Interdigital infection is a spectrum of relatively common, troubling, and neglected disorders [3], especially in the humid and rainy climates of southern China. It was first described as a specific dermatologic entity by Amonette and Rosenberg in 1973 [5]. A variety of terms have been used to describe this disorder, including gram-negative bacterial toe web infection, which is characterized as an acute bacterial infection of the toe web spaces due to Pseudomonas spp. alone, or with other gram-negative bacteria. However, gram-positive bacteria, such as E. faecalis and S. aureus, have also been isolated as pathogenic germs from interdigital infections [4, 6]. Meanwhile, the prevalence of this disease caused by gram-positive bacteria has been rising in recent years [2, 7]. In the present study, we examined 57 cases of pathological toe webs and found P. aeruginosa to be the causative agent in 40.35% of cases and S. aureus in 36.84% of cases. We suggest that gram-positive bacteria are present in a considerable number of cases of interdigital infection; thus, as a possible pathogen, they are worthy of attention. Consequently, the original term “gram-negative bacterial toe web infection” might not cover all causative infectious agents; perhaps interdigital infection is the most suitable term for this disease.
The interdigital space is typically colonized by polymicrobial flora [8, 9]. We found 40 patients with positive microscopic fungi before and after antibiotic treatment. Previous studies have suggested that a strong correlation exists between dermatophytes and bacteria [8]. The former organism seems to act as a triggering factor, while the latter organism may play a role in the symptoms associated with interdigital infections. Dermatophytes initiate damage to the stratum corneum, thus, facilitating bacterial invasion; consequently, penicillin- and streptomycin-like antibacterial substances are produced, favoring specific colonization and overgrowth of the physiological skin flora by gram-negative bacteria and antibiotic-resistant gram-positive bacteria [10, 11]. Moreover, we observed that the positive rate of fungal cultures was significantly lower in acute interdigital infections that were not treated with antibiotics. Fifteen patients were found to be positive for microscopic fungi even with effective antibiotic therapy. It is postulated that the cases of unsuccessful fungal tests may be due to factors caused by bacterial overgrowth and the inflammatory response [11–13]. As fungal infections might be a triggering factor for interdigital infections and may cause false-negative results during the acute phase, it is recommended to repeat testing for fungi at the end of the antibiotic treatment.
Interestingly, we noted that the incidence of interdigital infections caused by different types of bacteria had obvious seasonal and age distribution characteristics. We speculated that the seasonal peaks of interdigital infections may be mainly related to air temperature, humidity, proliferation of fungi, and personal habits required for bacterial production. P. aeruginosa interdigital infections were frequently seen to increase during spring and autumn. The rate of fungal infections accompanying P. aeruginosa interdigital infections was found to be up to 96.65%. We speculated that the morbidity of interdigital infections caused by P. aeruginosa might be related to the proliferation of fungi. The optimal culture temperature of dermatophytes ranges from 22℃ to 28℃, and these temperatures usually appear in spring and autumn in Shanghai; furthermore, gram-negative bacteria, particularly P. aeruginosa, thrive in a warm, moist, and occluded environment [14–16]. As people prefer to wear porous sandals in the summer, leading to the feet being in less moist occlusive conditions, P. aeruginosa interdigital infections become uncommon. In addition, the most common age group affected by P. aeruginosa interdigital infections was the 60–69 years group. This corresponds with the age of onset of tinea pedis [17]. Furthermore, other predisposing factors, such as scratching or rubbing, unreasonable treatment, and hot-water scalding are more common in this age group (data not shown). Interdigital infection caused by S. aureus was more liable to occur in summer and might be associated with more frequent attendance at swimming pools and public baths and prolonged exposure to rain. These conditions increase the risk of foot exposure to S. aureus. Moreover, the prevalence of occupations by interdigital infections caused by S. aureus may be explained by professional and non-professional reasons or the practice of strenuous exercise, all of which are prone to skin damage and increase the risk of S. aureus infection.
With high recurrence rates and possible complications, such as immobility, cellulitis, wound healing disorders, and autosensitization dermatitis, interdigital infections present a frequent therapeutic challenge in clinical practice [4, 5, 18]. However, data supporting a standardized regimen remain insufficient [19]. Therefore, we theorized a therapeutic algorithm for interdigital infections through a year-long cross-sectional study (Fig. 4). In order to prevent relapse and reduce complications, we suggest the following: first, levofloxacin may be a preferred first-line therapy administered before drug sensitivity testing because it was much more effective and provided excellent results against both gram-negative and gram-positive bacterial interdigital infections [1]. Subsequently, the therapeutic regimen could be adjusted based on drug sensitivity tests. Second, effective monitoring of local primary skin diseases, such as tinea pedis, onychomycosis, and allergic dermatitis, might be a critical factor in reducing relapse of interdigital infections. Third, health education for patients, such as lifestyle changes, has generally been ignored and may prove to be a simple method for prevention. Finally, recent literature has confirmed that immersing feet in 1% acetic acid is a relatively effective adjuvant treatment to significantly reduce relapse of interdigital infections caused by P. aeruginosa [20–22].
The present study has some limitations. First, only acute symptomatic cases were included in this study. It was difficult to research asymptomatic, mild cases since these cases usually fail to attract attention from patients. However, we believe that the prognosis of interdigital infections is favorable if there is an early, accurate diagnosis and appropriate treatment. Topical antibiotics alone or in combination with topical antifungal agents, produce good curative effects for mild cases. Second, because of the small sample size and limited study period, future research should include larger sample sizes with longer study periods to better elucidate the epidemiological characteristics of interdigital infections.