In this large-scale nationwide study of the Korean population, the data of 4,532,665 subjects aged between 20 to 40 years were analyzed to determine the risk factors associated with the development of dermatophytosis in Korea. Notably, women were less likely to be infected by dermatophytes than men, while those who were older and those with lower incomes; higher blood pressure values; higher low-density lipoprotein cholesterol levels; lower high-density lipoprotein cholesterol levels; and more comorbidities, such as diabetes, dyslipidemia, and hypertension, were determined to be more susceptible to dermatophytosis. Also, drinking more alcohol, performing exercise more frequently, having a history of smoking, and having a greater waist circumference also increased the risk of developing a dermatophyte infection, even after adjusting the confounding factors.
It has been widely reported that dermatophytosis is associated with male sex; a systemic or local immunocompromised status, such as diabetes mellitus; and long-term use of topical steroids9,10. Previous known risk factors of tinea pedis and onychomycosis, the most common types of tinea, accounting for 33–40% of all dermatophytosis, includes advanced age; male sex; nail trauma; peripheral vascular insufficiency; and immunosuppression status, such as diabetes mellitus and human immunodeficiency virus infection9,11. Warm and humid foot conditions secondary to wearing tight shoes; performing specific sporting activities, such as swimming; smoking; and obesity were also reported to increase the risk of tinea pedis and onychomycosis11–13. On the other hand, tinea capitis mainly occurs in children between the ages of three and 14 fours, and known risk factors include male sex, low socioeconomic status, overcrowding, and keeping pets at home14,15. Meanwhile, tinea capitis in the adult population is highly associated with comorbidities, such as rheumatoid arthritis, human immunodeficiency virus infection, diabetes mellitus, leukemia, and kidney failure15.
In this study, male sex, advanced age, obesity, alcohol consumption, and moderate to heavy exercise were shown as risk factors for the development of dermatophytosis. Among these risk factors, sex was the most influential variable. Tinea cruris is the most known sex-influenced dermatophytosis, found to be three times more common in men than in women14. In addition, men also have a higher risk of developing tinea pedis than women, which also may be due to their more common exposure to moist environments due to wearing of occlusive footwear and more frequent physical exercise14. These aforementioned lifestyle characteristics more commonly associated with male sex are thought to be responsible for the high prevalence of dermatophytosis in men; however, it is not yet understood whether male sex itself increases the susceptibility to infection10,14.
In our study, advanced age was shown to be the risk factor for developing dermatophytosis. Yoon et al.4 reported that the prevalence of dermatophytosis in Korea continuously increases with age, and the highest prevalence is among those aged between 60 and 69 years old. Among the types of dermatophytosis, onychomycosis, which is the most common superficial fungal infection, has been confirmed to increase with age16. Eleweski and Charif17 reported that approximately 40% of elderly patients have onychomycosis. The presumed cause for this high prevalence was to be that older individuals find it more difficult to exercise and care for feet and nails, increasing their susceptibility to colonization by infectious organisms16,17. Predisposing conditions, such as diabetes and compromised peripheral circulation, were also thought to be contributing causes16,17. These contributing factors, however, are thought to increase the prevalence of infection by all kinds of dermatophytes.
Obesity, expressed through the BMI index and waist circumference, was also found to be a significant contributing factor to the development of dermatophytosis. Onychomycosis has been relatively well studied regarding its relationship with obesity18–20. In a study of more than 1,000 patients randomly screened to examine their feet, obesity was found to be one of the most prevalent predisposing factors among patients with fungal nail disease18. Onychomycosis was significantly increased in patients with obesity (odds ratio, 2.13; 95% CI, 1.45–3.13) in a study of 1,245 diabetic Taiwanese patients19. In addition, topical antifungal treatment was shown to be less effective in patients who were overweight or obese20. Complete cure rates were 15.9% in obese patients and 22.0% in patients with healthy BMIs after 52 weeks of applying topical efinaconzole20. Several factors have been proposed to explain the potential mechanism behind the results. Firstly, obesity might make the skin more hospitable to fungal growth. Humid and warm conditions are important for fungal growth and survival. Thick layers of subcutaneous fat with deep skin folds may cause profuse sweating, and the trapped moisture and warmth may provide an optimal environment for the colonization of the dermatophytes21,22. Secondly, increased adipose tissue itself may further contribute to the increased risk for infection23–25. It has been recently recognized that adipose tissue participates actively in immunity through producing a variety of cytokines, such as leptin and adiponectin24,25. Leptin is a pro-inflammatory cytokine that activates polymorphonuclear neutrophils and T lymphocytes and regulates the activation of monocytes and macrophages24,25. However, obese patients often show leptin resistance, making them more vulnerable to infection23. Also, obese patients are more likely to have comorbidities, such as diabetes mellitus, which further contributes to the development of dermatophytosis21,23.
Lastly, engaging in routine exercise was also associated with an increased incidence of dermatophytosis. This is thought to be due to the wet environment caused by sweating from physical activities. However, as obesity is critical for the development of dermatophytosis and exercise is essential for maintaining a healthy weight, exercise should not be discouraged to manage the risk of developing dermatophytosis itself; instead, self-hygiene activities to keep the body clean and dry following each exercise session should be highlighted to reduce the risk of dermatophytosis after exercise.
This study also had some limitations. Firstly, dermatophytosis and comorbidities, such as hypertension, diabetes mellitus, and dyslipidemia, were identified using ICD-10 codes from claims databases. However, a validation study of the diagnostic codes of the KNHIS claims database revealed that only approximately 70% of the diagnosis codes coincided with those from medical records26. Secondly, dermatophytosis has some different clinical characteristics depending on the anatomic sites of infection, but this study did not distinguish these subtypes and was conducted while considering all subtypes of dermatophytosis as a whole. However, it is still meaningful to find out the risk factors for dermatophytosis as a whole without losing the significance since the fungi invading the skin share similar common characteristics. In contrast, the strengths of this study are its large sample size and nationally representative study population. Moreover, the national database contained information on socio-demographic characteristics, such as smoking status, alcohol consumption, physical activity, household income, and BMI, which were all found to be significant variables in the risk of developing a dermatophyte infection.
Taken together, our results demonstrated a significant positive association between the incidence of dermatophytosis and increasing BMI. In addition to male sex and increasing age, an elevated waist circumference, drinking, and exercise all contributed to the development of dermatophytosis. It is emphasized that lifestyle corrections directed at managing weight, drinking less alcohol, and keeping the body clean and dry after exercise might contribute to the prevention of dermatophytosis.