarian or disaster relief missions that can impact mission readiness and capacity. Implementation of a vigilant surveillance system in coordination with point-ofcare molecular diagnostics allowed for early detection and aggressive infection control measures to mitigate transmission and limit the infectious burden during a NoV outbreak aboard USNS Mercy.