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Wildland Firefighter Health & Safety Report

Fall 2000 MTDC No. 2

Featured Topic

Wildland Firefighting and the Immune Response
Steve Wood, Ph.D., R.D.

Wildland firefighters perform their jobs in a number of conditions that may suppress the immune system and increase the risk of illness. This paper examines the influences of physical and psychological stresses on immune function and presents a military model, containing many of the stresses found in wildland firefighting, of a field nutrition study where immune suppression was minimized.

The Immune System—The immune system is an intricate system of organs, tissues, cells, and molecules that maintain balance between the environment and the body. Stresses such as physical exertion, sleep deprivation, malnutrition, extreme temperatures, smoke, psychological pressures, and endocrine changes influence the body's defense system and alter its equilibrium. The environment we live in has a variety of infectious microbes (viruses, bacteria, fungi, parasites, and protozoa). If these multiply unchecked, they can cause sickness, disease, and death. Our bodies combat these microorganisms through immune responses. The skin is the first line of defense. While few infectious agents can penetrate intact skin, they may gain access across epithelial tissues, or through the gastrointestinal or urogenital tracts. Once an organism has gained access, the immune system must recognize the pathogen or foreign material and mount an attack (immune response).

Immune responses are coordinated primarily by white blood cells (leukocytes). Once the white blood cells identify the foreign material, the immune system mounts an attack to bind and destroy it (phagocytosis). The immune response is much like an orchestra. When cells and organs are working together, the immune system functions effectively; however, if cells and organs are not working harmoniously, the risk of disease and illness increases.

Energy and sleep deprivation, mental stress, and intense physical exertion affect the immune system. It is possible to measure overall immune system effectiveness by using clinical measures such as delayed-type hypersensitivity (DTH), the rate of infection or response to pathogen or vaccine. DTH tests can be administered easily by placing small amounts of proteins that cause a reaction (antigens) into the epidermis and superficial dermal tissue (Table 1).

Table 1—Delayed-Type Hypersensitivity (DTH), infection, and death rate.
Infection Rate
(percent)
Death Rate of
infected patients
(percent)
Reactive 7 19
Relative Anergy* 16 32
Anergy 25 51
*Anergy, the lack of response, indicates impairment of the immune system. Based on skin testing of 4,289 hospitalized patients (Christou and others, 1995).

Persons who are DTH reactive have a much lower rate of infection than those who are anergic (not responsive). If persons who are anergic become infected, they have a poor prognosis. A reduction in cell-mediated immune responses can limit the development of an effective immune response against intracellular pathogens, including viruses, bacteria, fungi, and protozoa. DTH has also been used to evaluate the immune function of patients who are malnourished, whose immune system is suppressed, or who have experienced trauma.

Firefighting Stress—Wildland firefighters are exposed to many stresses that can affect their health and influence their immune systems. In addition to the psychological challenges of firefighting, a number of physical challenges (such as environmental toxins, injuries, smoke inhalation, and burns) could cause the immune system to be suppressed. Although many studies have evaluated the effectiveness of equipment for wildland firefighters, none has focused directly on the health of the firefighter's immune system. Therefore, we will examine research conducted in similar situations to evaluate factors that may influence the wildland firefighter's immune system.

Stress and Infection—What happens to the immune system as a result of stress? Do changes in the immune system translate into increased susceptibility to disease? In 1991 Peterson and others reviewed stress and the pathogenesis of infectious disease. The authors reviewed many studies in which stressors (exertion, electric shock, isolation or crowding, and exposure to cold temperatures) were imposed on animals.

Several epidemiological studies have documented an increased incidence of upper respiratory tract infection after strenuous exercise. On the other hand, individuals who exercise moderately have a lower risk of infection than those who are sedentary. Regular moderate exercise appears to convey enhanced immunological responses. More chronic and severe stress, such as that experienced by persons training in the U.S. Army Special Forces Assessment and Selection School (SFAS), decreases immune function and increases susceptibility to environmental pathogens (Figure 1).

A Physical Activity Plot showing the relationship of physical activity to immune function and risk of infection.
Figure 1—The relationship of physical
activity to immune function and
risk of infection.

Ambient conditions also can influence immune function and susceptibility to infection. Animals maintained at low temperatures experience higher rates of infection. Animal studies found females more resistant to the effects of crowding. Lack of sleep has been shown to reduce immune function. Rats that are deprived of sleep drastically increase their food consumption (hyperphagia) but they lose weight (malnutrition-like symptoms) and may succumb to lethal systemic infections.

Readiness and Impact on Time Lost—The effect of decreased immune function on firefighter readiness and lost work hours has not been reported. However, corollaries from military studies can be used to establish the effects. For example, in several major U.S. military conflicts disease and nonbattle injuries (DNBI) have accounted for significant loss of manpower.

Exposure to pathogens from environmental factors and crowded living quarters contribute to disease susceptibility in military units even during routine peacetime operations. Seay and others found that 777 cases of gastroenteritis were reported after a 10-day port visit. The illness affected 15 percent of the work force and cost 462 days of lost work. Seay stated, "Lost duty days, and the burden of providing medical confinement may be more critical than fatal illnesses. Fatalities may be replaced but sick soldiers continue to occupy positions, decrement performance, and consume large quantities of medical supplies." While many factors may contribute to susceptibility to infection and disease during military operations, the effect of physical and psychological stress undoubtedly have a significant impact.

Physical and Nonphysical Stresses—Stressors, such as exertion and extreme environmental temperatures, can affect the immune system. The changes are similar to nonphysical stresses (such as isolation). In human studies, nonphysical and physical stress can produce immunological changes.

Chemicals in the smoke wildland firefighters may be exposed to can affect the immune system. Smoke exposure has been a recognized health issue for many years. A few studies of firefighters in the 1970's hinted that smoke exposure contributed to fatigue and injury. The main inhalation hazards are carbon monoxide, aldehydes, benzene, acrolein, and particulate matter small enough to reach the lungs.

Nutrition—Starvation has been shown to affect the immune system. Nutritional deficiencies or suboptimal nutrient levels can be the result of eating too little food, decreased absorption of nutrients, or increased use of nutrients. Many nutrients are required for optimal functioning of the immune system. The effect of infection on nutritional status is characterized by wasting of peripheral tissues, particularly lean body mass (muscle). Nutrients (vitamins and minerals) are used as substrate and cofactors for immune cells. Once an infection takes place, immune cells have a higher requirement for nutrients. After infection, particularly of the gastrointestinal tract, the gut's mucosal lining may be damaged and the microflora of the gut may change, decreasing the small intestine's absorptive capacity. The mucosal and microflora changes can also be influenced by antibiotics and medications, further decreasing the effectiveness of nutrient absorption. An infection may also increase use of nutrients. Nutrient supplementation can prevent malnutrition and provide the essential factors for immune cells to function optimally.

The era of nutritional manipulation of the immune system has finally dawned and it brings with it the promise of using diet and nutrition as innovative powerful tools to reduce illness and death caused by infection.
—R.K. Chandra, Proceedings of the National Academy of Science.

Nutrient supplementation can improve immune function, counteracting the stress-induced immune changes. Supplementation to minimize or prevent immune changes and protect individuals who are undergoing physical stress has been studied to a limited extent. Nutrients such as vitamin C may play a role in immune function and may prevent immune suppression in physically stressed individuals. Peters and others supplemented the diets of runners with 600 mg vitamin C per day or a placebo for 21 days before an ultramarathon. Sixty-eight percent of the runners who consumed the placebo reported upper respiratory tract infections compared with 33 percent of those who consumed the vitamin C.

Giuliani and Cesaro report that physical activity raises oxygen demand severalfold, increasing the formation of oxygen radical species or free radicals. As oxygen radical species are formed, they adversely affect immune function. Theoretically, providing antioxidants in sufficient amounts during exercise-induced oxidative stress may maintain the immune system. Several reviews have been published on the importance of dietary antioxidants and their influence on the immune response.

Army Special Forces and Ranger Training—Ross Products Division of Abbott Laboratories has investigated potential benefits of nutrient supplementation and nutritional strategies to minimize immunologic changes in a variety of conditions. The military stresses (carrying heavy packs and equipment, strenuous physical exertion, exposure to extreme temperatures, and psychological stress) seem to be similar to those experienced by wildland firefighters.

Historically, military deployment has been associated with an increased incidence of infectious diseases. Furthermore, soldiers in military training have incurred more outbreaks of infectious disease than would be expected, pointing toward an underlying susceptibility. The United States Army Special Forces Assessment and Selection Course (SFAS) is highly demanding, physically and emotionally. Fairbrother and others documented a negative energy balance (1,379 kcals/day) in soldiers who participated in the 21-day course; however, no significant clinical manifestations of vitamin or mineral deficiencies were noted. One observation from that study was a reduced lymphocyte proliferation response. This observation suggested that multiple stressors (negative energy balance, sleep deprivation, physical activity, and psychological stress) may have negative effects on immune function and the ability of lymphocytes to combat infections. Bernton and others found that during Ranger training DTH skin anergy increased over time. Those trainees who did respond had a decreased response.

Nutritional intervention with this group allowed for an evaluation of nutritional supplementation on immune function. Two hundred soldiers participating in an SFAS course volunteered to participate in this prospective, randomized, blinded, placebo-controlled study. The test group (n = 100 soldiers) consumed their regular diet consisting mainly of meals ready-to-eat (MRE) plus a novel ready-to-eat treatment product (8 oz two times per day) containing antioxidants, minerals, a structured lipid (from long- and medium-chain fatty acids), and indigestible carbohydrates. The control group (n = 100 soldiers) was given a ready-to-eat product similar to the treatment product in taste and appearance with a similar amount of energy but without antioxidants, minerals, and the structured lipid. Dietary intake, body weight, and immune function were measured before and after the physically and psychologically demanding 21-day training course.

Fifty percent of the soldiers in the treatment group completed the SFAS course compared to 57 percent in the control group. Subjects lost about 6.4 pounds in 3 weeks. Combining the dietary intake with the weight loss data, soldiers had an estimated energy expenditure of 5,040 kcals per day. The DTH test after training suggested that subjects consuming the treatment product had a greater response (P = 0.07). Fewer subjects in the treatment group were DTH anergic (18 percent) to the skin test compared to the control group (39 percent), suggesting a lower risk for infections in the treatment group. Overall, the treatment formula appeared to minimize the immunologic changes associated with the stresses of SFAS training.

A similar 62-day study was conducted during Ranger training, in which trainees received a nutrition bar with a nutrient profile similar to the ready-to-drink formula used in the SFAS study. The treatment bar lessened some of the immunological changes and appeared to help maintain weight better than the placebo product.

Conclusions

The immune system is an intricate and highly regulated network of cells, tissues and molecules that help ward off infections and disease. Although the effectiveness of the immune response can be influenced by a number of stressors, specialized nutrition can help maintain the immune system.

No studies have been conducted to evaluate immune function in wildland firefighters; however it is likely that they experience some level of immune suppression from the physical and emotional stresses. If there is an increased rate of infection, it would be valuable to study strategies (such as nutritional supplementation) that might influence the immune systems under these conditions. Nutritional formulations tested in military training, where the physical and psychological stress may be similar to wildland firefighting, may also be of value.

Steven Wood, Ph.D., RD, is senior clinical project leader associated with Ross Products Division, Abbott Laboratories. The complete text of this paper may be found in Wildland Firefighter Health and Safety: Recommendations of the April 1999 Conference, 9951-2841-MTDC. The paper contains references and a list of nutrients and their influence on immune function.

Firefighter Immunity

Mark Vore of the Idaho Panhandle National Forests analyzed 1994 Northern Rockies fire camp medical records. He found that 30 to 50 percent of the visits to medical tents were for upper respiratory problems, including coughs, colds, and sore throats. Similar results were noted by a team investigating fatigue and stress during the 2000 fire season. A number of factors in the firefighting environment influence immune function and the body's susceptibility to respiratory and other illnesses. Upper respiratory problems can be due to fatigue, stress, smoke exposure, inadequate nutrition, or a combination of stresses. Rapid weight loss and sleep deprivation have also been associated with decreased immune function.

Fire managers and crew leaders should do all they can to avoid exhausting firefighters and exposing them to excess smoke. Managers can reduce stress by providing up-to-date information, by soliciting input from crew members, and by minimizing crowding, waiting, and other factors that contribute to stress . They can enhance rest and recuperation by providing adequate time and conditions for sleep. And they can ensure firefighter fitness with early-season conditioning and heat acclimatization. Firefighters are responsible for preseason conditioning and nutritional food choices. Fire camp meals provide adequate energy and nutrients. Firefighters must consume adequate calories to maintain energy, and eat adequate servings of fruits and vegetables or consider multi-vitamin/mineral supplements.