Infection as an Exertional Heat Illness Risk Factor: A Prospective Cohort Study.

Abstract

Strenuous physical activity for work or leisure increases the risk of exertional heat illness (EHI), which can be fatal. Respiratory infection may increase EHI risk, but empirical evidence is equivocal and relies upon clinical case reviews that lack objective measures and comparator controls. This prospective cohort study investigated the association between respiratory infection and EHI. N = 807 UK infantry recruits (M = 805/F = 2) completed a 6.4-mile loaded march between spring and fall (2021-2024, wet bulb globe temperature, 11.2°C ± 3.6°C). Participants completed the Jackson Common Cold Questionnaire on each of the 3 d preceding and before the loaded march on day 0. Additional measures included clinical pathology on throat swabs (day -3 and -1), serum C-reactive protein (CRP; day -1), and gastrointestinal temperature ( Tgi , day 0). EHI was classified as mild (exercise-induced headache, dizziness, or nausea) or severe (CNS disturbance plus hyperthermia and end-organ damage). Logistic regression examined the association between respiratory infection and EHI after full adjustment for widely considered EHI risk factors. N = 118 participants were classified as mild (15%) and N = 40 as severe EHI (5%). The likelihood of severe EHI was increased four-fold with respiratory infection symptoms on day -1 and 0 (OR = 4.09; 95% confidence intervals = 1.29-12.90, P = 0.016) and three-fold when restricting analysis to symptoms on day 0 (OR = 2.83, 95% confidence intervals = 1.02-7.86, P = 0.046). Participants with respiratory infection symptoms exhibited increased pathogen expression, systemic inflammation (CRP > 3 mg·L -1 ), and preloaded march Tgi (+0.3°C, P = 0.023). Respiratory infection symptoms were not associated with mild EHI susceptibility. Ongoing respiratory infection was associated with an increased likelihood of severe EHI. Individuals at risk of EHI (e.g., athletes) should avoid strenuous physical activity when suffering respiratory infection symptoms.