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Military Health System

Heat Illness, Active Component, U.S. Armed Forces, 2021

Image of Airmen participate in the 13th Annual Fallen Defender Ruck March at Joint Base San Antonio, Nov. 6, 2020. The event honors 186 fallen security forces, security police and air police members who have made the ultimate sacrifice. Photo By: Sarayuth Pinthong, Air Force. Airmen participate in the 13th Annual Fallen Defender Ruck March at Joint Base San Antonio, Nov. 6, 2020. The event honors 186 fallen security forces, security police and air police members who have made the ultimate sacrifice. Photo By: Sarayuth Pinthong, Air Force.

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Medical Surveillance Monthly Report

Abstract

In 2021, there were 488 incident cases of heat stroke and 1,864 incident cases of heat exhaustion among active component service members of the U.S. Armed Forces. The unadjusted annual rates of incident heat stroke and heat exhaustion peaked in 2018 and then declined in 2019 and 2020. Between 2020 and 2021, the rate of incident heat stroke was relatively stable (0.37 cases per 1,000 person-years [p-yrs]) while the rate of heat exhaustion increased slightly (1.40 cases per 1,000 p-yrs). In 2021, subgroup-specific rates of incident heat stroke and heat exhaustion were highest among male service members, those less than 20 years old, Marine Corps and Army members, recruit trainees, and those in combat-specific occupations. During 2017–2021, a total of 312 heat illnesses were documented among service members in the U.S. Central Command (CENTCOM) area of responsibility (AOR); 6.4% (n=20) were diagnosed as heat stroke. Commanders, small unit leaders, training cadre, and supporting medical personnel must ensure that the military members whom they supervise and support are informed about the risks, preventive countermeasures, early signs and symptoms, and first-responder actions related to heat illnesses.

What are the new findings?

From 2020 to 2021, the rate of incident heat stroke was relatively stable while the rate of heat exhaustion increased slightly. The annual numbers of heat illnesses diagnosed in the CENTCOM AOR have generally trended downward since 2017. Sizeable proportions of heat stroke and heat exhaustion cases were not identified by way of mandatory reports through the Disease Reporting System internet (DRSi).

What is the Impact on Readiness and Force Health Protection?

Heat illness can degrade operational readiness by causing considerable morbidity, particularly during training of recruits and of soldiers and Marines in combat arms specialties. This analysis demonstrates again the magnitude of risks of heat illnesses among active component service members and the enhanced risks associated with sex, age, location of assignment, and occupational categories. Recognition of these risk factors should inform the preventive measures that military leaders, trainers, and service members routinely employ.

Background

Heat illness refers to a group of disorders that occur when the elevation of core body temperature surpasses the compensatory limits of thermoregulation.1 Heat illness is the result of environmental heat stress and/or exertion and represents a set of conditions that exist along a continuum from less severe (heat exhaustion) to potentially life threatening (heat stroke).

Heat exhaustion is caused by the inability to maintain adequate cardiac output because of strenuous physical exertion and environmental heat stress.1,2 Acute dehydration often accompanies heat exhaustion but is not required for the diagnosis.3 The clinical criteria for heat exhaustion include a core body temperature greater than 100.5ºF/38ºC and less than 104ºF/40ºC at the time of or immediately after exertion and/or heat exposure, physical collapse at the time of or shortly after physical exertion, and no significant dysfunction of the central nervous system. If any central nervous system dysfunction develops (e.g., dizziness or headache), it is mild and rapidly resolves with rest and cooling measures (e.g., removal of unnecessary clothing, relocation to a cooled environment, and oral hydration with cooled, slightly hypotonic solutions).1–4 

Heat stroke is a debilitating illness characterized clinically by severe hyperthermia (i.e., a core body temperature of 104ºF/40ºC or greater), profound central nervous system dysfunction (e.g., delirium, seizures, or coma), and additional organ and tissue damage.1,4,5 The onset of heat stroke should prompt aggressive clinical treatments, including rapid cooling and supportive therapies such as fluid resuscitation to stabilize organ function.1,5 The observed pathologic changes in several organ systems are thought to occur through a complex interaction between heat cytotoxicity, coagulopathies, and a severe systemic inflammatory response.1,5 Multiorgan system failure is the ultimate cause of mortality due to heat stroke.5

Timely medical intervention can prevent milder cases of heat illness (e.g., heat exhaustion) from becoming severe (e.g., heat stroke) and potentially life threatening. However, even with medical intervention, heat stroke may have lasting effects, including damage to the nervous system and other vital organs and decreased heat tolerance, making an individual more susceptible to subsequent episodes of heat illness.6–8 Furthermore, the continued manifestation of multiorgan system dysfunction after heat stroke increases patients’ risk of mortality during the ensuing months and years.9,10 

Strenuous physical activity for extended durations in occupational settings as well as during military operational and training exercises exposes service members to considerable heat stress because of high environmental heat and/or a high rate of metabolic heat production.11,12 In some military settings, wearing needed protective clothing or equipment may make it biophysically difficult to dissipate body heat.13,14 The resulting body heat burden and associated cardiovascular strain reduce exercise performance and increase the risk of heat-related illness.11,15 

Over many decades, lessons learned during military training and operations in hot environments as well as a substantial body of research findings have resulted in doctrine, equipment, and preventive measures that can significantly reduce the adverse health effects of military activities in hot weather.16–22 Although numerous effective countermeasures are available, heat-related illness remains a significant threat to the health and operational effectiveness of military members and their units and accounts for considerable morbidity, particularly during recruit training in the U.S. military.11,23,24 Moreover, with the projected rise in the intensity and frequency of extreme heat conditions associated with global climate change, heat-related illnesses will likely represent an increasing challenge to the military.25–28

In the U.S. Military Health System (MHS), the most serious types of heat-related illness are considered notifiable medical events. Notifiable cases of heat illness include heat exhaustion and heat stroke. All cases of heat illness that require medical intervention or result in change of duty status are reportable.

This report summarizes reportable medical events of heat illness as well as heat illness-related hospitalizations and ambulatory visits among active component service members during 2021 and compares them to the previous 4 years. Episodes of heat stroke and heat exhaustion are summarized separately.

Methods

The surveillance period was January 2017 through December 2021. The surveillance population included all individuals who served in the active component of the Army, Navy, Air Force, or Marine Corps at any time during the surveillance period. All data used to determine incident heat illness diagnoses were derived from records routinely maintained in the Defense Medical Surveillance System (DMSS). These records document both ambulatory health care encounters and hospitalizations of active component service members of the U.S. Armed Forces in fixed military and civilian (if reimbursed through the MHS) treatment facilities worldwide. In-theater diagnoses of heat illness were identified from medical records of service members deployed to Southwest Asia or the Middle East and whose health care encounters were documented in the Theater Medical Data Store. Because heat illnesses represent a threat to the health of individual service members and to military training and operations, the Armed Forces require expeditious reporting of these reportable medical events through any of the service-specific electronic reporting systems; these reports are routinely transmitted and incorporated into the DMSS. 

For this analysis, a case of heat illness was defined as an individual with 1) a hospitalization or outpatient medical encounter with a primary (first-listed) or secondary (second-listed) diagnosis of heat stroke (International Classification of Diseases, 9th Revision [ICD-9]: 992.0; International Classification of Diseases, 10th Revision [ICD-10]: T67.0*) or heat exhaustion (ICD-9: 992.3–992.5; ICD-10: T67.3*–T67.5*) or 2) a reportable medical event record of heat exhaustion or heat stroke.29 Because of an update to the Disease Reporting System internet (DRSi) medical event reporting system in July 2017, the type of reportable medical events for heat illness (i.e., heat stroke or heat exhaustion) could not be distinguished using reportable medical event records in DMSS data. Instead, information on the type of reportable medical event for heat illness during the entire 2017–2021 surveillance period was extracted directly from the records of the DRSi. It is important to note that MSMR analyses carried out before 2018 included diagnosis codes for other and unspecified effects of heat and light (ICD-9: 992.8 and 992.9; ICD-10: T67.8* and T67.9*) within the heat illness category “other heat illnesses.” These codes were excluded from the current analysis and the April MSMR analyses of 2018–2021. 

Each individual could be considered an incident case of heat illness only once per calendar year. If an individual had a diagnosis for both heat stroke and heat exhaustion during a given year, only 1 diagnosis was selected, prioritizing heat stroke over heat exhaustion. Encounters for each individual within each calendar year then were prioritized in terms of record source with hospitalizations prioritized over reportable events, which were prioritized over ambulatory visits. Incidence rates were calculated as incident cases of heat illness per 1,000 person-years (p-yrs) of active component service. Percent change in incidence was calculated using unrounded rates. 

For surveillance purposes, recruit trainees were identified as active component members who were assigned to service-specific training locations during the relevant basic training periods (e.g., 8 weeks for Navy basic training). Recruit trainees were considered a separate category of enlisted service members in summaries of heat illnesses by military grade overall. 

Records of medical evacuations from the U.S. Central Command (CENTCOM) area of responsibility (AOR) (i.e., Southwest Asia/Middle East) to a medical treatment facility outside the CENTCOM AOR were analyzed separately. Evacuations were considered case defining if affected service members had at least 1 inpatient or outpatient heat illness medical encounter in a permanent military medical facility in the U.S. or Europe from 5 days before to 10 days after their evacuation dates.

It should be noted that medical data from sites that were using the new electronic health record for the Military Health System, MHS GENESIS, between July 2017 and October 2019 are not available in the DMSS. These sites include Naval Hospital Oak Harbor, Naval Hospital Bremerton, Air Force Medical Services Fairchild, and Madigan Army Medical Center. Therefore, medical encounter data for individuals seeking care at any of these facilities from July 2017 through October 2019 were not included in the current analysis.

Results

In 2021, there were 488 incident cases of heat stroke and 1,864 incident cases of heat exhaustion among active component service members (Table 1). The crude overall incidence rates of heat stroke and heat exhaustion were 0.37 and 1.40 per 1,000 p-yrs, respectively. In 2021, subgroup-specific incidence rates of heat stroke were highest among male service members, those less than 20 years old, those of other/unknown race/ethnicity (includes American Indian/Alaska Native service members, Asian/Pacific Islander service members, and those of unknown race/ethnicity), Marine Corps and Army members, recruit trainees, and those in combat-specific occupations (Table 1). The overall rate of heat stroke among female service members was 43.1% lower than the rate among male service members. The overall rates of incident heat stroke among Marine Corps and Army members were more than 8 times the rates among Air Force and Navy members. There were only 25 cases of heat stroke reported among recruit trainees, but their incidence rate was more than 2.5 times that of other enlisted service members and officers. 

The crude overall incidence rate of heat exhaustion among female service members was slightly (8.1%) lower than the rate among males (Table 1). In 2021, compared to their respective counterparts, service members less than 20 years old, Marine Corps and Army members, recruit trainees, and service members in combat-specific occupations had notably higher overall rates of incident heat exhaustion. 

Crude (unadjusted) annual incidence rates of heat stroke increased slightly from 0.42 per 1,000 p-yrs in 2017 to 0.46 cases per 1,000 p-yrs in 2018, but then dropped to 0.36 cases per 1,000 p-yrs in 2020 before leveling off at 0.37 per 1,000 p-yrs in 2021 (Figure 1). In the last year of the surveillance period, there were fewer heat stroke-related ambulatory visits than in any of the previous 4 years. The proportions of total heat stroke cases from hospitalizations remained relatively stable during 2017–2021 (range: 23.6%–28.1%). The proportions of total heat stroke cases from reportable medical events ranged from 24.9% to 34.5% and the proportions from ambulatory visits varied between 41.8% and 48.8%.

Crude annual rates of incident heat exhaustion increased from 1.47 per 1,000 p-yrs in 2017 to a peak of 1.74 per 1,000 p-yrs in 2018, fell to 1.26 per 1,000 p-yrs in 2020, and then increased to 1.40 per 1,000 p-yrs in 2021. (Figure 2). During the 5-year surveillance period, the proportions of total heat exhaustion cases from reportable medical events fluctuated between 33.5% and 40.9% and the proportions of cases from ambulatory visits varied between 57.4% and 65.0%. However, the proportions of heat exhaustion cases from hospitalizations remained relatively stable (range: 1.4%–3.2%). 

Heat Illnesses by Location

During the 5-year surveillance period, a total of 12,477 heat-related illnesses were diagnosed at more than 250 military installations and geographic locations worldwide (Table 2). Of the total heat illness cases, 5.6% occurred outside of the U.S., including 284 in Okinawa and 418 at 59 other locations in Europe, East Asia, Southwest Asia, Africa, and Cuba. Four Army installations in the U.S. accounted for more than one-third (35.1%) of all heat illnesses during the period: Fort Benning, GA (n=2,033); Fort Bragg, NC (n=936); Fort Campbell, KY (n=792); and Fort Polk, LA (n=614). Seven other locations accounted for an additional 29.1% of heat illness events: Marine Corps Base (MCB) Camp Lejeune/Cherry Point, NC (n=1,038); Marine Corps Recruit Depot Parris Island/Beaufort, SC (n=530); Naval Medical Center San Diego, CA (n=526); MCB Camp Pendleton, CA (n=457); Fort Hood, TX (n=407); MCB Quantico, VA (n=340); and Joint Base San Antonio-Lackland Air Force Base, TX (n=331). Of these 11 locations with the most heat illness events, 7 are located in the southeastern U.S. The 21 locations with more than 100 cases of heat illness accounted for over three-quarters (78.1%) of all active component cases during 2017–2021.

Heat Illnesses in the CENTCOM AOR

During the 5-year surveillance period, a total of 312 heat illnesses were diagnosed and treated in the CENTCOM AOR (i.e., Southwest Asia/Middle East) (Figure 3). Of the total cases of heat illness, 6.4%(n=20) were diagnosed as heat stroke. Deployed service members who were affected by heat illnesses were most frequently male (n=241; 77.2%); non-Hispanic White (n=183; 58.7%); 20–24 years old (n=168; 53.8%); in the Army (n=138; 44.2%); enlisted (n=301; 96.5%); and in repair/engineering (n=101; 32.4%) or communications/intelligence (n=65; 20.8%) occupations (data not shown). During the surveillance period, 2 service members were medically evacuated for heat illnesses from the CENTCOM AOR; 1 of the evacuations took place in spring (May 2017) and 1 in November 2020.

Editorial Comment

This annual update of heat illnesses among service members in the active component documented that the unadjusted annual rates of incident heat stroke and heat exhaustion peaked in 2018 and then declined in 2019 and 2020. Between 2020 and 2021, the rate of incident heat stroke was relatively stable while the rate of heat exhaustion increased slightly. 

There are significant limitations to this update that should be considered when interpreting the results. Similar heat-related clinical illnesses are likely managed differently and reported with different diagnostic codes at different locations and in different clinical settings. Such differences undermine the validity of direct comparisons of rates of nominal heat stroke and heat exhaustion events across locations and settings. Also, heat illnesses during training exercises and deployments that are treated in field medical facilities are not completely ascertained as cases for this report. In addition, recruit trainees were identified using an algorithm based on age, rank, loca­tion, and time in service. This method is only an approximation and likely resulted in some misclassification of recruit train­ing status. Moreover, it should be noted that the guidelines for mandatory reporting of heat illnesses were modified in the 2017 revision of the Armed Forces guidelines and case definitions for reportable medical events and carried into the 2020 revision.4 In this updated version of the guidelines and case definitions, the heat injury category was removed, leaving only case classifications for heat stroke and heat exhaustion. To compensate for such possible variation in reporting, the analysis for this update, as in previous years, included cases identified in DMSS records of ambulatory care and hospitalizations using a consistent set of ICD-9/ICD-10 codes for the entire surveillance period. However, it also is important to note that the exclusion of diagnosis codes for other and unspecified effects of heat and light (formerly included within the heat illness category “other heat illnesses”) in the current analysis precludes the direct comparison of numbers and rates of cases of heat exhaustion to the numbers and rates of “other heat illnesses” reported in MSMR updates before 2018. 

As has been noted in previous MSMR heat illness updates, results indicate that a sizable proportion of cases identified through DMSS records of ambulatory visits did not prompt mandatory reports through the reporting system.23 However, this study did not directly ascertain the overlap between hospitalizations and reportable events and the overlap between reportable events and outpatient encounters. It is possible that cases of heat illness, whether diagnosed during an inpatient or outpatient encounter, were not documented as reportable medical events because treatment providers were not attentive to the criteria for reporting or because of ambiguity in interpreting the criteria (e.g., the heat illness did not result in a change in duty status or the core body temperature measured during/immediately after exertion or heat exposure was not available). Underreporting is especially concerning for cases of heat stroke because it may reflect insufficient attentiveness to the need for prompt recognition of cases of this dangerous illness and for timely intervention at the local level to prevent additional cases. 

In spite of its limitations, this report demonstrates that heat illnesses continue to be a significant and persistent threat to both the health of U.S. military members and the effectiveness of military operations. Of all military members, the youngest and most inexperienced Marine Corps and Army members (particularly those training at installations in the southeastern U.S.) are at highest risk of heat illnesses, including heat stroke, exertional hyponatremia, and exertional rhabdomyolysis (see the other articles in this issue of the MSMR). 

Commanders, small unit leaders, training cadre, and supporting medical personnel—particularly at recruit training centers and installations with large combat troop populations—must ensure that the military members whom they supervise and support are informed regarding the risks, preventive countermeasures (e.g., water consumption), early signs and symptoms, and first-responder actions related to heat illnesses.16–22,30–32 Leaders should be aware of the dangers of insufficient hydration on the one hand and excessive water intake on the other; they must have detailed knowledge of, and rigidly enforce countermeasures against, all types of heat illnesses. 

Policies, guidance, and other information related to heat illness prevention and sun safety among U.S. military members are available online through the Army Public Health Center website at: https://phc.amedd.army.mil/topics/discond/hipss/Pages/default.aspx.

References

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4. Armed Forces Health Surveillance Branch, Defense Health Agency. In collaboration with U.S. Air Force School of Aerospace Medicine, Army Public Health Center, and Navy and Marine Corps Public Health Center. Armed Forces Reportable Medical Events. Guidelines and Case Definitions, January 2020. Accessed 3 March 2022. https://health.mil/Reference-Center/Publications/2020/01/01/Armed-Forces-Reportable-Medical-Events-Guidelines

5. Epstein Y, Yanovich R. Heatstroke. N Engl J Med. 2019;380(25):2449–2459.

6. Epstein Y. Heat intolerance: predisposing factor or residual injury? Med Sci Sports Exerc. 1990;22(1):29–35.

7. O’Connor FG, Casa DJ, Bergeron MF, et al. American College of Sports Medicine roundtable on exertional heat stroke—return to duty/return to play: conference proceedings. Curr Sports Med Rep. 2010;9(5):314–321.

8. Shapiro Y, Magazanik A, Udassin R, Ben-Baruch G, Shvartz E, Shoenfeld Y. Heat intolerance in former heatstroke patients. Ann Intern Med. 1979;90(6):913–916.

9. Dematte JE, O’Mara K, Buescher J, et al. Near-fatal heat stroke during the 1995 heat wave in Chicago. Ann Intern Med. 1998;129(3):173–181.

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11. Carter R 3rd, Cheuvront SN, Williams JO, et al. Epidemiology of hospitalizations and deaths from heat illness in soldiers. Med Sci Sports Exerc. 2005;37(8):1338–1344.

12. Hancock PA, Ross JM, Szalma JL. A meta-analysis of performance response under thermal stressors. Hum Factors. 2007;49(5):851–877.

13. Caldwell JN, Engelen L, van der Henst C, Patterson MJ, Taylor NA. The interaction of body armor, low-intensity exercise, and hot-humid conditions on physiological strain and cognitive function. Mil Med. 2011;176(5):488–493.

14. Maynard SL, Kao R, Craig DG. Impact of personal protective equipment on clinical output and perceived exertion. J R Army Med Corps. 2016;162(3):180–183.

15. Sawka MN, Cheuvront SN, Kenefick RW. High skin temperature and hypohydration impair aerobic performance. Exp Physiol. 2012;97(3):327–332.

16. Goldman RF. Introduction to heat-related problems in military operations. In: Lounsbury DE, Bellamy RF, Zajtchuk R, eds. Textbook of Military Medicine: Medical Aspects of Harsh Environments, Volume 1. Falls Church, VA: Office of the Surgeon General; 2001:3–49.

17. Sonna LA. Practical medical aspects of military operations in the heat. In: Lounsbury DE, Bellamy RF, Zajtchuk R, eds. Textbook of Military Medicine: Medical Aspects of Harsh Environments, Volume 1. Falls Church, VA: Office of the Surgeon General; 2001:293–309.

18. Headquarters, Department of the Army and Air Force. Technical Bulletin, Medical 507, Air Force Pamphlet 48-152. Heat Stress Control and Heat Casualty Management. 7 March 2003. 

19. Headquarters, United States Marine Corps, Department of the Navy. Marine Corps Order 6200.1E. Marine Corps Heat Injury Prevention Program. Washington DC: Department of the Navy; 6 June 2002. 

20. Navy Environmental Health Center. Technical Manual NEHC-TM-OEM 6260.6A. Prevention and Treatment of Heat and Cold Stress Injuries. Published June 2007.

21. Webber BJ, Casa DJ, Beutler AI, Nye NS, Trueblood WE, O'Connor FG. Preventing exertional death in military trainees: recommendations and treatment algorithms from a multidisciplinary working group. Mil Med. 2016;181(4):311–318. 

22. Lee JK, Kenefick RW, Cheuvront SN. Novel cooling strategies for military training and operations. J Strength Cond Res. 2015;29(suppl 11):S77–S81. 

23. Armed Forces Health Surveillance Branch. Update: Heat illness, active component, U.S. Armed Forces, 2020. MSMR. 2021;26(4):10–15.

24. Alele FO, Bunmi SM, Aduli EOM, Crow MJ. Epidemiology of exertional heat illness in the military: A systematic review of observational studies. Int J Environ Res Public Health. 2020;17(19):7037.

25. Dahl K, Licker R, Abatzoglou JT, Declet-Barreto J. Increased frequency of and population exposure to extreme heat index days in the United States during the 21st century. Environ Res Commun. 2019;1:075002.

26. Mukherjee S, Mishra AK, Mann ME, Raymona C. Anthropogenic warming and population growth may double US heat stress by the late 21st century. Earth’s Future. 2021;9:e2020EF001886.

27. Parsons IT, Stacey MJ, Woods DR. Heat adaptation in military personnel: mitigating risk, maximizing performance. Front Physiol. 2019;10:1485.

28. Kenny GP, Notley SR, Flouris AD, Grundstein A. Climate change and heat exposure: impact on health in occupational and general populations. In: Adams W, Jardine J, eds. Exertional Heat Illness: A Clinical and Evidence-Based Guide. Cham, Switzerland: Springer Nature; 2020:225–261.

29. Armed Forces Health Surveillance Branch. Surveillance case definition: Heat illness. Accessed on 3 March 2022. https://health.mil/Reference-Center/Publications/2019/10/01/Heat-Injuries 

30. Headquarters, Department of the Army Training and Doctrine Command. Memorandum. TRADOC Heat Illness Prevention Program 2018. 8 January 2018.

31. Kazman JB, O’Connor FG, Nelson DA, Deuster PA. Exertional heat illness in the military: risk mitigation. In: Hosokawa Y, ed. Human Health and Physical Activity During Heat Exposure. Cham, Switzerland: SpringerBriefs in Medical Earth Sciences; 2018:59–71.

32. Nye NS, O’Connor FG. Exertional heat illness considerations in the military. In: Adams W, Jardine J, eds. Exertional Heat Illness: A Clinical and Evidence-Based Guide. Cham, Switzerland: Springer Nature; 2020:181–210.

FIGURE 1. Incident casesa and incidence rates of heat stroke, by source of report and year of diagnosis, active component, U.S. Armed Forces, 2017–2021

FIGURE 2. Incident casesa and incidence rates of heat exhaustion, by source of report and year of diagnosis, active component, U.S. Armed Forces, 2017–2021

FIGURE 3. Numbers of heat illnesses diagnosed in the CENTCOM AOR, active component, U.S. Armed Forces, 2017–2021

TABLE 1. Incident casesa and incidence ratesb of heat illness, active component members, U.S. Army, Navy, Air Force, and Marine Corps, 2021

 

TABLE 2. Heat illness eventsa by location of diagnosis/report (with at least 100 cases during the period), active component, U.S. Armed Forces, 2017-2021

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4/1/2019
Drink water the day before and during physical activity or if heat is going to become a factor. (Photo Courtesy: U.S. Air Force)

In 2018, there were 578 incident diagnoses of heat stroke and 2,214 incident diagnoses of heat exhaustion among active component service members. The overall crude incidence rates of heat stroke and heat exhaustion diagnoses were 0.45 cases and 1.71 cases per 1,000 person-years, respectively. In 2018, subgroup-specific rates of incident heat stroke diagnoses were highest among males and service members less than 20 years old, Asian/Pacific Islanders, Marine Corps and Army members, recruit trainees, and those in combat-specific occupations. Subgroup-specific incidence rates of heat exhaustion diagnoses in 2018 were notably higher among service members less than 20 years old, Asian/Pacific Islanders, Army and Marine Corps members, recruit trainees, and service members in combat-specific occupations. During 2014–2018, a total of 325 heat illnesses were documented among service members in Iraq and Afghanistan; 8.6% (n=28) were diagnosed as heat stroke. Commanders, small unit leaders, training cadre, and supporting medical personnel must ensure that the military members whom they supervise and support are informed about the risks, preventive countermeasures, early signs and symptoms, and first-responder actions related to heat illnesses.

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Brief Report: Male Infertility, Active Component, U.S. Armed Forces, 2013–2017

Article
3/1/2019
Sperm is the male reproductive cell  Photo: iStock

Infertility, defined as the inability to achieve a successful pregnancy after 1 year or more of unprotected sexual intercourse or therapeutic donor insemination, affects approximately 15% of all couples. Male infertility is diagnosed when, after testing both partners, reproductive problems have been found in the male. A male factor contributes in part or whole to about 50% of cases of infertility. However, determining the true prevalence of male infertility remains elusive, as most estimates are derived from couples seeking assistive reproductive technology in tertiary care or referral centers, population-based surveys, or high-risk occupational cohorts, all of which are likely to underestimate the prevalence of the condition in the general U.S. population.

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Medical Surveillance Monthly Report

Sexually Transmitted Infections, Active Component, U.S. Armed Forces, 2010–2018

Article
3/1/2019
Anopheles merus

This report summarizes incidence rates of the 5 most common sexually transmitted infections (STIs) among active component service members of the U.S. Armed Forces during 2010–2018. Infections with chlamydia were the most common, followed in decreasing order of frequency by infections with genital human papillomavirus (HPV), gonorrhea, genital herpes simplex virus (HSV), and syphilis. Compared to men, women had higher rates of all STIs except for syphilis. In general, compared to their respective counterparts, younger service members, non-Hispanic blacks, soldiers, and enlisted members had higher incidence rates of STIs. During the latter half of the surveillance period, the incidence of chlamydia and gonorrhea increased among both male and female service members. Rates of syphilis increased for male service members but remained relatively stable among female service members. In contrast, the incidence of genital HPV and HSV decreased among both male and female service members. Similarities to and differences from the findings of the last MSMR update on STIs are discussed.

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Medical Surveillance Monthly Report

Vasectomy and Vasectomy Reversals, Active Component, U.S. Armed Forces, 2000–2017

Article
3/1/2019
Sperm is the male reproductive cell  Photo: iStock

During 2000–2017, a total of 170,878 active component service members underwent a first-occurring vasectomy, for a crude overall incidence rate of 8.6 cases per 1,000 person-years (p-yrs). Among the men who underwent incident vasectomy, 2.2% had another vasectomy performed during the surveillance period. Compared to their respective counterparts, the overall rates of vasectomy were highest among service men aged 30–39 years, non-Hispanic whites, married men, and those in pilot/air crew occupations. Male Air Force members had the highest overall incidence of vasectomy and men in the Marine Corps, the lowest. Crude annual vasectomy rates among service men increased slightly between 2000 and 2017. The largest increases in rates over the 18-year period occurred among service men aged 35–49 years and among men working as pilots/air crew. Among those who underwent vasectomy, 1.8% also had at least 1 vasectomy reversal during the surveillance period. The likelihood of vasectomy reversal decreased with advancing age. Non-Hispanic black and Hispanic service men were more likely than those of other race/ethnicity groups to undergo vasectomy reversals.

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Testosterone Replacement Therapy Use Among Active Component Service Men, 2017

Article
3/1/2019
Image of Marines carrying a wooden log for physical fitness. Click to open a larger version of the image.

This analysis summarizes the prevalence of testosterone replacement therapy (TRT) during 2017 among active component service men by demographic and military characteristics. This analysis also determines the percentage of those receiving TRT in 2017 who had an indication for receiving TRT using the 2018 American Urological Association (AUA) clinical practice guidelines. In 2017, 5,093 of 1,076,633 active component service men filled a prescription for TRT, for a period prevalence of 4.7 per 1,000 male service members. After adjustment for covariates, the prevalence of TRT use remained highest among Army members, senior enlisted members, warrant officers, non-Hispanic whites, American Indians/Alaska Natives, those in combat arms occupations, healthcare workers, those who were married, and those with other/unknown marital status. Among active component male service members who received TRT in 2017, only 44.5% met the 2018 AUA clinical practice guidelines for receiving TRT.

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Outbreak of Acute Respiratory Illness Associated with Adenovirus Type 4 at the U.S. Naval Academy, 2016

Article
2/1/2019
Malaria case definition

Human adenoviruses (HAdVs) are known to cause respiratory illness outbreaks at basic military training (BMT) sites. HAdV type-4 and -7 vaccines are routinely administered at enlisted BMT sites, but not at military academies. During Aug.–Sept. 2016, U.S. Naval Academy clinical staff noted an increase in students presenting with acute respiratory illness (ARI). An investigation was conducted to determine the extent and cause of the outbreak. During 22 Aug.–11 Sept. 2016, 652 clinic visits for ARI were identified using electronic health records. HAdV-4 was confirmed by real-time polymerase chain reaction assay in 18 out of 33 patient specimens collected and 1 additional HAdV case was detected from hospital records. Two HAdV-4 positive patients were treated for pneumonia including 1 hospitalized patient. Molecular analysis of 4 HAdV-4 isolates identified genome type 4a1, which is considered vaccine-preventable. Understanding the impact of HAdV in congregate settings other than enlisted BMT sites is necessary to inform discussions regarding future HAdV vaccine strategy.

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Update: Malaria, U.S. Armed Forces, 2018

Article
2/1/2019
Anopheles merus

Malaria infection remains an important health threat to U.S. service mem­bers who are located in endemic areas because of long-term duty assign­ments, participation in shorter-term contingency operations, or personal travel. In 2018, a total of 58 service members were diagnosed with or reported to have malaria. This represents a 65.7% increase from the 35 cases identi­fied in 2017. The relatively low numbers of cases during 2012–2018 mainly reflect decreases in cases acquired in Afghanistan, a reduction due largely to the progressive withdrawal of U.S. forces from that country. The percentage of cases of malaria caused by unspecified agents (63.8%; n=37) in 2018 was the highest during any given year of the surveillance period. The percent­age of cases identified as having been caused by Plasmodium vivax (10.3%; n=6) in 2018 was the lowest observed during the 10-year surveillance period. The percentage of malaria cases attributed to P. falciparum (25.9 %) in 2018 was similar to that observed in 2017 (25.7%), although the number of cases increased. Malaria was diagnosed at or reported from 31 different medical facilities in the U.S., Afghanistan, Italy, Germany, Djibouti, and Korea. Pro­viders of medical care to military members should be knowledgeable of and vigilant for clinical manifestations of malaria outside of endemic areas.

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Update: Incidence of Glaucoma Diagnoses, Active Component, U.S. Armed Forces, 2013–2017

Article
2/1/2019
Glaucoma

Glaucoma is an eye disease that involves progressive optic nerve damage and vision loss, leading to blindness if undetected or untreated. This report describes an analysis using the Defense Medical Surveillance System to identify all active component service members with an incident diagnosis of glaucoma during the period between 2013 and 2017. The analysis identified 37,718 incident cases of glaucoma and an overall incidence rate of 5.9 cases per 1,000 person-years (p-yrs). The majority of cases (97.6%) were diagnosed at an early stage as borderline glaucoma; of these borderline cases, 2.2% progressed to open-angle glaucoma during the study period. No incident cases of absolute glaucoma, or total blindness, were identified. Rates of glaucoma were higher among non-Hispanic black (11.0 per 1,000 p-yrs), Asian/Pacific Islander (9.5), and Hispanic (6.9) service members, compared with non-Hispanic white (4.0) service members. Rates among female service members (6.6 per 1,000 p-yrs) were higher than those among male service members (5.8). Between 2013 and 2017, incidence rates of glaucoma diagnoses increased by 75.4% among all service members.

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Re-evaluation of the MSMR Case Definition for Incident Cases of Malaria

Article
2/1/2019
Anopheles merus

The MSMR has been publishing the results of surveillance studies of malaria since 1995. The standard MSMR case definition uses Medical Event Reports and records of hospitalizations in counting cases of malaria. This report summarizes the performance of the standard MSMR case definition in estimating incident cases of malaria from 2015 through 2017. Also explored was the potential surveillance value of including outpatient encounters with diagnoses of malaria or positive laboratory tests for malaria in the case definition. The study corroborated the relative accuracy of the MSMR case definition in estimating malaria incidence and provided the basis for updating the case definition in 2019 to include positive laboratory tests for malaria antigen within 30 days of an outpatient diagnosis.

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Thyroid Disorders, Active Component, U.S. Armed Forces, 2008–2017

Article
12/1/2018
A U.S. naval officer listens through his stethoscope to hear his patient’s lungs at Camp Schwab in Okinawa, Japan in 2018. (Photo courtesy of U.S. Marine Corps) photo by Lance Cpl. Cameron Parks)

This analysis describes the incidence and prevalence of five thyroid disorders (goiter, thyrotoxicosis, primary/not otherwise specified [NOS] hypothyroidism, thyroiditis, and other disorders of the thyroid) among active component service members between 2008 and 2017. During the 10-year surveillance period, the most common incident thyroid disorder among male and female service members was primary/NOS hypothyroidism and the least common were thyroiditis and other disorders of thyroid. Primary/NOS hypothyroidism was diagnosed among 8,641 females (incidence rate: 43.7 per 10,000 person-years [p-yrs]) and 11,656 males (incidence rate: 10.2 per 10,000 p-yrs). Overall incidence rates of all thyroid disorders were 3 to 5 times higher among females compared to males. Among both males and females, incidence of primary/NOS hypothyroidism was higher among non-Hispanic white service members compared with service members in other race/ethnicity groups. The incidence of most thyroid disorders remained stable or decreased during the surveillance period. Overall, the prevalence of most thyroid disorders increased during the first part of the surveillance period and then either decreased or leveled off.31.6 per 100,000 active component service members in 2017. Validation of ICD-9/ICD-10 diagnostic codes for MetS using the National Cholesterol Education Program Adult Treatment Panel III criteria is needed to establish the level of agreement between the two methods for identifying this condition.

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Incidence and Prevalence of the Metabolic Syndrome Using ICD-9 and ICD-10 Diagnostic Codes, Active Component, U.S. Armed Forces, 2002–2017

Article
12/1/2018

This report uses ICD-9 and ICD-10 codes (277.7 and E88.81, respectively) for the metabolic syndrome (MetS) to summarize trends in the incidence and prevalence of this condition among active component members of the U.S. Armed Forces between 2002 and 2017. During this period, the crude overall incidence rate of MetS was 7.5 cases per 100,000 person-years (p-yrs). Compared to their respective counterparts, overall incidence rates were highest among Asian/Pacific Islanders, Air Force members, and warrant officers and were lowest among those of other/unknown race/ethnicity, Marine Corps members, and junior enlisted personnel and officers. During 2002–2017, the annual incidence rates of MetS peaked in 2009 at 11.6 cases per 100,000 p-yrs and decreased to 5.9 cases per 100,000 p-yrs in 2017. Annual prevalence rates of MetS increased steadily during the first 11 years of the surveillance period reaching a high of 38.9 per 100,000 active component service members in 2012, after which rates declined slightly to 31.6 per 100,000 active component service members in 2017. Validation of ICD-9/ICD-10 diagnostic codes for MetS using the National Cholesterol Education Program Adult Treatment Panel III criteria is needed to establish the level of agreement between the two methods for identifying this condition.

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Medical Surveillance Monthly Report
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Last Updated: June 21, 2022
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