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|a The occurrence of illness, particularly upper respiratory tract symptom (URTS) episodes, is common in elite team-sport athletes. Recurrent or severe URTS is known to impair performance; despite this, research in elite team-sport athletes is lacking, with most exercise immunology studies focusing on endurance athletes. Therefore, this thesis used a multifaceted/holistic approach to investigate which factors (if any) meaningfully influence URTS risk in elite team-sport athletes during real-life training and competition periods. A series of four related studies were conducted involving three groups of elite team-sport athletes; namely, rugby union, rugby league and field hockey players. In the first study (Chapter 5), URTS incidence and possible predictors of URTS were compared between elite rugby union and league players during an intensive pre-season training period. URTS incidence was similar between the rugby codes; however, predictors of URTS risk differed. Strongest predictors of URTS risk were reduced salivary secretory immunoglobulin (SIgA) (Hazard ratio (HR): 0.997, p = 0.094) in rugby union players, and decreased total wellness (HR: 0.731, p = 0.004) and sleep quality (HR: 0.345, p = 0.001) in rugby league players. These findings suggest that factors influencing URTS risk are perhaps sport and/or cohort specific. The second study (Chapter 6) examined potential factors influencing URTS risk in elite rugby union players during an entire sporting season. Rest weeks were identified as periods of increased risk for URTS; whereas URTS risk was reduced during weeks involving international travel. Household illness incidence was found to be the strongest predictor of URTS risk (HR: 2.902, p = 0.002), and a trend for an inverse association between SIgA concentration and URTS incidence was also observed (HR: 0.998, p = 0.070). These findings suggest that self-reported household illness can be measured in surrogate of SIgA to predict URTS risk. In the third study (Chapter 7), mucosal immunoendocrine, self-reported wellness, hydration and URTS data were repeatedly measured in elite field hockey players during a 3-week overseas tour, involving a congested competition period, long-haul travel, extreme environmental conditions and hypohydration. Despite multi-stressor exposure, all measures remained stable and only one player experienced an URTS episode. The low URTS incidence may be explained by the unique traits of elite athletes' immune systems, and/or the practitioners' management of tour stressors to prevent URTS. Finally, in the fourth study (Chapter 8), factors influencing URTS risk were examined in elite field hockey players during an 8-week training and competition period that simulated the expected preparatory and competition phases of the 2020 Tokyo Olympics. Illness in players' households (HR: 4.9; p < 0.001) and increased self-reported stress (HR: 0.63; p = 0.043) predicted greater URTS risk. Additionally, low baseline resting SIgA concentration predicted players 'potential' URTS risk (p = 0.021). It is therefore recommended that practitioners screen SIgA and regularly monitor self-reported lifestyle and behavioural data to predict URTS risk in athletes. In conclusion, the findings add to the body of knowledge with regards to factors influencing URTS risk in elite team-sport athletes. Specifically, lifestyle and behavioural factors outside of the team environment may influence URTS risk to a greater extent than sport-related stressors. Non-biological self-reported data were also found to be more effective than biomarkers in predicting team-sport athletes URTS risk. Therefore, monitoring of self-reported data, specifically household illness incidence and wellness indicators (i.e., sleep quality and stress), offer a practical and accessible method that team-sport practitioners can use in surrogate of biomarkers to identify elite athletes' risk for URTS.
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