Protothecosis is a rare infection, caused by the genus Prototheca, achlorophyllous algae which are saprophytes in soil, and occasionally affect dogs and cats1. It is believed that they may infect humans by traumatic inoculation2.

A 74-year-old woman visited our clinic with prickling erythematous umbilicated papules on the anterior neck and upper chest which developed 2 months prior on top of an erythematous telangiectatic patch which had been present for 2 years (Fig. 1A, B). She had been treated at another clinic. with topical corticosteroids for 2 years for suspected eczema aggravated by rubbing. She had no history of trauma or other medical disease. She had lived with a pet dog for 18 years.

The patient's complete blood count, renal and liver function, and chest X-ray study were all normal. The HBs Ag, HCV Ab, HIV Ab, and anti-nuclear Ab tests were negative. The venereal disease research laboratory test (VDRL) was nonreactive. The histopathology revealed a granulomatous inflammation with necrosis in the dermis (Fig. 2A), containing many non-budding spherical organisms. Multiple sporangia containing endospores were observed with the morula appearance that is the characteristic feature of the Prototheca species (Fig. 2B). The Periodic acid-Schiff staining was positive and showed characteristic internal septation and thick cell walls (Fig. 2C). Treatment with oral itraconazole (200 mg/day) and topical sertaconazole for 12 weeks resulted in marked improvement of the skin lesions (Fig. 1C).

Cutaneous protothecosis mainly involves the extremities and its presentation is variable2. It can manifest as an erythematous plaque, or less commonly, in ulcerative or purulent forms345. Our patient presented with erythematous umbilicated papules on chest, which is a less common34.

Prototheca have low virulence; hence, systemic or local predisposing factors are commonly found in affected patients, such as steroid use, malignancy, diabetes mellitus, and organ transplantation3. Our patient had intact immunity but was predisposed to local immunosuppression by long-term topical steroid application, skin damage by frequent rubbing, and the presence of a pet dog, which may have been the source of the algae. Prototheca wickerhamii and P.zopfii can mainly cause human protothecosis, and P.wickerhamii is the most commonly responsible34. The sporangia of P.wickerhamii are round with symmetrical morula formation and a diameter of 7~ 13 µm, while the sporangia of P.zopfii are oval with relatively asymmetric random septation and a diameter of 14~16 µm. P.wickerhamii is susceptible to antifungal agents and is easily treated, but P.zopfii is resistant to treatment.

In Korea, 11 cases of cutaneous protothecosis have been reported2. Most of the reported cases involved patients who were over the age of 60 years. The most commonly involved sites were the extremities, except one patient whose cheek was affected. Most patients had systemic predisposing factors and 2 patients had local predisposing factors (long-term topical steroid use). All of these cases were confirmed with diagnostic tests; P.wickerhamii was identified in 10 cases and P.zopfii was identified in 1 case2.

We report a rare case of cutaneous protothecosis in an immunocompetent patient who presented with erythematous umbilicated papules on the chest after long-term topical corticosteroid use, traumatic rubbing, and exposure to a pet dog.