Gastrointestinal illness (GII) in military personnel can have a serious impact on missions both in and out of combat zones. In late 2016, a Salmonella outbreak at a major Army installation, coupled with multiple reports of Salmonella at other Army installations, led to heightened concern surrounding GII among US Army Medical Command leadership. Cases of GII are reported to the Army Public Health Center daily and compiled into weekly and monthly reports, but no threshold exists to detect outbreaks. The objective of this analysis was to determine the burden of gastrointestinal illness among US Army soldiers over the past 4 years and to develop an epidemic threshold to warrant a formal outbreak investigation and response efforts.
GII case counts among Active Duty Army soldiers were obtained from the Disease Reporting System Internet (DRSi), the Army’s disease surveillance system, for the period of January 2013 to December 2016. Incidence rates among Army soldiers were compared to national rates published by the Centers for Disease Control and Prevention. The probability of seeing a specific number of cases was calculated by dividing the number of times that each case count was observed by the of number of time points in the time series. For each of the three most common GI pathogens, null Poisson and negative binomial regression models were fit and compared in order to establish alarm thresholds indicating statistically abnormal high weekly case count levels that could be suggestive of an outbreak event.
There were a total of 635 cases of GII among Army soldiers reported in DRSi during the study period. The majority of the cases were Campylobacter (42.4%), closely followed by Salmonella (39.5%). The weekly case count that fall within the top 10th percentile for Campylobacter, Salmonella, and Shigella are 3 cases, 3 cases, and 2 cases, respectively. The weekly case count that would fall within the top 5th percentile response for Campylobacter, Salmonella, and Shigella are 4 cases, 4 cases, and 2 cases, respectively.
Public health practitioners can use these thresholds, in collaboration with clinicians at Military Treatment Facilities, to improve GII surveillance and outbreak response protocols.