Onychomycosis is often mistaken by both physicians and patients as being not only a cosmetic entity but also of secondary or tertiary importance in the patient’s list of concerns. This book has strived to show that it is an infection which affects a wide range of patients. It may not present itself as a carbuncle or a circumscribed area of cellulitis, but its simple presence can indicate something more sinister such as the link to carotid atherosclerosis in diabetics or create an environment where a dermatophyte can infect the interdigital space leading to a possible bacterial superinfection [1, 2].

Cellulitis, commonly seen on the lower extremity in older adults with diabetes, is a diagnosis that requires a hospital admission in cases that are potentially limb and life threatening [3]. The link to lower extremity cellulitis from a mycotic toenail may not seem logical at first, but in some cases, that fungal foot infection provided the initial spark for a problematic chain reaction.

The presence of onychomycosis generally co-presents with tinea pedis, specifically interdigital tinea pedis. It can be difficult to know which caused the other, but a general statement can be made that the same dermatophyte, whether starting from the nail or the interdigital space, is creating both the infection in the nail bed and the plantar/toe-web skin. When examining a patient with onychomycosis, it is worthwhile for the practitioner to examine the skin around the nail, interdigital space, and plantar foot for the serpiginous scale characteristic of tinea pedis. The patient may not have symptoms of pruritus, but in some cases, the skin will not only present with the characteristic scale but may also present with mild inflammation and fissuring [3]. The fissures can act as a portal for bacteria to invade and create an infectious and inflammatory cascade.

Bristow and Spruce reviewed the literature to determine the potentiality of a fungal foot infection causing cellulitis, especially in the diabetic population [3]. One of the articles they reviewed was Roujeau et al.’s case-controlled study of 243 patients with acute bacterial cellulitis of the leg that aimed to find any association with mycology-proven foot dermatomycosis (i.e., both tinea pedis and onychomycosis) [4]. The presence of a fungal foot infection was a significant risk factor in developing cellulitis with an odds ratio of 2.4, p < 0.001, confidence interval 95 % [4]. Another prospective case-controlled study of 100 subjects concluded that risk factors for acute bacterial cellulitis in hospitalized patients were sites of pathogen entry on legs and toe webs. Therefore, management and treatment of toe-web intertrigo may reduce cellulitis incidence [5]. Likewise, Dupuy et al. assessed risk factors for erysipelas of the leg, cellulitis, through seven hospital centers in France [6]. One hundred and sixty seven patients were admitted for erysipelas of the leg, and through multivariate analysis, they showed that disruption of the cutaneous barrier secondary to a macerated interdigital space was a risk factor in developing cellulitis (odds ratio 6.6, 95 % confidence interval). A site of entry through the skin was found in almost all cases. In the same study, the risk of developing leg cellulitis increased when more than one interdigital space was affected (odds ratio 19.5, 95 % confidence interval).