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Traveler's Diarrhea Among Soldiers Linked to Gut Microbiome Changes

NEW YORK – Soldiers who contracted traveler's diarrhea had different levels of microbes present in their gut even before they fell ill, a new study has found.

In addition to affecting tourists, traveler's diarrhea affects soldiers posted around the world. About 40 percent of US soldiers deployed to Egypt in 2005, for instance, reported bouts of traveler's diarrhea, while nearly 80 percent of US troops in Iraq and 54 percent of US troops in Afghanistan between 2003 and 2004 reported gastrointestinal illness.

As a person's gut microbiome is thought to influence a number of diseases including traveler's diarrhea, researchers from the US Naval Medical Research Center and elsewhere collected data on the prevalence of gastrointestinal illness among troops deployed to Honduras in 2015 and 2016 and analyzed stool samples from soldiers with and without traveler's diarrhea. As they reported Wednesday in PLOS One, the researchers found that enteroaggregative Escherichia coli (EAEC), enterotoxigenic E. coli (ETEC), and enteropathogenic E. coli (EPEC) were common among the gut microbiomes of afflicted soldiers. In addition, a longitudinal analysis of soldiers' gut microbiomes found the presence of certain bacteria appears protective.

"Gaining a better understanding of how microbial community composition affects military personnel experiencing diarrhea during deployment is important for strategic decisions regarding potential medical interventions, such as use of probiotics as prophylaxis and/or post-infectious therapy," senior author Mark Simons from the Naval Medical Research Center and his colleagues wrote in their paper.

The researchers initially surveyed soldiers following the end of their deployments to Honduras to inquire whether they had experienced symptoms of traveler's diarrhea. Of the 1,173 respondents, a quarter reported experiencing traveler's diarrhea during their deployment.

At the same time, the researchers conducted a passive surveillance study of 152 soldiers who reported to the clinic with signs of gastrointestinal illness. Stool cultures from these individuals uncovered two cases of Shigella and one case of Salmonella infections. Additional testing using Biofire's FilmArray GI assay uncovered diarrheagenic E. coli pathogens among these samples, including EAEC, ETEC, EPEC, and Shiga-like toxin-producing E. coli. Shigella was found in 18 cases through this nucleic acid-based approach, as compared to the two discovered via culture.

They further recruited 67 soldiers — all of whom lived on base — to an active surveillance study at the start of their deployment. As part of this analysis, the soldiers were queried weekly about any symptoms of gastrointestinal illness and, if they had any, were asked to provide a stool sample for analysis. Seventeen cases of diarrhea were reported in this way, for an incidence rate of nearly three cases per person-month. Similar to the passive surveillance study, diarrheagenic E. coli pathogens were most commonly present, though the researchers noted co-infections in the majority of cases, suggesting a complex disease etiology.

Eleven soldiers were additionally enrolled into a microbiome study in which the researchers collected weekly stool samples for 16S rRNA sequencing, regardless of the appearance of any symptoms. During the study timeframe, four of the 11 individuals in the study developed traveler's diarrhea.

In general, the researchers noted that each individual's microbial signature remained consistent over time. When they focused on two individuals with the most diarrhea-related clinic visits, they noted that Enterobacteriaceae levels were high in the weeks with bouts of traveler's diarrhea, as the researchers noted would be expected with Escherichia infections. 

They also compared the microbiomes of the soldiers who developed traveler's diarrhea to those who did not to find a higher Firmicutes-to-Bacteroidetes ratio among those who remained healthy. Additionally, taxa in the Ruminococcaceae family were differentially abundant between healthy individuals and those who fell ill. As Ruminiclostridium produces short-chain fatty acids, the researchers suspect that it can boost epithelial integrity and protect against disease.

"These findings illustrate the complex etiology of diarrhea amongst military personnel in deployed settings and its impacts on job performance," Simons and his colleagues wrote. "Potential factors of resistance or susceptibility can provide a foundation for future clinical trials to evaluate prevention and treatment strategies."

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