Featured Topic

• Wildland Firefighting and the Immune Response
• Measuring Immune Response
• Field Studies
• Operational Strategies

Wildland Firefighting and the Immune Response

The human immune system is a complex arrangement of organs, tissues, cells, and molecules that protect the body from infectious microbes (including viruses, bacteria, fungi, parasites, and others). Fire camp records indicate that 30 to 50 percent of visits to first aid stations are for upper respiratory problems, including coughs, colds, and sore throats. A number of factors in the firefighting environment are known to compromise immune function. They include fatigue, exhaustion, stress, inadequate energy and nutrition, dehydration, smoke, and sleep deprivation. Wildland firefighting clearly presents a challenge to immune function and health. The goals of this phase of the project are to:

• Identify operational methods for assessing immune response in the field.
  
  
• Conduct field studies of immune response during fire suppression activities.
  
  
• Examine the use of immune function as an objective measure for the study of work, rest, and fatigue.
  
  
• Identify and test operational strategies for maintaining immune function in the field.
  
  
• Implement effective operational strategies in the field.

Measuring Immune Response

Dozens of tests are used to measure different aspects of the immune response. Skin-sensitivity tests measure the reaction of the skin to foreign assaults. Blood tests indicate the levels of white blood cells (leukocytes), cytokines, natural killer cells, T cells, immunoglobulin (A, G, and M), and other elements of the immune response. We needed a relatively noninvasive test that could be used in the field, without disrupting the work or rest of firefighters. Study of the literature and conversations with researchers led to a decision to conduct trials using salivary immunoglobulin A (sIgA) as an indicator of an individual’s immune response.

SIgA—The secretory immune system of the upper respiratory tract’s mucosal tissues is considered the body’s first line of defense against pathogens. SIgA, the major component of that system, inhibits attachment and replication of pathogens, preventing their entry into the body. Numerous studies have shown that sIgA decreases significantly after a bout of prolonged, intense effort, such as a marathon run. As many as 25 percent of the finishers in a marathon will experience an upper respiratory tract infection within 2 weeks after the event. After longer races, more than 50 percent of the finishers may experience an upper respiratory tract infection.

This decline in sIgA after arduous effort is viewed as a period of decreased immunity, when viruses and bacteria can gain a foothold. The data suggest that the stress of prolonged effort suppresses the immune system, decreasing host protection. After successful laboratory trials, sIgA levels were used as an indicator of immune function in field studies of wildland firefighters.

Field Studies

The initial field study looked at sIgA before and after a 14-h shift on the fireline. Hotshot firefighters provided saliva samples before and immediately after the shift and before work the following day. Values fell dramatically (from 76.9 µg/min to 14 µg/min) in the sample taken after a shift, but recovered to baseline levels (82.4 µg/min) the following morning. When sIgA was measured before and after a shift of 21 h, the immune response declined, failed to fully recover the following morning, and remained depressed during 5 additional days of work (figure 1).

Figure 1—Changes in salivary IgA secretion rate
in response to a single work shift of 21.4 ± 0.3 h (n = 11)
followed by 5 d of 14 ± 0.7-h shifts.

Also measured in this study was a fatigue index calculated from heart rates recorded after a standardized stepping test. Fatigue measured after the 14-h shift rose significantly, but returned to baseline levels before the next day’s work. Measures of immune function went down when measures of fatigue went up, as reported by Ruby and others in Wildland Firefighter Health and Safety Report, No. 5 (0251–2815–MTDC).

The second study investigated the effects of energy expenditure and consecutive days of work on immune function (as measured by sIgA). The results indicated the immune function’s ability to recover after shifts with moderate energy expenditure, its failure to recover after hard shifts, and its progressive decline with consecutive days of hard work. As before, the fatigue index was inversely related to immune function. A summary of the study is included in this report.

Operational Strategies

During studies of immune function in difficult field conditions, such as during military training, researchers have investigated the effects of nutritional supplements on various aspects of the immune response. Nutrients that appear to play a role in immune function include the amino acids, glutamine and arginine, certain vitamins and antioxidants, proteins, and some fatty acids. However, supplementing diets with these nutrients has not provided consistent significant benefits to immune function during periods of prolonged arduous work. Total energy intake appears to play the greatest role in maintaining the immune function. Researchers recommend optimizing soldiers’ nutritional status before deployment and maintaining nutritional status during the operation.

Studies of endurance events and military training have shown consistent benefits from liquid and solid carbohydrate supplements, suggesting that total energy intake needs to be maintained during periods of prolonged arduous work. Based on these studies, we decided to investigate the effects of liquid carbohydrate supplementation on work output, fatigue, and immune function in wildland firefighters. The results, reported in Wildland Firefighter Health and Safety Report, No. 6, (0251–2837–MTDC), showed that carbohydrate supplementation improved blood glucose levels, increased energy expenditure and work rate, and reduced fatigue, especially later in a work shift.

The sIgA levels of firefighters who received supplemental carbohydrates fell significantly less and recovered better 12 h after a shift (table 1).

Table 1—The effect of carbohydrate supplementation on sIgA levels of wildland firefighters. The values show the percent sIgA levels were above or below baseline values.

Time	Carbohydrate (percent)	Placebo (percent)
End of shift	-32	-47
12 h after shift	3	-23

Carbohydrate supplements provide energy for working muscles and the brain and nervous system. When blood glucose levels are maintained, the body secretes less cortisol, a stress hormone that suppresses the immune response. Without carbohydrate supplementation, sIgA remained depressed at the beginning of the next work shift (see page 3).

Researchers from the University of Montana and MTDC have summarized aspects of these findings in the tech tip, Feeding the Wildland Firefighter (0251–2323–MTDC). The dietary recommendations are based on energy and nutritional requirements, field studies of firefighter energy balance, analysis of the energy and nutrients available in fire camp meals, and published studies of athletes, soldiers, and wildland firefighters. The tech tip includes recommendations for liquid and solid carbohydrate supplementation during and after work. The field notes section of this report provides practical guidelines for selecting liquid and solid carbohydrate supplements.