Onychomycosis is a common fungal infection of the nail, representing up to 50% of all nail diseases¹. This infection may be caused by dermatophytes, yeast, or non-dermatophyte molds. However, Trichophyton rubrum is the primary causative pathogen². Fungal cutaneous infection is a major skin disease, causing troublesome health problems³. Risk factors include old age, immunocompromized condition, diabetes mellitus, polyneuropathy, peripheral arterial disease, chronic venous insufficiency, tinea pedis, and trauma⁴. Hyperhidrosis, occlusive shoes, and participation in sports may also be the predisposing factors⁵. Onychomycosis may result in severe complications in patients with diabetes⁶.

Previous studies have demonstrated all-type fungal infections, total cutaneous fungal infections, or subtypes of fungal species in patients with onychomycosis. Cutaneous fungal infections account for 10~20% of all dermatologic outpatient cases reported in Korea⁷. In 2010, the prevalence of cutaneous fungal infection was 6.51% for dermatophytosis and 0.31% for other superficial mycoses⁸. Yoon et al. reported that the prevalence of all-type fungal infections among the Korean population was approximately 7.1%⁹.

Currently, there is a lack of nationwide studies exploring the prevalence of onychomycosis in Korea. Thus, in this population-based study, we have analyzed the prevalence of onychomycosis using data obtained from the Korean National Health Insurance Service (NHIS) in 2015.

1. Data source

We utilized data from the NHIS database obtained between January and December, 2015. The Korean NHIS monitors all medical services in Korea; 100% of the Korean population is covered under the NHIS. This computerized database includes all claim data, such as patient demographics, dates of hospital visits, residence, socioeconomic status, and claimed treatment details based on the International Classification of Disease (ICD)-10 codes¹⁰.

2. Study sample

We analyzed data for patients who visited clinics or hospitals with a diagnostic code (ICD-10) of onychomycosis (Tinea ungium) (B351) more than once between January and December, 2015. Patients with other types of dermatophytosis, including Tinea pedis and those with missing data, were excluded from the analysis. This study was approved by the institutional review boards of the Korean National Institute for Bioethics Policy (NHIS-2017-1-002), the Korea Centers for Disease Control and Prevention, and the Catholic University of Korea (No. KC16EISE0923).

3. Study variables

NHIS data were interpreted to calculate the national prevalence of onychomycosis in each city or province of Korea. Patients were classified into nine age groups as follows: 0~9, 10~19, 20~29, 30~39, 40~49, 50~59, 60~69, 70~79, and >80 years. The region of residence for each patient was defined as urban (Seoul, Busan, Daegu, Incheon, Gwangju, Daejeon, Ulsan, Sejong, and Gyeonggi-do) or rural (Gangwon-do, Chungcheongbuk-do, Chungcheongnam-do, Jeollabuk-do, Jeollanam-do, Gyeongsangbuk-do, Gyeongsangnam-do, and Jeju-do)¹¹. Further, assessment of the influence of the month of hospital visit on the prevalence of onychomycosis was also performed.

4. Statistical analysis

Statistical analyses were performed using the SAS version 9.4 (SAS Institute, Cary, NC, USA) statistical software. Age-specific rates of prevalence of onychomycosis were calculated by dividing the number of patients with onychomycosis by the total Korean population according to the 2010 Population and Housing Census. p-values < 0.05 were considered statistically significant.

1. Characteristics of patients with onycho- mycosis

In 2015, total 1,917,588 patients with onychomycosis were covered under the NHIS and the prevalence of onychomycosis was 3.49%. The prevalence of onychomycosis tended to in- crease with age (Table 1). The prevalence of onychomycosis among patients aged 60~69 years [odds ratio (OR) 5.148, 95% confidence interval (CI) 5.113~5.183)] and 70~79 years (OR 5.466, 95% CI 5.427~5.505) were significantly higher than that reported in patients aged 20~29 years (reference age group). In addition, the prevalence of onychomycosis was higher in females than in males across all age groups. The prevalence was 3.59% for males and 3.84% for females. Females aged 70~79 years had the highest prevalence of onychomycosis (OR 6.597, 95% CI 6.527~6.668). However, the prevalence of onychomycosis among patients aged 0~9 and 10~19 years was 0.088 (0.086~0.090) and 0.405 (0.401~ 0.410), respectively, which is lower than that reported in the reference group.

2. Monthly prevalence

The highest prevalence of onychomycosis was observed in July and August. The lowest prevalence of onychomycosis was noted in January and February (Figure 1).

Figure 1. Monthly prevalence of onychomycosis in 2015

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3. Place of residence

When the age-standardized prevalence was calculated using the 2010 Census Korean population, Jeollanam-do (4.29%), Gyeongsangnam-do (3.99%), Ulsan (3.89%), and Jeju-do (3.88%) showed a much higher prevalence of onychomycosis than other areas. Conversely, the age-standardized prevalence in Seoul (3.24%) and Gyeonggi-do (3.31%) was relatively low (Figure 2).

Figure 2. Regional prevalence of onychomycosis in Korea The prevalence for each region was based on diagnostic codes.

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This was the first large-scale population-based study, evaluating the epidemiology of onychomycosis in Korea. The prevalence of onychomycosis was shown to be dependent on age and region.

The impact of age on the prevalence of onychomycosis in a population is very important. There is a positive correlation between onychomycosis prevalence and age¹². In the current study, the prevalence of onychomycosis increased with age, i.e., the highest prevalence was observed in patients aged 60~69 and 70~79 years. Advanced age is considered a risk factor of onychomycosis owing to diminished blood circulation, longer exposure to fungi, slower growth of nails and nail thickening, and increased susceptibility to infection.

We observed a female predominance in the prevalence of onychomycosis. This finding was consistent with that of a previous study in Spain showing that the prevalence of onychomycosis among females was higher than that reported in males (1.8% vs. 0.8%, respectively)¹³. However, other studies have demonstrated that onychomycosis is more prevalent in males than in males^14-17. In the United Kingdom, a population survey showed a prevalence of dermatophyte nail infection of 2.8% in males and 2.6% in females¹⁴. In Finland, the prevalence of onychomycosis identified as dermatophyte culture-positive cases was 13% in males and 4.3% in females¹⁵. In Iceland, based on photograph evaluations by patients, 195 males (17.5%) and 187 females (13.7%) identified themselves as having onychomycotic nails¹⁶. In another study in Spain, the prevalence of onychomycosis was 3.1% (CI 95% 2.12~4.38), 4.4% (CI 95% 2.7~6.7), and 1.9% (CI 95% 0.9~ 3.4), in total, males, and females, respectively¹⁷. The discrepancies among the results of studies may be explained by the finding that Korean females are more likely to seek medical attention than Korean males. Another possible explanation is that susceptibility to such infections varies between ethnic groups.

Moreover, the results of the current study suggest that the prevalence of onychomycosis in Korea is related to environmental factors. Firstly, we observed seasonal differences in the month of hospital visit among the studied groups. Secondly, we observed regional differences in the prevalence of onychomycosis, with higher prevalence reported in lower latitudes, such as Jeollanam-do, Gyeongsangnam-do, Ulsan, and Jeju-do. Although the reason for this regional difference remains unclear, we hypothesize that environmental factors such as high temperature and humidity may play key roles. These factors may increase sweating and provoke the development of onychomycosis.

The limitations of this study must be acknowledged. Firstly, access to medical care differs from region to region. Secondly, we did not make mycologic diagnoses using potassium hydroxide or fungus culture. Despite these limitations, this was the first nationally representative, population-based study, evaluating patients with onychomycosis in Korea.

The results of the current study suggest that onychomycosis is most prevalent in elderly patients living at lower latitudes. This high prevalence may be because of the hot, humid weather in such provinces in South Korea.

In relation to this article, I declare that there is no conflict of interest.