Abstract

From July 2017 through June 2018, a total of 478 members of the active (n=402) and reserve (n=76) components had at least one medical encounter with a primary diagnosis of cold injury. The crude overall incidence rate of cold injury for all active component service members in 2017–2018 was 19.6% higher than the rate for the 2016–2017 cold season and was the highest rate since the 2013–2014 season. Frostbite was the most common type of cold injury among active component service members in 2017–2018. Among active component members during the 2013–2018 cold seasons, overall rates of cold injuries were generally highest among males, non-Hispanic black service members, the youngest (less than 20 years old), and those who were enlisted. As noted in prior MSMR updates, the rate of all cold injuries among active component Army members was considerably higher in females than in males due to a much higher rate of frostbite among female soldiers. The numbers of cold injuries associated with overseas deployments have fallen precipitously in the past three cold seasons and included 17 cases in the most recent year.

What Are the New Findings?

Cold weather injuries increased by 20% in 2017-2018 from the previous year. Rates were highest among exposed Army and Marine Corps personnel. Frostbite remained the most common cold weather injury.

What Is the Impact on Readiness and Force Health Protection?

Prevention of cold injuries is the responsibility of commanders at all levels and cold weather injuries such as hypothermia, frostbite, and trench foot can be avoided during cold weather exercises. Preparation for cold weather operations involves advance planning by leaders, individual compliance, and supervisory follow-through.

Since 2004, the MSMR has published annual updates on the incidence of cold weather injuries that affected U.S. military members during the five most recent cold seasons.¹ The content of this 2018 report addresses the occurrence of such injuries during the cold seasons from July 2013 through June 2018. The timing of the annual updates is intended to call attention to the recurring risks of such injuries as winter approaches in the Northern Hemisphere, where most members of the U.S. Armed Forces are assigned. For many years, the U.S. Armed Forces have developed and improved robust training, doctrine, procedures, and protective equipment and clothing to counter the threat from cold environments.^2-4 Although these measures are highly effective, cold injuries have continued to affect hundreds of service members each year because of exposure to cold and wet environments.⁵ Such environmental conditions pose the threat of hypothermia, frostbite, and nonfreezing cold injury such as immersion injury. The human physiologic response to cold exposure is to retard heat loss and preserve core body temperature, but this response may not be sufficient to prevent hypothermia if heat loss is prolonged.⁶ Moreover, the response includes constriction of the peripheral (superficial) vascular system, which may result in non-freezing injuries or hasten the onset of actual freezing of tissues (frostbite).⁶ Traditional measures to counter the dangers associated with cold environments include minimizing loss of body heat and protecting superficial tissues through such means as protective clothing, shelter, physical activity, and nutrition. However, military training or mission requirements in cold and wet weather may place service members in situations where they may be unable to be physically active, find warm shelter, or change wet or damp clothing.^2,3

Military history has well documented the toll of cold weather injuries. Continuous surveillance of these injuries is essential to inform steps to reduce their impact as well as to remind leaders of the predictable threat of cold injuries. This update summarizes the frequencies, incidence rates, and correlates of risk of cold injuries among members of both active and reserve components of the U.S. Armed Forces during the past 5 years.

Methods

The surveillance period was 1 July 2013 through 30 June 2018. The surveillance population included all individuals who served in the active or reserve component of the U.S. Armed Forces at any time during the surveillance period. For analysis purposes, "cold years" or "cold seasons" were defined as 1 July through 30 June intervals so that complete cold weather seasons could be represented in year-to-year summaries and comparisons.

Because cold weather injuries 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 (RMEs) via one of the service-specific electronic reporting systems; these reports are routinely incorporated into the Defense Medical Surveillance System (DMSS). For this analysis, the DMSS and the Theater Medical Data Store (which maintains electronic records of medical encounters of deployed service members) were searched for records of RMEs and inpatient and outpatient care for the diagnoses of interest (frostbite, immersion injury, and hypothermia). A case was defined by the presence of an RME or of any qualifying ICD-9 or ICD-10 code in the first diagnostic position of a record of a health care encounter (Table 1). The DOD guidelines for RMEs require the reporting of cases of hypothermia, frostbite, and immersion injuries but not "other specified/unspecified effects of reduced temperature." Seven cases of chilblains are not included in this report because the condition is common, infrequently diagnosed, usually mild in severity, and thought to have minimal medical, public health, or military impacts.

To estimate the number of unique individuals who suffered a cold injury each cold season, and to avoid counting follow-up health care encounters after single episodes of cold injury, only one cold injury per individual per cold season was included. A slightly different approach was taken for summaries of the incidence of the different types of cold injury diagnoses. In counting types of diagnoses, one of each type of cold injury per individual per cold season was included. For example, if an individual was diagnosed with immersion foot at one point during a cold season and then with frostbite later during the same cold season, each of those different types of injury would be counted in the tally of injuries. If a service member had multiple medical encounters for cold injuries on the same day, only one encounter was used for analysis (hospitalizations were prioritized over ambulatory visits which were prioritized over RMEs). Annual incidence rates of cold injuries among active component service members were calculated as incident cold injury diagnoses per 100,000 person-years (p-yrs) of service. Annual rates of cold injuries among reservists were calculated as cases per 100,000 persons using the total number of reserve component service members for each year of the surveillance period. Counts of persons were used as the denominator in these calculations because information on the start and end dates of active duty service periods of reserve component members was not available.

The numbers of cold injuries were summarized by the locations at which service members were treated for these injuries as identified by the Defense Medical Information System Identifier (DMIS ID) recorded in the medical records of the cold injuries. Because such injuries may be sustained during field training exercises, temporary duty, or other instances for which a service member may not be located at his/her usual duty station, DMIS ID was used as a proxy for the location where the cold injury occurred.

The new electronic health record for the Military Health System, MHS GENESIS, was implemented at several military treatment facilities during 2017. Medical data from sites using MHS GENESIS 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 and person-time data for individuals seeking care at one of these facilities during 2017 were not included in this analysis.

Results

2017-2018 cold season

From July 2017 through June 2018, a total of 478 members of the active (n=402) and reserve (n=76) components had at least one medical encounter with a primary diagnosis of cold injury (Table 2). The crude overall incidence rate of cold injury for all active component service members in 2017–2018 (32.9 per 100,000 p-yrs) was 19.6% higher than the rate for the 2016– 2017 cold season (27.5 per 100,000 p-yrs) and was the highest rate since the 2013–2014 season (Table 2, Figure 1). Throughout the surveillance period, the cold injury rates were consistently higher among active component members of the Army or the Marine Corps than among those in the Air Force or Navy. In 2017–2018, the service-specific incidence rate for active component Army members (54.6 per 100,000 p-yrs) was 26.5% higher than the 2016– 2017 Army rate (43.2 per 100,000 p-yrs). The Army contributed slightly more than three-fifths (60.9%; n=245) of all cold injury diagnoses in the active component during the 2017–2018 cold season. For the Marine Corps, the active component rate for 2017–2018 was 15.9% higher than the rate for the previous season. The 85 members of the Marine Corps diagnosed with a cold injury in 2017–2018 represented 21.1% of all affected active component service members. Navy service members (n=27) had the lowest service-specific rate of cold injuries during the 2017–2018 cold season (9.8 per 100,000 p-yrs) (Table 2, Figure 1).

This update for 2017–2018 represents the second year that annual rates of cold injuries for members of the reserve component were estimated. Army personnel (n=51) accounted for 67.1% of all reserve component service members (n=76) affected by cold injuries during 2017–2018 (Table 2). As was true for the active component, service-specific rates among reserve component members were higher among those in the Army or Marine Corps than among those in the Air Force or Navy (Figure 2). For the 2017–2018 cold season, the overall rate of cold injuries for the reserve than in the 2016–2017 season. Among reserve component members, the most pronounced increase in service-specific rates between the 2016–2017 and 2017– 2018 seasons was seen in the Marine Corps.

When all injuries were considered, not just the numbers of individuals affected, frostbite was the most common type of cold injury (n=247; 60.8% of all cold injuries) among active component service members in 2017–2018 (Tables 3a–3d). In the Air Force and Army respectively, 84.4% and 65.0% of all cold injuries were frostbite, whereas the proportions in the Navy (55.6%) and Marine Corps (38.6%) were much lower. For the Army and Marine Corps, the 2017–2018 numbers and rates of frostbite injuries among active component service members were the highest of the past 4 years. For all active component service members during 2017–2018, the proportions of all cold weather injuries that were hypothermia and immersion injuries were 18.7% and 20.4%, respectively (data not shown). Among active component Navy members, the numbers and rates of hypothermia cases and immersion injuries in 2017–2018 were the lowest of the 5-year surveillance period and of the past 4 years, respectively (Table 3b). The number and rate of immersion injury cases in 2017–2018 in the Air Force were the lowest of the surveillance period (Table 3c).

Five cold seasons: July 2013-June 2018

During the 5-year surveillance period, the rates of cold injuries among members of the active components of the Navy, Air Force, and Marine Corps were higher among males than females. Among active component Army members, there was a striking difference between the rates for females (61.3 per 100,000 p-yrs) and males (48.5 per 100,000 p-yrs). In all of the services during 2013–2018, females had lower rates of immersion injury and hypothermia than did males but higher rates of frostbite (except in the Air Force) (Tables 3a–3d). For active component service members in all four services combined, the overall rate of cold injury was slightly higher among males (32.6 per 100,000 p-yrs) than among females (29.4 per 100,000 p-yrs) (data not shown).

In all of the services, overall rates of cold injuries were higher among non-Hispanic black service members than among those of the other race/ethnicity groups. In particular, within the Marine Corps and Army, and for all services combined, rates of cold injuries were more than twice as high among non-Hispanic black service members than among either non-Hispanic white service members or those in the "other/unknown" race/ethnicity group (Tables 3a–3d). The major underlying factor in these differences is that rates of frostbite among non-Hispanic black members of all services were 1.5 or more times higher than those of the other race/ethnicity groups across the active components of all services during 2013–2018, non-Hispanic black service members had incidence rates of cold injuries greater than the rates of other race/ethnicity groups in nearly every military occupational category (data not shown).

Rates of cold injuries were generally highest among the youngest service members (less than 20 years old) and tended to be lower with each succeeding older age group. Enlisted members of the Army, Air Force, and Navy had higher rates than officers, but the opposite was true of Marine Corps members (Tables 3a–3d). In the Army and Air Force, rates of all cold injuries combined were highest among service members in combat-specific occupations (infantry/artillery/combat engineering/armor) (Tables 3a, 3c).

During the 5-year surveillance period, the 2,405 service members who were affected by any cold injury included 2,056 from the active component and 349 from the reserve component. Of all affected reserve component members, 70.5% (n=246) were members of the Army (Table 2). Overall, soldiers accounted for the majority (60.0%) of all cold injuries affecting active and reserve component service members (Table 2, Figure 3).

Of all active component service members who were diagnosed with a cold injury (n=2,056), 195 (9.5% of the total) were affected during basic training. The Army (n=79) and Marine Corps (n=107) accounted for 95.4% of all basic trainees who suffered a cold injury (data not shown). Additionally, during the surveillance period, 73 service members who were diagnosed with cold injuries (3.6% of the total) were hospitalized, and most (91.8%) of the hospitalized cases were members of either the Army (n=40) or Marine Corps (n=27) (data not shown).

Cold injuries during deployments

During the 5-year surveillance period, a total of 77 cold injuries were diagnosed and treated in service members deployed outside of the U.S. Of these, 38 (49.4%) were immersion injuries; 26 (33.8%) were frostbite; and 13 (16.9%) were hypothermia. Of all 77 cold injuries during the surveillance period, nearly one-third (32.5%) occurred in the first cold season. There were 25 cold injuries during cold season 2013–2014 but only 13 during 2014–2015, 11 during 2015–2016, 11 during 2016–2017, and 17 during 2017–2018 (data not shown).

Cold injuries by location

During the 5-year surveillance period, 21 military locations had at least 30 incident cold injuries (one per person per year) among active and reserve component service members (data not shown). Among these locations, those with the highest counts of five-year injuries were Fort Wainwright, AK (n=155); Bavaria (Vilseck/Grafenwoehr), Germany (117); Marine Corps Recruit Depot Parris Island/Beaufort, SC (100); Fort Benning, GA (86); San Diego, CA (78); Fort Carson, CO (67); and Fort Campbell, KY (65). During the 2017–2018 cold season, the numbers of incident cases of cold injuries were 2016–2017 cold season at 13 of the 21 locations (data not shown). The most noteworthy increases were found at the Army's Fort Benning and Fort Campbell, where there were 16 total cases diagnosed at each location in 2017–2018, compared to just five and six, respectively, the year before (data not shown). Figure 4 shows the numbers of cold injuries during 2017–2018 and the median numbers of cases for the previous 4 years for those locations that had at least 30 cases during the surveillance period. For nine of the 21 installations, the numbers of cases in 2017–2018 were below the median counts for the previous 4 years.

Editorial Comment

Overall incidence rates of cold injuries among U.S. service members increased in 2017–2018 compared with the previous winter. Across all services, the number of cold injury cases in 2017–2018 was the highest count of the past 3 years.

In 2017–2018, frostbite was the most common type of cold injury among active component service members in all the services except for the Marine Corps, in which immersion injury was the most common. Compared to their respective counterparts, overall rates of cold injuries were generally higher among males, non-Hispanic black service members, the youngest (less than 20 years old), and those who were enlisted. Increased rates of cold injuries affected nearly all enlisted and officer occupations among non-Hispanic black service members. Of note, rates of frostbite were markedly higher among non-Hispanic blacks compared to non-Hispanic whites and those in the other/unknown race/ethnicity group. These differences have been noted in prior MSMR updates and the results of several studies suggest that other factors (e.g., physiologic differences and/or previous cold weather experience) are possible explanations for increased susceptibility.^8-11

The numbers of cold injuries associated with deployment have fallen precipitously in the past four cold seasons. This reduction in the number of cases is almost certainly a result of the dramatic decline in the numbers of service members deployed to Iraq and Afghanistan and of changes in the nature of military operations there.

Policies and procedures are in place to protect service members against cold weather injuries. Modern cold weather uniforms and equipment provide excellent protection against the cold when used correctly. However, in spite of these safeguards, a significant number of individuals within all military services continue to be affected by cold weather injuries each year. It is important that awareness, policies, and procedures continue to be emphasized to reduce the toll of such injuries. In addition, enhancements in protective technologies deserve continued research. It should be noted that this analysis of cold injuries was unable to distinguish between injuries sustained during official military duties (training or operations) and injuries associated with personal activities not related to official duties. To provide for all circumstances that pose the threat of cold weather injury, service members should know well the signs of cold injury and how to protect themselves against such injuries they are training, operating, fighting, or recreating under wet and freezing conditions.

The most current cold injury prevention materials are available at:
https://phc.amedd.army.mil/topics/discond/cip/Pages/Cold-Weather-Casualties-and-Injuries.aspx

References

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