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INCIDENCE, ETIOLOGY, AND IMPACT OF DIARRHEA AMONG LONG-TERM TRAVELERS (US MILITARY AND SIMILAR POPULATIONS): A SYSTEMATIC REVIEW

ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
To determine regional estimates of pathogen-specific prevalence and incidence, as well as, describe morbidity associated with diarrhea among deployed US military and similar populations, a systematic review was conducted for publications between January 1990 to June 2005. Point estimates and confidence intervals of pathogen prevalence and travelers’ diarrhea incidence were combined in a random effects model and assessed for heterogeneity. In total, 262 studies were identified for potential inclusion, of which 52 fulfilled inclusion criteria. Overall, 38% were from the Middle East, 29% from Southeast Asia, 27% from Latin America/Caribbean, and 6% from sub-Saharan Africa. Median duration of travel was 1.5 months (interquartile range, 1–3 months). Enterotoxigenic Escherichia coli (ETEC), Campylobacter, and Shigella were identified as causing 38–45% of diarrhea, with regional and population differences. Incidence based on self-report was higher than studies using passive surveillance or clinic-based methods (29 versus 7 versus 6 episodes per 100 person-months, respectively) without regional differences.

INTRODUCTION

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Infectious diarrhea continues to be one of the most common problems facing travelers abroad. A distinction is sometimes made in risk and/or pathogen distribution between short-term (< 2 weeks) travelers and populations living overseas for extended periods, such as military personnel, expatriates, students, and Peace Corps volunteers.^1–4 Epidemiologic studies of infectious diarrhea in deployed military account for a majority of the published experience given the well-recognized continued threat.^5–7 Studies evaluating disease and non-battle injury rates in recent peacetime and combat operational settings have consistently identified infectious gastrointestinal illness in the top five reasons for clinic visits.^8–14 Because the increasingly global economy has led to both an increase in short-term travelers and an increase in populations from developed countries moving to and residing for lengthier stays in developing countries, it is important to determine whether there are differences in the epidemiology of diarrhea in these groups.

Black¹⁵ summarized pathogen etiology and attack rates by select geographic regions. This review was not limited to military and similar long-term traveler populations, nor did it report on disease morbidity. Furthermore, due to the date of the review, diagnostic sensitivities now available with the development of PCR and other molecular diagnostic techniques for enteric pathogen identification were lacking.^15,16 Furthermore, no studies have attempted to use a systematic methodology to combine estimates of disease incidence, morbidity, and treatment outcomes to summarize and quantify pathogen-specific disease burden in selected geographic regions. The primary objectives of this study were to determine updated regional estimates of diarrheal disease incidence and pathogen-specific prevalence and describe morbidity and treatment outcomes among long-term travelers, including US military and similar populations, through the use of a systematic review of the scientific literature.

MATERIALS AND METHODS

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The study design is a systematic review of the scientific literature based on accepted principles of good methodological design.^17,18 The systematic review included eligibility criteria for available evidence, standardized data abstraction, critical appraisal of the quality of the evidence, and standard methods of data analysis. Further description of these methodological components is as follows.

Search strategy and study selection. A comprehensive retrieval of information was conducted using a stepwise procedure of searching personal files, perform searches on electronic bibliographic databases (including MEDLINE, EMBASE, CINAHL, and the Cochrane Library), and hand-searching bibliographies of retrieved articles, technical reports (including Defense Technical Information Center, National Technical Information Service), and doctoral dissertations. All searches were conducted starting with the term travelers’ diarrhea or diarrhea and then followed by the addition of the following terms: epidemiology, etiology, military, peace corps, expatriate, incidence, burden, morbidity, and treatment. In addition, MEDLINE searches were conducted using major MeSH headings (medical subject headings) determined from articles known to be eligible. All publications and reports published between January 1, 1990 and June 30, 2005 were screened by a single reviewer to determine if they met the eligibility criteria. Those deemed to be irrelevant were excluded, and reasons for exclusion were noted. When the information provided by the titles and/or abstracts was inadequate to decide on eligibility, the full-text article was retrieved and evaluated. Review articles were obtained for the purpose of screening reference lists.

Based on the goal and specific aims of this systematic review original research in the form of observational cohorts, surveys, database analyses, or clinical trials published in English and conducted on long-term travelers (including US military or other similar traveler populations) were considered for inclusion. Similar traveler populations were defined as expatriates of a developed country living abroad in an under-developed country, as well as any traveler in country for a month or longer. All studies involving US military, regardless of travel duration, were eligible. Studies involving tourists and short-term business travelers were excluded. Justification for the criteria of study selection for these eligible populations was based on the primary interests of developing stable estimates of disease incidence among long-term traveler populations finding (e.g., living conditions, risk profile). Studies were categorized to geographic regions of sub-Saharan Africa, Latin America and the Caribbean, Southeast Asia, and the Middle East based on logical geographic regions and the convention of previous reviews. Only studies detailing information for variables of interest were included.

Data abstraction/validation. Data from obtained articles and reports were abstracted using a pre-tested, standard data abstraction form. Bibliographic information, study design description, study years, geographic location, population characteristics, primary outcome measures, and other study characteristics (e.g., follow-up period, case definition used) necessary to answer the key questions and to evaluate heterogeneity were included in the data abstraction form. Pathogen prevalence was abstracted as a percent of total cases along with the study denominator that was used to compute the prevalence. For consistency, pathogen prevalence was abstracted based on tables or text reporting the number of diarrheic samples in which a particular pathogen was isolated. Because it is difficult to determine the exact etiology when more than one pathogen is found, prevalence was reported as number of cases infected with a particular agent inclusive of cases with multiple pathogens. In studies that were clinical trials involving antimicrobial prophylaxis, the placebo control arm was used to estimate pathogen prevalence. Prevalence of multiple pathogens was also abstracted when available. Incidence was abstracted as an event number and person-time as the denominator when available. The source of the event number was also recorded as either based on self-report, clinic-based case series, or disease and non-battle injury (DNBI). For studies conducted over a period of more than 1 year, the mid-point of the study period was used for analysis.

To evaluate how the validity of study design may affect interpretation of the results, each article was scored for quality by two reviewers using a standardized grading criteria that was specifically developed for prevalence and incidence systematic reviews.¹⁹ These grading criteria placed primary emphasis on domains of study design and sampling method, sample size, standardization and unbiased collection of outcome measures, adequate response rate, appropriate analysis, and applicability of the study population. For each validity domain, an ordinal score of 0, 1, or 2 depending on whether the criteria was "not met," "partially met," or "fully met" was assigned. All domains were assumed to be of equal importance to the validity of the study and were summed to create an overall quality score. Inter-rater reliability between scorers where assessed using a quadratic weighted statistic. Abstraction and quality scoring were not blinded. Accuracy of data abstraction was reviewed and validated for all articles and abstraction forms by duplicate review. Data was entered into a database, and a 100% check for accuracy of entry was performed by visual confirmation of each abstraction form.

Analysis. The analysis of pathogen prevalence and incidence was stratified by region as geographic differences have been previously described.^8,15 A primary goal of this study was to define point estimates and confidence intervals for pathogen prevalence and incidence to be used later in an economic analysis. Furthermore, because of known variations in study design, methodologies, population characteristics, and other factors, heterogeneity of prevalence and incidence estimates across studies was expected and assessed graphically through the use of Forest plots and statistically through the use of heterogeneity statistics and non-parametric methods. For purpose of summary, point estimates and standard 95% confidence intervals were combined using a random effects model with methodology developed by DerSimonian and Laird²⁰ and reporting point estimates with 99% confidence intervals. This method is considered more conservative compared with a fixed effects model and weights studies by both sample size and between-study variance. The use of 99% confidence intervals also assures a more robust estimate of any given prevalence or incidence.

Heterogeneity was tested using a ² heterogeneity statistic, and potential sources of heterogeneity were assessed graphically by Forest plots and using non-parametric methods (e.g., Kruskal-Wallis, Mann-Whitney U test) to compare differences in prevalence or incidence between two or more groups of a given population or study characteristic. In the case of parameters where only a few studies were found (e.g., probabilities and outcomes associated diarrhea and treatment), a median and range of estimates are reported. As a principle purpose of this systematic review was to summarize studies reporting pathogen prevalence and diarrhea incidence (not an evaluation of intervention effectiveness), publication bias was not assessed, because the concern for non-published findings caused by negative studies or disappointing results was considered to be minimal.

All analyses were conducted using Stata V9 (College Station, TX).

RESULTS

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In total, 262 studies were identified as eligible, of which 49 articles fulfilled all criteria and were suitable for inclusion in the analysis, abstracted, and scored for quality. The study selection process is further detailed in Figure 1. One study reported incidence estimates and treatment probabilities stratified by three regions and one study reported pathogen prevalence distributions stratified by two regions. These studies were abstracted separately for each region and included as if they were individual studies in the analysis. Table 1 provides descriptive details of the 52 included studies.

View larger version (26K): [in this window] [in a new window]       FIGURE 1. Flow diagram of study selection for inclusion in the systematic review.

Study quality. Agreement of quality scores assigned by two observers were compared using a quadratic weighted and was found to be good ( = 0.73), with scores ranging from a low of 3 to a maximum of 14 (of 16), with a median of 8 (IQR, 6–10) for both reviewers. Quality scores were averaged between observers for the remaining analysis. Quality domains that consistently scored well across studies (median values > 1) were the use of standard outcome measures and applicability of study population, whereas the analysis quality domain was lower across all studies (median values < 1). Overall study quality scores were found to be associated with factors related to study design and study population. Studies using a mixed design (N = 5) had a better overall median scores (11; IQR, 10–11) compared with other study designs (8; IQR, 6–9.5; Mann-Whitney U, P = 0.01). Studies conducted among US military populations had higher median total quality scores compared with non--US military studies (median, 9 versus 7.5; Mann-Whitney U, P = 0.007). There were no differences in overall quality score by geographic region or year of publication.

Pathogen prevalence. Summary estimates of pathogen prevalence by region are detailed in Table 2. Overall, we found regional differences in pathogen distributions of ETEC (P = 0.02), Campylobacter (P = 0.001), and Salmonella (Kruskal-Wallis, P = 0.001). The differences seem to be because of Southeast Asia having a relatively lower prevalence of ETEC and a higher prevalence of Campylobacter and Salmonella compared with other regions. ETEC was the most common pathogen identified in Latin America and the Caribbean and the Middle East, accounting for 29% and 28% of cases, respectively. Whereas Campylobacter accounted for nearly one quarter of all cases in Southeast Asia, ETEC was also quite common, accounting for nearly one of every six cases. The two studies from sub-Saharan Africa describe ETEC and Shigella as important pathogens, accounting for approximately 16% and 9% of pathogens, respectively. Salmonella was reported in a majority of studies in each of the regions and was highest in Southeast Asia (11%) compared with regions of the Middle East (1%) and Latin America and the Caribbean (3%). Other bacterial and viral pathogens were inconsistently reported across studies within regions; however, pooled summary estimates of prevalence for EAEC was 8–19%, norovirus was 4–13%, and rotavirus was 2–6%. Multiple pathogens were also common and higher in Southeast Asia, accounting for 16%, compared with a frequency of 7–9% in the other regions (excluding sub-Saharan Africa, which reported 4% and 13% in two studies), although this difference was not statistically significant.

Incidence. Incidence estimates were extracted for 32 studies. As with pathogen prevalence, there was considerable heterogeneity between studies used to estimate diarrhea incidence (² heterogeneity statistic, P < 0.001). Table 2 describes the summary incidence estimates stratified by passive (clinic-based studies, DNBI) and active (cohort studies, self-report) surveillance ascertainment methodology by region, for which there did not seem to be any association between incidences graphically or statistically (data not shown). There was a higher incidence based on how the incidence measurements were ascertained, with pooled summary incidence estimates among studies that based incident events on self-report (e.g., post-deployment/travel questionnaires and cohort studies) being higher (29 cases per 100 person-months) compared with DNBI-based (7 cases per 100 person-months) and case-surveillance study estimates (6 cases per 100 person-months; Kruskal-Wallis test, P = 0.001).

Additionally, an association was noted with higher incidence in populations that were not in the military compared with other foreign and US military populations (Kruskal-Wallis, P = 0.04). However, this association may be confounded because more non-military population studies ascertained incidence estimates based on designs using self-report. No association of differential incidence with other variables such as study design, quality, use of standard definition, or duration of travel were identified.

Twelve studies reported data to estimate the probability that an individual might seek treatment if they became ill with diarrhea (Table 3). Eight of these included estimates of self-reported incidence and clinic-based incidence (visits to a medical treatment facility). Overall, it seemed that a median of 23% (IQR, 12–29%) of individuals who became ill with diarrhea sought treatment at a medical treatment facility. No differences in the probability of seeking treatment could be explained.

DISCUSSION

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An additional important finding was that studies conducted in the Southeast Asia region showed a trend toward having higher pathogen recovery rates compared with the other regions (61% versus 50%, P > 0.05). This finding could be explained by factors involving the characteristics of studies in this region or other factors inherent to the region. There were no differences between study design or quality by region, but there were more US military studies conducted in the Southeast Asia region compared with other regions (80% versus 56%, P = 0.19), and this may confound the trend toward finding an association of higher pathogen recovery rates in Southeast Asia compared with other regions. This suggestive regional association might also be explained by regional differences in pathogen etiologies. Campylobacter is known to cause more severe disease than most other common diarrheal pathogens.^31,36,40,41 Therefore, it could be hypothesized in regions where there is a predominance of Campylobacter infections (or more severe diarrhea), more patients may be presenting for treatment, and subsequently, a pathogen, particularly Campylobacter, is more often identified. Extractable data on severity of diarrheal disease were not available for all studies; thus, an assessment for severity of disease could not be evaluated for association with prevalence of pathogen identified.

While there were regional differences in pathogen prevalence, no differences in diarrhea incidence by region was found. We did find that method of case ascertainment was associated with differential estimates of incidence. Not unexpectedly, incidence based on self-report was much higher than incidence based on studies using passive surveillance data (DNBI) or clinic-based case series (29 versus 7 versus 6 episodes per 100 person-months, respectively). This finding is corroborated by studies that reported both self-report and clinic-based estimates of incidence. From these studies, it seems that less than one quarter of all episodes of diarrhea that occur among deployed US military personnel and similar traveler populations are seen by a health care provider.

The self-reported incidence in the long-term traveler population that we describe is comparatively lower to estimates reported from business/leisure travelers and the previously reported review.¹⁵ Compared with the review of Black, which reported a summary incidence rate of 60 cases/100 person-months (95% CI: 47–73 cases/100 person-months), our finding of 29 cases/100 person-months (among cohort and self-reported incidence data) is much lower (Kruskall-Wallis, P < 0.0001, data not shown). Possible factors that could explain this difference are differences in populations between the two studies and/or changes risk behavior of travelers’ over time. Our current review consists of studies with relatively more US military, no tourists, and longer travel durations compared with the study of Black. Given these differences, high attack rates among populations with shorter travel durations may explain the differences in the incidence estimates. Also, military populations, with their often controlled food and water distribution systems, may account for the lower incidence compared with other travelers. Changes in risk behavior over time because of increased traveler education with the advent of pre-travel counseling and recognition of travel medicine as an independent discipline may also help to explain a decrease in incidence over time. In fact, when studies from the article of Black and this study are combined, we find an inverse association between year of study (published) and incidence (Spearman = –0.61, P < 0.001)—a trend that persists with the exclusion of US military studies (Spearman = –0.33, P = 0.07; data not shown).

Specific to the US military, there are a number of possible reasons to suggest why a person with diarrhea may not seek care at a treatment facility, including lack of access to care, less severe disease, self-treatment, or a belief that there is nothing to be done to treat the condition. None of the studies reported reasons why individuals chose not to seek care. However, Hyams et al.,⁷ in their report of US military troops in the first Gulf War, found that, of those that did not seek treatment, 20% used antibiotics, suggesting self-treatment may play a role. In our review, nine studies described the self-reported total duration of illness among those individuals not seeking care to be about 3 days (IQR, 2.6–3.5 days). Travelers’ diarrhea is generally thought to have a median illness duration of 3–4 days; thus, it does not seem that the diarrhea illness experienced by those not seeking treatment is any less severe, although further studies defining these disease episodes (e.g., etiology and impact) that are not encountered in medical treatment facilities are warranted.⁴²

ETEC, Campylobacter, and Shigella continue to be identified as important pathogens, causing anywhere from 38% to 45% of diarrhea cases among US military and similar traveler populations. However, this review also highlights the importance of other pathogens, including norovirus, rotavirus, and enteroaggregative Escherichia coli (EAEC), which were responsible for ~20% of identified pathogens recovered. Furthermore, because the case definition of most studies focused on illness with diarrhea or vomiting, but not vomiting alone, this review may have underestimated the burden of acute enteric infectious disease caused by norovirus, rotavirus, and other enteric viruses that often cause a vomiting predominant illness. This burden of these enteric viruses is beginning to be understood, but more surveillance is likely needed to further describe the incidence and morbidity of disease associated with these agents in comparison to travelers’ diarrhea (TD) and other infectious diseases of military importance.^43–46

While only a few studies reported on outcomes of disease and treatment, important findings regarding the clinical significance of diarrhea disease are noted. The finding that one quarter of individuals seeking treatment are reported to be incapacitated because of this illness is significant. The estimates of 10% requiring hospitalization are alarming and need to be further evaluated. Admittedly, this estimate seems to be quite high based on the experience of a number of the study authors who have experience in treatment of diarrhea in field settings. These estimates could be overstated because of the limited number of studies that reported this finding (N = 2) and the fact that these estimates were based on provider estimates and not population-based hospitalization data. A 0–15% probability of requiring intravenous fluids for treatment of disease is consistent with practice patterns of military treatment of diarrhea and aggressive rehydration therapy that is often instituted to assure timely recovery of those that become ill. Too few studies were available to review for estimation of the pathogen-specific disease outcomes, probabilities, and treatment response caused by ETEC, Campylobacter, and Shigella. The increased severity and duration of illness caused by Campylobacter and Shigella compared with other pathogens were noted in only a few studies and need further description to assess their importance in these traveler populations, although it is consistent with what has been described in other studies on the epidemiology of these potentially invasive enteric pathogens.^{31,36,40,41,47}

This review includes a comprehensive literature search, prospective inclusion and exclusion criteria, standardized data abstraction, quality scoring, and current analytic methods, all of which reduce the potential bias in the resultant population of studies used for analysis. Limitations of this review include the significant heterogeneity in the prevalence and incidence estimates among many different study designs, populations, and across regions, and sparseness in some data, particularly pathogen-specific disease probabilities and outcome. While a number of independent variables were found to explain some of the heterogeneity, small study numbers precluded further assessment of what factors may be associated with differential pathogen prevalence and incidence. Caution should be taken in generalizing the estimates to an entire region, because many of the articles came from serial studies of the same populations in same countries of a particular region (e.g., Bright Star Exercises in Egypt, Cobra Gold Exercises in Thailand, student populations in Mexico). Furthermore, the exclusion of leisure and business travelers’ should be considered in generalizing the results to these populations. However, a review of excluded studies based on population non-eligibility does not find appreciable differences in estimates than what are described herein.^48–52 In addition, as this review shows, the collapsing of US military with other similar populations describing the epidemiology of diarrhea among long-term travelers presents a challenge. Also, clearly there is a gap of epidemiologic data from important regions of India, China, Oceana, and sub-Saharan Africa.

Additionally, this systematic review focused primarily on endemic (sporadic) diarrheal disease that occurs in these populations. While these are important and contribute to a large burden of disease, pathogens that have the potential to cause epidemic disease also need to be considered, particularly for military populations. Bacterial and viral agents having the potential to cause explosive and debilitating outbreaks may be as important, from a military perspective, because these agents cause the heavy burden of endemic disease.^43,53,54 In this respect, a study conducted among Israeli Defense Force troops during a routine deployment period found that, while sporadic cases of disease were caused by a number of different pathogens, most outbreaks were associated with Shigella, norovirus, and Salmonella.⁵⁵ Furthermore, impact of these agents with epidemic potential have been anecdotally described in a number of studies, including a study among United States Air Force personnel in this review that reported that onset of diarrheal illness in 5 of 222 airmen on 1 day had an adverse affect on operations,³⁶ and another study reported a flight mission was aborted mid-flight because of sudden onset of gastrointestinal illness in the pilot.³⁷ Last, more epidemiologic studies in sub-Saharan Africa need to be conducted to better describe the regional incidence and pathogen prevalence in these geographic regions.

CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
This review of studies on diarrhea in long-term travelers (US military and similar populations) provides some certain conclusions. First, diarrhea is frequent, and a large burden of disease is not seen by a health care provider. It remains to be known whether this illness is milder illness or is illness that is being successfully self-treated. Second, ETEC, Campylobacter, and Shigella bacteria are significant pathogens globally, and the latter two seem to also be associated with more severe symptoms with often longer duration. Third, a number of other bacterial, viral, and parasitic pathogens, including EAEC, Salmonella, norovirus, and rotavirus, should continue to be considered as important pathogens causing disease in these populations. Last, the combination of disease incidence requiring treatment, disease incidence among individuals who do not seek treatment, and incapacitation caused by these illnesses should be considered an important health threat and be addressed with further studies evaluating timely and effective management, as well as other strategies including the evaluation of vaccines to prevent these infections.

Received September 15, 2005. Accepted for publication December 29, 2005.

^* Address correspondence to Mark S. Riddle, NAMRU 3, PSC 452, Box 105, FPO AE 09835. E-mail: markriddlemd{at}hotmail.com

Authors’ addresses: Mark S. Riddle, NAMRU-3, PSC 452, Box 105, FPO AE 09835, E-mail: markriddlemd{at}hotmail.com. John W. Sanders, Infectious Disease Service, National Naval Medical Center, 8901 Wisconsin Avenue, Bethesda, MD 20889, E-mail: jwsanders{at}bethesda.med.navy.mil. Shannon D. Putnam, U.S. Embassy Jakarta, NAMRU 2, Box 8132, FPO AP 96520, E-mail: sha8299{at}hotmail.com. David R. Tribble, Naval Medical Research Center, 503 Robert Grant Avenue, Silver Spring, MD 20910-7500, E-mail: tribbled{at}nmrc.med.navy.mil.

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