Fungal infection of the toenails and the skin of the foot is so common in diabetes that it is often dismissed as a cosmetic nuisance. The evidence suggests otherwise. Onychomycosis and tinea pedis alter the barrier function of the skin and nail, create portals of entry for bacteria, and appear repeatedly in risk models for diabetic foot ulceration. Understanding where the evidence is strong — and where it remains uncertain — helps clarify why these infections belong in the diabetic foot examination rather than at the margins of it.
How Common Is Fungal Infection in the Diabetic Foot?
A systematic review by Navarro-Pérez and colleagues, published in Mycoses in 2023, pooled ten studies enrolling 5,664 people with diabetes and found that 29.18% had onychomycosis, most frequently caused by Trichophyton rubrum. The review identified significant associations between onychomycosis and two variables familiar to anyone working in diabetic foot care: the presence of diabetic neuropathy (p = 0.012) and elevated glycated haemoglobin (p = 0.039).
Notably, that same review did not find a statistically significant association between onychomycosis and ulceration (p = 0.185). The authors were candid about why: the available literature was heterogeneous, most studies carried a low level of evidence, and many relied on clinical appearance alone rather than laboratory confirmation of the diagnosis. A thickened, discoloured nail may be fungal — or it may be traumatic onychodystrophy, psoriasis, or lichen planus. Without mycological confirmation, prevalence and risk estimates are difficult to interpret.
Fungal Infection as an Independent Ulcer Risk Factor
Larger administrative datasets have produced a different signal. Fan and colleagues, writing in Frontiers in Endocrinology in 2024, analysed 161,834 patients with neurovascular complications of diabetes drawn from the United States National Inpatient Sample for 2017–2019. The prevalence of diabetic foot ulcer in this cohort was 29.4%. In multivariable logistic regression, both tinea pedis (odds ratio 1.89; 95% CI 1.59–2.26) and onychomycosis (OR 1.26; 95% CI 1.13–1.42) emerged as independent predictors of ulceration, alongside peripheral vascular disease (OR 1.69), osteomyelitis (OR 7.10), sepsis (OR 1.24), and obesity (OR 1.08).
These are association data from a hospitalised population and cannot establish causation. What they do establish is that fungal foot infection is not an isolated finding — it clusters with the other markers of a foot at risk.
A Plausible Biological Mechanism
Laboratory work offers a mechanistic explanation for how fungal colonisation might impair healing. Aruga, Takehara and Kametaka, publishing in the International Wound Journal in 2024, inoculated full-thickness excisional wounds in ob/ob (diabetic) mice with Trichophyton mentagrophytes spores. Wound healing was significantly delayed. Histopathology showed reduced collagen fibre formation and inhibited granulation tissue formation, with spores and elongated mycelia persisting in the wound bed and along the wound edges. The authors concluded that neglected tinea pedis represents a potential risk factor for delayed healing and progression to refractory wounds.
Animal models do not translate directly to human wounds. But the finding aligns with the long-standing clinical observation that interdigital maceration and fissuring provide an entry point for bacterial pathogens, a pathway reviewed in detail by Gupta and colleagues in the Journal of Fungi in 2024. Their review argued that reducing the burden of superficial fungal infection is a reasonable component of a strategy to reduce diabetic foot ulcer and diabetic foot infection complications.
What the Treatment Evidence Shows
The most rigorous synthesis of topical therapy comes from a 2020 Cochrane review by Foley and colleagues, which included 56 randomised trials and 12,501 participants with toenail onychomycosis. High-quality evidence supported efinaconazole 10% solution over vehicle for complete cure (risk ratio 3.54; 95% CI 2.24–5.60) and clinical cure (RR 3.07; 95% CI 2.08–4.53). Moderate-quality evidence supported tavaborole 5% solution (RR 7.40 for complete cure) and P-3051 ciclopirox 8% hydrolacquer; evidence for ciclopirox 8% lacquer was of low quality. Adverse events, mostly local application-site reactions, were modestly more frequent with the active agents.
Two caveats matter clinically. First, as the Cochrane authors emphasised, absolute complete cure rates with topical therapy remain relatively low even where the relative effect is convincing. Second, evidence for device-based treatments was thin: for 1064-nm Nd:YAG laser, there may be little or no difference in mycological cure at 52 weeks compared with no treatment or sham, and the certainty around adverse events was very low.
Clinical Summary
Roughly three in ten people with diabetes carry onychomycosis, and its presence tracks with neuropathy and poorer glycaemic control. Large observational datasets identify both onychomycosis and tinea pedis as independent correlates of foot ulceration, and experimental work provides a plausible mechanism through impaired granulation and collagen deposition. The causal link between fungal infection and ulceration has not been proven, and the treatment literature shows that complete cure is achievable but not reliable. Confirming the diagnosis mycologically rather than visually, and treating interdigital tinea alongside nail disease, are the steps most consistently supported by the current evidence. The 2023 IWGDF prevention guideline places this work within a broader framework: annual screening for loss of protective sensation and peripheral artery disease, more frequent review at higher risk levels, education in foot self-care, and treatment of any pre-ulcerative lesion on the foot.
References
- Navarro-Pérez D, Tardáguila-García A, García-Oreja S, López-Moral M, García-Madrid M, Lázaro-Martínez JL. Onychomycosis associated with diabetic foot syndrome: A systematic review. Mycoses. 2023;66(6):459–466.
- Fan Z, Huang J, Liu Y, Xie H, Yang Q, Liang Y, Ding H. Multifactorial analysis of risk factors for foot ulcers in patients with neurovascular complications of diabetes. Frontiers in Endocrinology. 2024;15:1399924.
- Aruga K, Takehara K, Kametaka S. Trichophyton mentagrophytes delays wound healing in ob/ob mice. International Wound Journal. 2024;21(12):e70118.
- Gupta AK, Shemer A, Economopoulos V, Talukder M. Diabetic foot and fungal infections: etiology and management from a dermatologic perspective. Journal of Fungi. 2024;10(8):577.
- Foley K, Gupta AK, Versteeg S, Mays R, Villanueva E, John D. Topical and device-based treatments for fungal infections of the toenails. Cochrane Database of Systematic Reviews. 2020;1(1):CD012093.
- Bus SA, Sacco ICN, Monteiro-Soares M, Raspovic A, Paton J, Rasmussen A, Lavery LA, van Netten JJ. Guidelines on the prevention of foot ulcers in persons with diabetes (IWGDF 2023 update). Diabetes/Metabolism Research and Reviews. 2024;40(3):e3651.
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