A Painful Eruption Following Multiple Scratches From a Pet Rat

To the Editors:

A 17-year-old female presented to the Emergency Department (ED) with a rash. Seven days before, she sustained scratches from her pet rat. At the site of each scratch, painful, pruritic, circular and erythematous patches occurred. On day 3 of the rash, she visited her primary care provider, who suspected a bacterial infection, and empirically treated her with mupirocin and doxycycline. These therapies were ineffective. On day 5, she visited the ED. Physical examination revealed multiple erythematous, scaly, circular patches along the neck, arms and chest. Associated with the rash were tender cervical and axillary lymphadenopathy (Fig. 1) Her vital signs were normal. Laboratory tests including a Complete Blood Count, Erythrocyte Sedimentation Rate and C Reactive Protein were normal. The plaques were rubbed with swabs for bacterial and fungal cultures.

FIGURE 1.: Six erythematous, pruritic plaques on the right side of our patient’s neck.

The fungal culture grew Trichophyton mentagrophytes, which commonly colonizes the nails of rats. Bacterial culture was negative. Empiric treatment with oral terbinafine and topical clotrimazole was initiated. After 14 days of therapy, the erythematous plaques started to fade. The painful local adenopathy persisted for 2 additional weeks.

Tinea, a fungal infection of keratinized skin and appendages, is caused by dermatophytes (Trichophyton, Microsporum and Epidermophyton). Certain tinea species have adapted to humans (anthrophilic) or animals (zoophilic).^1,2 Zoonotic dermatophyte infections in humans can be intensely inflammatory, as seen in our patient.²

The most common anthrophilic causes of tinea are from the species T. rubrum and T. tonsurans,² characterized by a slowly progressive ring of scale (ringworm) with little inflammation. The lesions in our patient were characterized by an acute and rapid progression of patches with scale and adenopathy, fitting a zoonotic dermatophyte infection.

Zoonotic dermatophyte infections in humans are usually a result of contact with an animal but may also be acquired indirectly through a contaminated environment. Depending on geographic location and rural versus urban areas, incidence of certain zoonotic dermatophyte infections can vary.¹ Reported cases of animal-human dermatophyte transmission include Trichophyton simii from monkeys, Trichophyton equinum from horses, Microsporum nanum from pigs, Microsporum gallinae from chickens, Tricophyton verrucosum from cattle, Microsporum canis from pet cats or dogs and T. mentagrophytes from rodents and rabbits.¹ Rodents serve as main reservoir for T. mentagrophytes as well as for Trichophyton benhamiae and Microsporum persicolor; however, they can also serve as alternate hosts, carriers, or passenger hosts for other infections.³ The exposure history is fundamental for making the diagnosis of a zoonotic dermatophyte infection.

Zoonotic dermatophyte infections in humans usually require systemic and local anti-fungal therapy and may be slow to resolve, as occurred in our case. Terbinafine, fluconazole or itraconazole are preferred over griseofulvin, as griseofulvin takes extended times to resolve zoonotic infections. It is noteworthy that T. mentagrophyte resistance to griseofulvin, terbinafine, fluconazole and/or itraconazole has been reported.^4,5 Our T. mentagrophytes isolate was tested, and it was susceptible to griseofulvin, terbinafine, fluconazole and itraconazole.

Human tinea skin infections are typically slowing enlarging, mildly erythematous, circular skin lesions with scale. Zoonotic fungal skin infections may differ, as they can be rapidly expanding, painful and severely erythematous.