A case of Tinea nigra associated to a bite from a European rabbit (Oryctolagus cuniculus, Leporidae): the role of dermoscopy in diagnosis

Abstract

We report a case of Tinea nigra in an adolescent living in Itapema, Santa Catarina, Brazil, who presented a hyperchromic macule on the palm of the left hand, close to another erythematous macule caused by a rabbit bite. The patient received guidance on accidents and animal bites and evolved well treated with topical butenafine for the dermatomycosis. The authors also highlight the efficacy of the dermoscopic exam in diagnosing Tinea nigra with animal bite lesions and other traumas.

Tinea nigra (TN) manifests as an asymptomatic hyperchromic macule in the palmar region, and eventually the plantar or other atypical areas.^1-5 Dermoscopy exam reveals its unique characteristic pattern, but aetiology is established by the mycologic examination.^6-9 Treatment response is good to topical antifungals with reports of spontaneous cure.^2-6

The patient was a thirteen year old female with two asymptomatic marks measuring 1.0 x 2.0 cm in the left palmar region, one black spotted and the other erythematous, of geographic shape and scaly border, caused by a European rabbit (Oryctolagus cuniculus) bite while playing with the animal (Figure 1).

FIGURE 1.

Detail of two macules, both sized approximately 1.0 x 2.0 cm and located on the internal edge of the left palmar region, one black spotted and not scaly, and one erythematous of geographic shape, poorly delimited borders, and with discrete scaling on the distal border

Dermoscopy of the hyperchromic macule showed darkened irregular puntiform pigmentation, a pattern typical to TN (Figure 2). The erythematous macule had a red homogenous colour without red spots or crown (Figure 3). In both there was an absence of typical pigmented melanocyte characteristics such as pigment network, globules, and stripes. The images were captured with a DermLite DL1 3Gen connected in a 5 megapixels iPhone 4S camera.

FIGURE 2.

Dermoscopy of the hyperchromic macule showed darkened pigmentation, distributed in an irregular spotty shape without the standard characteristics of melanocytic pigmented lesions such as pigment network, globules, and stripes. (Magnification 10x)

FIGURE 3.

Dermoscopy of the erythematous macule showing an area of reddened homogenous regularly distributed colouration with imprecise limits, an absence of red spots crown. (Magnification 10x)

Mycological exam of the hyperchromic macule revealed septate ramified dematiaceous hyphae (Figure 4). In Sabouraud Agar culture, wrinkled pigmented colonies were grown and H. werneckii was identified in microculture. The macule regressed after application of topical butenafine twice a day for four weeks with no recurrence in six months.

FIGURE 4.

Mycological exam cleared with 20% potassium hydroxide, showing irregular septate and ramified dematiaceous hyphae.

Dermascopy in TN, initially described in 1997, is a simple conclusive examination which was effective in this case.^6-10 The growing habit of keeping animals such as rabbits can occasionally lead to accidental bites and the resulting cutaneous lesion could be confused with TN.