The Double-Edged Sword of Military Response to Societal Disruptions: A Systematic Review of the Evidence for Military Personnel as Pathogen Transmitters

Abstract

Given their lack of immunity and increased exposure, military personnel have the potential to serve as carriers or reservoirs for infectious diseases into or out of the deployment areas, but, to our knowledge, the historical evidence for such transmission events has not previously been reviewed. Using PubMed, we performed a systematic review of published literature between 1955 and 2018, which documented evidence for military personnel transporting infectious pathogens into or out of deployment areas. Of the 439 articles screened, 67 were included for final qualitative and quantitative review. The data extracted from these articles described numerous instances in which thousands of military service members demonstrated potential or actual transmission and transportation of multiple diverse pathogens. These data underscore the immense importance preventive medical professionals play in mitigating such risk, how their public health efforts must be supported, and the importance of surveillance in protecting both military and civilian populations.

Military Medicine, Communicable Diseases, Military Personnel, Global Health

Today, perhaps more than ever before, military personnel throughout the world are being deployed to diverse geographic areas in response to many types of societal disruptions. These include humanitarian assistance and disaster relief after earthquakes, hurricanes, floods, political strife, terrorism, and epidemics, and most recently, for border control [1–3]. As the world becomes more connected, global health threats will continue to intensify, whether they are naturally occurring, accidental, or deliberate. Military units are well suited to such response roles, given their ability to respond rapidly with ready international transportation, effective logistics and supply support, and strategies for security in areas threatened by strife. Military responses are likely to increase as the Global Health Security Agenda calls for partner nations to develop military medical capabilities to perform such response activities, engaging in important health research and development, and developing surveillance systems to prevent, detect, and respond to emerging health threats around the world [4, 5].

This increased tempo of US military personnel foreign deployment in support of noncombat operations can result in service members (SMs) being put at high risk for acquiring novel infectious diseases for which they have no previous immunity. In addition, arduous working and living conditions, as well as enhanced deployment-related psychological stress [6] may increase infectious disease morbidity and transmission. Military personnel may inadvertently become carriers of pathogens, causing infections among close contacts within their military units or among community members at home or abroad. Hence, this double-edged sword of mitigating one problem while creating another may not be considered when decisions are made to deploy young, immunologically naive SMs to areas far from their homes.

In this systematic review, we sought to examine the medical literature with a goal of documenting instances when deployed SMs acquired infectious diseases, transmitted infectious diseases to others, and/or transported infectious pathogens to geographic locations. In doing so, we hoped to illustrate the nature of such infectious disease risks and describe the extent to which this has been reported, such that preventive measures might be considered.

METHODS

Literature Search

In February 2018, we searched the literature published between 1955 and February of 2018 on PubMed. This review followed procedures standardized by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The time period restriction was chosen to capture infectious disease events documented after the end of the Korean War, representing the modern military era. Specific details of the literature search, abstract screening, and article review process are recorded in the Supplementary Material.

Article Review

During full article reviews, data of interest included the number of infected military personnel and/or civilians, suspected mode of transmission, laboratory or clinical evidence used in identifying the infectious pathogen(s), and/or geographic area of acquisition and detection. In this process, a number of additional articles were discovered and similarly examined for meeting the inclusion criteria. Articles were selected for inclusion in this analysis based on evidence that military SMs transmitted (or demonstrated the potential for transmission) novel bacterial, viruses, and parasites into or out of deployment zones.

RESULTS

The search (see Supplementary Material) yielded 436 citations, to which 6 additional citations were added from other sources. Three of these citations were removed as duplicates, leaving a total of 439 abstracts that were screened for inclusion (Figure 1). Eighty-two articles were chosen for full reviews. Another 28 citations were identified for complete review during this process. Of the 110 full articles reviewed, 43 were removed from consideration for ≥1 of the following reasons: (1) lack of evidence or indication of potential for military personnel serving as reservoir or carrier for the transmission of infectious diseases into or out of deployment zones (n = 31); (2) inability to find the original article in a language in which the reviewers were fluent (n = 3); or (3) no confirmation of the pathogen of interest through reliable laboratory testing, or the ambiguous clinical identification of the pathogen (n = 9).

Figure 1.

Flow chart of the literature search process. The authors conducted the search in February 2018, following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and procedures.

Relevant qualitative and quantitative data from the selected 67 articles were organized by the category of pathogen discussed (Table 1). Twenty-one articles described instances of bacterial pathogen transportation, or risk of transportation by military personnel. Twenty-three articles mentioned parasitic pathogens, and 23 discussed viral pathogens. Several additional articles, which did not fit our inclusion criteria but provide valuable context to understand the risk of transportation and transmission of pathogens by military personnel, were identified by coauthors during journal peer review and are described in the Results and Discussion sections in conjunction with the articles identified in the review process.

Table 1.

Instances of Deployed Military Personnel Transmitting or Transporting Pathogens to Others

References by Pathogen Type	Pathogen	Personnel Infected, No.a	Nationality of Personnel	Location of Deployment	Location of Detection
Bacterial (n = 21)
Dickerson (1966) [7]	Yersinia pestis	1	American	Vietnam	United States
Spotnitz et al (1967) [8]	Pseudomonas pseudomallei	6	American	Vietnam	United States
Holmes et al (1970) [9]	Neisseria gonorrhoeae	88	American	Philippines	US aircraft carriers
Hyams et al (1991) [10]	Escherichia coli, Shigella	432	American	Saudi Arabia	Saudi Arabia
Ferrante et al (1993) [11]	Coxiella burnetii	1	American	Saudi Arabia	United States
Emmons and Ljaamo (1999) [12]	Mycobacterium tuberculosis		American, Norwegian	Yugoslavia	Yugoslavia
Vincent et al (2000) [13]	Bordetella pertussis, Chlamydia pneumoniae, Mycoplasma pneumoniae	46	American	South Korea	South Korea
Cooper et al (2007) [14]	B. pertussis	21	British	Afghanistan	Germany
Scott et al (2007) [15]	Acinetobacter baumanii	70	American	Iraq	United States and Germany
Chambers et al (2009) [16]	Mycobacterium leprae	1	American	Southeast Asia	United States
Keen et al (2010) [17]	A. baumanii, Pseudomonas aeruginosa	2447	American	Inclusive of Iraq and Afghanistan	United States
Miceli et al (2010) [18]	C. burnetii	1	American	Iraq	United States
Anderson et al (2011) [19]	C. burnetii	88	American	Iraq	United States
Cravioto et al (2011) [20]	Vibrio cholerae	NS	South Asian	Haiti	Haiti
Eick et al (2011) [21]	B. pertussis, C. pneumoniae, M. pneumoniae	99	American	Afghanistan	United States
Hospenthal et al (2011) [22]	Acinetobacter spp., E. coli, Klebsiella spp., MRSA, Enterobacter cloacae, P. aeruginosa, Citrobacter spp.	1406	American	Afghanistan and Iraq	United States and Germany
Piarroux et al (2011) [23]	V. cholerae	NS	Nepalese	Haiti	Haiti
Mancuso et al (2013) [24]	M. tuberculosis	9	American	Iraq, Afghanistan, Japan, and Turkey	NS
Akers et al (2014) [25]	A. baumanii, P. aeruginosa, E. coli, Klebsiella pneumoniae, Staphylococcus aureus	25	American	Iraq and Afghanistan	United States and Germany
Newman et al (2014) [26]	C. burnetti, Rickettsia spp.	19	United Kingdom	Afghanistan	United Kingdom
Campbell et al (2017) [27]	E. coli, Acinetobacter spp., K. pneumoniae, Pseudomonas spp, Enterobacter spp.	245	American	Inclusive of Iraq and Afghanistan	United States
Parasitic (n = 23)
NCDC (1966) [28]	Plasmodium	449	American	Vietnam	United States
Magill et al (1992) [29]	Leishmania	7	American	Saudi Arabia	United States
Hepburn et al (1993) [30]	Leishmania	306	British	Belize	United Kingdom
Kreutzer et al (1993) [31]	Leishmania major, Leishmania tropica	28	American	Saudi Arabia	United States
Magill et al (1993) [32]	Leishmania donovani	8	American	Saudi Arabia	United States
Sergiev et al (1993) [33]	Plasmodium	NS	Russian	Afghanistan	USSR
Almeda et al (1994) [34]	Schistosoma haematobium	18	Angolan	Republic of Sao Tome and Principe	Republic of Sao Tome and Principe
Newton et al (1994) [35]	Plasmodium	112	American	Somalia	United States
Barat et al (1997) [36]	Plasmodium	278	American	Somalia and Honduras	United States
Kachur et al (1997) [37]	Plasmodium	38	American	NS	United States
Smoak et al (1997) [38]	Plasmodium	75	American	Somalia	United States
Kitchener et al (2000) [39]	Plasmodium	212	Australian	East Timor	Australia
CDC (2003) [40]	L. major	22	American	Afghanistan, Iraq, and Kuwait	United States
Foca et al (2004) [41]	Plasmodium	6	Italian	Somalia, Honduras	Italy
Charles et al (2005) [42]	Plasmodium	23	Australian	NS	Australia
Kotwal et al (2005) [43]	Plasmodium	38	American	Afghanistan and Iraq	United States and Afghanistan
Meneizel et al (2009) [44]	Plasmodium	500	Jordanian	Eritrea, East Africa	Jordan
CDC (2010) [45]	Plasmodium	2	American	Haiti	Haiti
El-Bahnasawy et al (2010) [46]	Plasmodium	20	Egyptian	Inclusive of Afghanistan and Sudan	Egypt
Premaratna et al (2011) [47]	Plasmodium	1	Sri Lankan	Sri Lanka	Sri Lanka
Matheson et al (2012) [48]	Leishmania	2	United Kingdom	Afghanistan	United Kingdom
Bart et al (2013) [49]	Leishmania	102	Dutch	Afghanistan, Beliz, and Suriname	The Netherlands
Kanani et al (2015) [50]	Plasmodium	811	Jordanian	Ivory Coast, Liberia, East Timor, Eritrea, Sierra Leone, Haiti, and other locations	Jordan
Viral (n = 23)
Cates et al (1975) [51]	Hepatitis B	8700	American	Germany
Scott et al (1981) [52]	Hepatitis A/B	22	American	Thailand	Thailand
Aronson et al (1988) [53]	Hepatitis A/B	177	American	South Korea	South Korea
Klontz et al (1989) [54]	Influenza A(H1N1)	60	American	Puerto Rico	United States
Buisson et al (1990) [55]	Hepatitis B	30	French	“Overseas” and “tropical areas”	France
Hepburn and Brooks (1992) [56]	Varicella	21	American	Saudi Arabia	Saudi Arabia
Brodine et al (1992) [57]	HTLV-1	3	American	Japan	Japan
Coursaget et al (1994) [58]	Hepatitis E	5	French	Morocco, Senegal, or La Reunion Island	Chad
Artenstein et al (1995) [59]	HIV	6	Uruguayan	Cambodia	Uruguay
Soeprapto et al (1995) [60]	HIV	11	American	Cambodia	United States
Drabick et al (1997) [61]	Hepatitis E	4	Bangladeshi	Haiti	Haiti
Brodine et al (1999) [62]	HIV	6	American	Thailand	United States
McCarthy et al (2000) [63]	Norwalk-like Virus	126	American	US aircraft carriers	US aircraft carriers
Crum-Cianflone et al (2009) [64]	Influenza A(H1N1)	32	American	United States	United States
WHO (2009) [65]	Influenza A(H1N1)	313	Multinational	Afghanistan	Afghanistan
Eick et al (2011) [21]	Adenovirus, PIV, RSV	59	American	Afghanistan	United States
Johns et al (2011) [66]	Influenza A(H1N1)	NS	American	Kuwait, Iraq	Kuwait, Iraq
Young et al (2011) [67]	Vaccinia virus	1	American	United States	United States
CDC (2012) [68]	Rabies	1	American	Afghanistan	United States
Farrell et al (2013) [69]	Influenza A(H1N1)	68	American	Kuwait	Kuwait
Newman et al (2014) [26]	Hantavirus, sandfly virus	20	United Kingdom	Afghanistan	United Kingdom
Kyei et al (2015) [70]	Lassa virus	2	Ghanaian	Liberia	Ghana
Lindholm et al (2017) [71]	Dengue, chikungunya	2	American	Mexico, Caribbean, Central American, or South America	United States

Abbreviations: CDC, Centers for Disease Control and Prevention; HIV, human immunodeficiency virus; HTLV-1, human T-lymphotrophic virus type 1; MRSA, methicillin-resistant S. aureus; NCDC, National Communicable Disease Center; NS, Not Stated; PIV, parainfluenza virus; RSV, respiratory syncytial virus; WHO, World Health Organization.

^aData in this table do not represent the complete population of military personnel described by each article, but rather the subset of military personnel that fit the inclusion criteria for our review.

We illustrated the geographic locations where the infectious pathogens were commonly acquired (Figure 2) by SMs and the paths of infectious agents transported by SMs (Figure 3). Common foreign locations for pathogen acquisition were, understandably, often where social disruption had recently occurred.

Figure 2.

Common sources of infectious pathogens acquired by military personnel, 1955–2018. Geographic locations where service members acquired their infectious diseases are shown, with frequency indicated by color on a spectrum from light yellow (approximately 2 records) to dark red (approximately 19 records). These data were extracted from the 67 full article reviews documented in Table 1.

Figure 3.

Transportation of infectious pathogens by military personnel, 1955–2018. The geographic origins and destinations are shown for infectious pathogens that were documented to have been transported by service members. Data were extracted from 51 full article reviews included in Table 1 and detail the country of origin and detection of pathogen transportation. Abbreviations: AO, Angola; BW, Botswana; BY, Belarus; CD, Democratic Republic of the Congo; CL, Chile; CM, Cameroon; DK, Denmark; EC, Ecuador; ES, Spain; ET, Ethiopia; FI, Finland; FR, France; GR, Greece; KE, Kenya; KG, Kyrgyzstan; LA, Lao People's Democratic Republic; LK, Sri Lanka; MC, Monaco; MG, Madagascar; MM, Myanmar; MV, Maldives; NG, Nigeria; NO, Norway; NP, Nepal; NZ, New Zealand; PG, Papua New Guinea; PK, Pakistan; PL, Poland; PY, Paraguay; RO, Romania; SE, Sweden; SN, Senegal; SS, South Sudan; TD, Chad; TH, Thailand; TZ, Tanzania; UA, Ukraine; UY, Uruguay; UZ, Uzbekistan; VN, Vietnam.

Secondary Transmission of Pathogens to Civilians or Other Military Units

The majority of articles in this review did not report instances of subsequent transmission of pathogens by infected military personnel to others. However, in a few tangible examples, the introduction of pathogen to a novel or different geography had severe consequences for local civilian and military populations.

Of particular importance, and having major implications as the source of hundreds of thousands of cases among host nation civilians, a report describing spatiotemporal analyses clearly supported the notion that the importation of Vibrio cholerae by Nepalese peacekeepers into the Artibonite River Valley in Haiti served as the source of a massive cholera outbreak in 2010 [23]. An estimated 697 000 humans became ill, 8500 died, and the pathogen still exists in endemic form to this day. Multiple reports [23, 61] have implicated Nepalese peacekeepers in this outbreak, which may have been facilitated by asymptomatic carriage of the bacteria after these SMs were based near an ongoing cholera outbreak in Kathmandu before their deployment to Haiti [72].

In another important instance, multinational military forces were connected with the introduction and subsequent outbreak of pandemic influenza A(H1N1)pdm09 in multiple countries in the Middle East [65, 69]. The first diagnosed case of influenza A(H1N1)pdm09 in Kuwait was documented among US military units, with similar mutations to a North American virus strain [69]. Later investigation determined that the importation of influenza A(H1N1)pdm09 into Kuwait and Iraq by US military personnel followed predeployment aggregation and training at power projection platforms in the United States [66].

Other studies reported secondary transmission of pathogens by infected military personnel to civilians or other military personnel units on a smaller scale. Thousands of Russian military personnel imported malaria into Russia after deployment in Afghanistan in the 1980s, with 36 documented instances of secondary transmission. In 2009, families of crew members on an aircraft carrier reported a 6% secondary attack rate after an outbreak of pandemic influenza A(H1N1)pdm09 that was believed to have been acquired after contact with a maintenance crew in Mexico [64]. In 2008, an incidence of suspected sexual transmission of Q fever by a US SM who returned from deployment in Iraq to the civilian spouse was described by Miceli et al [18].

Finally, we have included a report that describes cases of secondary and tertiary transmission of vaccinia virus from a US military SM preparing for deployment to 3 semiprofessional wrestlers and a household contact in New York State [67]. This report illustrates the transmission risk of pathogens from US military to civilians as part of the US military’s required vaccination programs and bioterrorism preparedness activities. This case report underscores the need to ensure that military vaccinees understand the hazards associated with contact transmission.

Transportation of Active Infections Across Boundaries

Every article in this review served as evidence that military personnel have the potential to serve as vessels for pathogens to travel across political and geographic boundaries. In some instances, such as the Haiti cholera outbreak of 2010 or the 2009 influenza A(H1N1)pdm09 global pandemic, the combination of military personnel with active infection and environments amenable to disease spread spurred a large outbreak. In many other articles, authors did not report actual secondary transmission events but described contexts which indicated the ever-present threat.

Of particular note are the records indicating transportation of multidrug-resistant organisms (MDROs) through the Western military medical system. In a 2003, multidrug-resistant Acinetobacter baumanii infections among military personnel returning from Iraq to medical facilities in the United States and Germany were indicated to have been nosocomially acquired from a field hospital [15]. Between 2005 and 2009, a total of 1285 cases of Acinetobacter species colonization were recorded among US military personnel at medical military centers after evacuation from Iraq and Afghanistan, including at least 170 instances of infection with MDROs in 2009 [22]. A large prospective cohort study between 2009 and 2014 documented 245 instances of multidrug-resistant gram-negative infections among US military personnel referred to tertiary care hospitals in the United States and Germany after deployment in Iraq and Afghanistan [27]. A retrospective review of the military health system found 1605 and 842 isolates of A. baumanii and Pseudomonas aeruginosa, respectively, by military personnel after deployment abroad, with a higher number of MDROs among US military personnel compared with host nation military personnel and civilians [17].

Given the challenges inherent in monitoring and containing MDRO infections in medical contexts, the lack of reported secondary transmission events related to the cases captured by the review indicates a likelihood of undetected or unreported secondary transmission, rather than the absence of their occurrence.

Acquisition and secondary transmission of sexually transmitted infections have played a role in military conflicts, and the presence of armies has been routinely associated with higher rates of sexually transmitted infections [73, 74]. As an example, during the war between the United States and Vietnam, emergence of penicillin-resistant gonococcal infections among SMs stationed in the Asia region with acquisition from commercial sex workers was well documented by military investigators [75]. The risk of acquisition of gonorrhea from infected commercial sex workers to military personnel was quantified to be about 20% [9].

In 1986, military investigators were the first to publish evidence supporting the notion that human immunodeficiency virus (HIV) could be transmitted through heterosexual contact with commercial sex workers [76]. Subsequently, Thai and US military investigators screened recruits to the Thai military for HIV infection from 1991 through 2000. Coauthors identified clear geographic “hot spots” of HIV acquisition and transmission in specific cities and districts, and, though no deployment-associated or secondary cases were discussed, implied the risk for continued transmission by Thai military personnel [77].

US military investigators have recognized that the use of penicillin, along with many other factors, eventually led to a significant decrease in the incidence of syphilis among US military populations from the 1960s through the end of the 20th century [78, 79]. Unfortunately, a recent increase in the incidence of syphilis has been documented in the US Military Health Systems, reflecting a similar increase among US civilian population [80]. Clark and Hunt [80] noted in their investigation that 25% of identified syphilis cases were also coinfected with HIV, but they did not present data regarding geography of acquisition or associations with deployment. Likely influenced by the limitations of this systematic review, we have found no reliable case reports in the literature reviewed that documented secondary syphilis transmission to military dependents or to civilian community members.

Pathogens that are not typically transmitted person to person, such as arboviruses, may pose a transmission risk as military personnel return to their homeland, if competent vectors are present. The lack of phlebotomine (sandfly) populations in the United Kingdom and the Netherlands reduced the risk of SM-related Leishmania transmission to others. However, the United States is home to competent Leishmania vectors, which led to United States SM returning from Middle East deployment to being disqualified from donating blood after returning from deployment [73]. In still other cases, as reported by Spotnitz et al [8], the importation of personnel with melioidosis, a pathogen largely acquired through environmental contact, represents a minor or negligible threat, although human-to-human transmission has been documented [81].

Seroconversion as Evidence of Continued Risk of Transportation

The physical transportation of pathogens by military personnel, indicated by instances in which the pathogens were acquired in one location and detected in another, in itself represents a risk for future infectious pathogens to follow the same route. Multiple studies in our review detailed seroconversion of military personnel to various pathogens, particularly those associated with respiratory infections, after returning from deployment. Eick et al [21] documented 37 instances of seroconversion against Bordetella pertussis among US military personnel deployed in Afghanistan. They also noted seroconversion of military personnel to Chlamydia pneumoniae (n = 33) and Mycoplasma pneumoniae (n = 29) after their return from Operation Enduring Freedom in Afghanistan [21]. Mixed infections of B. pertussis, M. pneumoniae, and C. pneumoniae were documented in a serological study of US military personnel with afebrile, nonproductive coughs during deployment in South Korea [13]. A retrospective review documented seroconversion to Coxiella burnetti by 88 US military personnel after their return from deployment in Iraq [19].

Although these cases demonstrate a more distant risk compared with studies demonstrating instances of actual secondary transmission, it is important to recognize the routes and opportunities for infectious pathogens to be transported and transmitted by military personnel when considering prevention and containment strategies.

Discussion

Targeted pre- and post-screening for SMs and active education of healthcare personnel in the detection of infectious pathogens are critical to the prevention of infection of SMs and transportation, as well as potential transmission, of these pathogens to others during and after military deployments. In particular, influenza and other highly transmissible respiratory pathogens are high risks for undetected transmission within a population, owing to variations in symptom presentation that defy case definitions, as well as low adherence to medical recommendations that those infected should seek care [64].

Considering that vaccination is widely known to be one of the most cost-effective and efficient methods of outbreak prevention, military vaccination programs are extremely important in protecting individual SMs, and when the disease is communicable, their fellow SMs and other personnel with whom they have contact. However, instances like the novel influenza A(H1N1) global pandemic in 2009 demonstrate the limitations of vaccines in protecting populations against emerging pathogens and highlight the need for multiple layers of considerations for both civilians and SMs.

In the case of the 2010 cholera outbreak in Haiti, the benefit of hindsight has allowed scholars to propose basic sanitation as a failsafe to prevent future outbreaks. All Nepalese peacekeepers had passed a predeployment screening for pathogens before arrival, and none displayed symptomatic presentations of infection [82]. In reducing the causes of outbreak to the environmental mechanisms by which the pathogen spread, Piarroux et al [23] suggested that, at least in this case, omissions of infected SMs by the military medical screening process might have been mitigated by following robust hygiene and waste management practices.

Farrell and coauthors [69] acknowledge that each pathogen and population require individualized prevention and containment strategies and specify that the management of influenza outbreaks in military contexts could benefit from individualized dynamic and developmental approaches, accommodating new information and continually revising strategic responses to best fit various situations. Continued vigilance and flexibility in detecting and responding to ongoing outbreaks within and near military populations may serve to prevent SMs from serving as vectors of infectious pathogens between and near deployment zones.

Underreporting and Reporting Biases

In instances in which investigators of articles captured in this review were not assessing for cases of further infection to outside populations, the impact on civilians and other military personnel remains unknown. Imported pathogens that do not have well-known histories in the region to which they are transported may not be recognized by clinicians based on symptomatic presentation. For example, cases of leishmaniasis, when presented for medical treatment, have been treated as bacterial infections until prescribed antibiotics prove ineffective [48]. Surveillance for nonendemic pathogens, such as Leishmania in the United States and Europe, is often stymied by the absence of reporting regulations [49]. This reporting bias must be acknowledged when considering the existing data for secondary transmission of pathogens from military personnel to others and instances of military personnel transporting pathogens to novel geographies.

Multiple other factors could lead to underreporting of infectious pathogen transmission from SMs to others. As previously described, the absence of described secondary infections can be attributed in part to the challenges inherent in tracking and detecting case-to-case infections in medical settings. There are risks of underreporting influenza and other highly transmissible respiratory infections if they are circulating as novel strains, or if the persons infected do not present for medical treatment.

The majority of articles in the current review pertained to SMs from Western countries, and transportation routes of pathogens events were most often between the United States and areas of US SM deployment (Figure 3). This fact could be due to the language restrictions placed on the literature search by the authors of our review but may also reflect trends in peer-reviewed literature in general, in which publications by Western scholars are overrepresented. The higher proportion of articles pertaining to SMs from Western countries may not demonstrate a higher risk for these SMs serving as vectors of pathogens to and from areas of deployment but rather a higher likelihood that such events will be detected, reported, and/or published.

The data presented in this systematic review detail instances of military personnel transporting or transmitting infectious pathogens that were published in peer-reviewed journals, but it is likely that these data underrepresent all transportation and transmission events that have occurred in the modern military era. With this in mind, the themes and data generated can be used to direct further research using data collected by the military of the United States or other nations. Such investigations may help the global health and military medicine community understand associations or underlying correlations of specific infections with deployment based on geography, type of deployment, or contextual factors related to global health security at the time of deployment. Deeper understanding of these associations may serve to direct interventions for preventing and detecting infectious disease outbreaks vectored by military personnel.

In conclusion, in this review we found numerous reports of military personnel serving as real or potential reservoirs for foreign pathogen transmission to others within or outside their military communities. Although these collections of reports document thousands of instances of SM-related bacterial, viral, and parasitic pathogen transmission to others, they very likely markedly underestimate the effects of such diseases. This reinforces the extreme importance of the role that disease prevention professionals play among deployed military units. These data also remind us that deploying healthy but immunologically naive SMs to distant lands poses significant infectious disease risk to them and those with whom they interact with after deployment. This double-edged sword of using SMs to respond to societal disruptions is a reality that policy makers must remember to consider, and that the military medical community must continue to anticipate.

Supplementary Data

Supplementary materials are available at The Journal of Infectious Diseases online. Consisting of data provided by the authors to benefit the reader, the posted materials are not copyedited and are the sole responsibility of the authors, so questions or comments should be addressed to the corresponding author.

Notes

Author contributions. This study was conceived by G. C. G. The literature search was conducted by J. N. Z. Title and abstract reviews were completed by J. N. Z., J. L. S., and G. C. G. equally. J. N. Z. performed an initial full review of selected articles. J. L. S., J. P., and G. C. G. performed a secondary full review of selected articles equally, with G. C. G. as a third-party reviewer for discrepant opinions on suitability for inclusion. J. N. Z. wrote the first draft of the manuscript with input from G. C. G., and J. N. Z. prepared the figures and table. All authors provided critical conceptual contributions, revisions, and final approval on the text.

Financial support. This work was supported in part by the US Naval Medical Research Center–Asia and Vysnova Partners (SC-2016-SABER-003-002 and SC-2017-SABER-010-001; principal investigator G. C. G.) and by Duke University’s Global Health Institute (principal investigator G. C. G.). J. P. was a Fulbright United States–ASEAN visiting scholar attached to Duke University (under G. C. G.) during the current study; his Fulbright grant was administered by the Bureau of Educational and Cultural affairs, US Department of State, with the cooperation of the Institute of International Education.

Potential conflicts of interest. All authors: No reported conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.