To our knowledge, this is the first prospective cohort study conducted among a US military population with the primary objective of studying diarrheal disease incidence. Our finding of an overall incidence of diarrhea of 25.2 episodes per 100 person-months (33.0 episodes/person per year) is in close comparison with a recent systematic review of 13 studies from the Middle East and North Africa region, which estimated incidence at 24.3 episodes per 100 person-months.4
Both figures are lower than estimates among non-military long-term traveler populations of Peace Corps volunteers in Guatemala (4.7 episodes/person per year) and expatriates in Nepal (5.9 episodes/person per year), likely because of a number of factors, including a higher public health infrastructure in the military population.10,11
This study confirmed our previous finding that officers were nearly two times as likely to develop diarrhea compared with the enlisted ranks (IRR = 1.99, 95% CI = 1.22–3.26).12,13
Although the exact reason for the increased risk is uncertain, it is likely related to an increased consumption of non-military–provided food. However, a general attitude survey question assessing frequency of eating off-base showed no significant difference between officers and enlisted personnel. We also noted that increase risk was associated with increased age (independent of rank), which is consistent with previous military studies.13,14
Interestingly, the age effect direction among military populations is different to what has been traditionally found in previous non-military cohort studies or among general travelers, where increased risk is associated with younger age and more adventurous travel (i.e., increased risk-taking behavior).10,15
Prior history of diarrhea was also found to be a risk factor for developing diarrhea during the study. Plausible explanations could include differences in susceptibilities of individuals or risk behaviors. The finding of increased diarrhea risk among Caucasian troops compared with their non-white counterparts is interesting and has not been extensively addressed in previous studies. One previous study found an association among US troops between higher levels of pre-deployment serum antibodies against Shigella
anti-lipopolysaccharide (LPS) and non-white race and ethnicity, and it was hypothesized that these persons may have lived in areas with higher levels of transmission of Shigella
spp. and possibly other enterobacteriaceae with cross-reacting antigens.16
A possible extrapolation to the current study could be that the non-white populations were at lower risk because of pre-existing immunity, a potential avenue of further study.
Although incidence estimates and risk seemed to be consistent with published data among US military populations, there were differences in pathogen etiology in this study, with a finding of higher recovery of ETEC and Campylobacter
, than has been previously described in the region.17
This could be because of real increases in prevalence of these particular pathogens or better methods of detection in the current study relative to previous studies in the region. However, the finding of no pathogen detected in 53% of cases is on par with previous studies among similar populations and settings.4
The finding of nearly one of four participants with evidence of protozoa (pathogen and non-pathogenic) suggests that the relatively poor hygienic conditions and fecal oral exposure continues to occur, despite efforts to improve hygiene in the deployed setting. In addition, the 90% fluoroquinolone resistance to Campylobacter
spp. is alarming and supports a continued trend to increasing resistance, which has been recently described among Campylobacter
recovered in Egypt over recent years.15
Self-reported diarrhea incidence was seven times more common than self-reported injury estimates and six times more common than acute respiratory illnesses, two of the most commonly reported health problems in the troops. Days lost were similar for diarrhea and non-combat injury (1.3 days per 100 person-months) and higher compared with respiratory illness (0.9 days per 100 person-months). The similarity in days lost between diarrhea and non-combat injury, despite differential incidence estimates, is likely to be explained by the prompt and effective treatment of diarrhea (with antibiotics and anti-motility agents) that mitigated the numbers of days lost because of diarrheal illness and the considerable morbidity that non-combat injuries can have during deployments.1
Although this study has the strength of prospective cohort design, it is not without limitations. Among the initial enrollees, there were differences in demographic features among those who completed at least one follow-up visit and those that did not, which may result in selection bias. Those who followed up were older and were more likely to be in the medical profession. The effect of this selection bias (for age and profession) is uncertain. Similar to this study, previous studies in the military have found increasing age to be associated with an increased risk of TD, which may represent more freedom to obtain exposure to local food sources.13,14
Although not studied in the military, it could be assumed that, compared with other occupations, those in the medical field are likely to have lower risk of infectious diarrhea (i.e., more familiar/adherent to precautions). Therefore, the selection bias in this study is uncertain; the older age of participants might bias to increased risk, whereas the preponderance of personnel in the medical occupations might bias to decreased risk. Furthermore, we only enrolled approximately 20% of the temporary duty population, which may also limit our ability to generalize these results. For these reasons, caution must be exercised in generalizing these findings, although the results seem to be consistent with the current knowledge of diarrhea epidemiology in deployed US military.
Another limitation included our ability to obtain stool specimens only while troops were at the main camp (number of episodes versus clinic episodes). Although it is assumed that the pathogen distribution between cases that occurred while on an outpost was similar to that in the main camp, this could not be tested. Our microbiological assessment was limited and did not include testing of enteroaggregative E. coli
(EAEC); instead, it relied on phenotypic identification of ETEC, which may have underestimated our detection of diarrheagenic E. coli
Furthermore, our methods of detection for viral gastroenteritidis relied on ELISA-based assays, which are known to be less sensitive than genotypic methods.21
In addition, study subjects often enrolled in the study after they had been deployed an average of 2 months, and therefore, subjects could have missed the period of highest risk for developing diarrhea. If there was differential diarrhea risk during deployment times, this might bias our estimates. However, our study included participants who were enrolled throughout the deployment cycle and thus, should have captured the risk. Lastly, our incidence estimate did not include any observation time during the months of February to April, a period, from our experience, that generally has been associated with lower risk of infectious diarrhea, and this may have resulted in a bias to a higher summary incidence.
Population-based studies on TD, specifically in military populations, are rare. These studies are difficult to perform, particularly in the situation of a military operation in foreign countries. However, to estimate disease risk and provide a platform for primary preventive interventions (e.g., prophylaxis or vaccines), trials and cohort studies need to be conducted. Although this study had limitations, the estimates derived are consistent with our understanding of TD among military populations in this region and provide further evidence that diarrhea among a deployed US military population in Egypt continues to be a common problem, with ETEC being the most frequently identified cause. Illness is reported to affect the ability to work and is seen as an impediment to readiness. Because treatment may not always be practically provided in a timely manner, continued efforts need to be pursued to prevent diarrheal incidence in deployed military settings.