Scalp folliculitis with Demodex: innocent observer or pathogen?

LETTER-TO-THE-EDITOR

Scalp folliculitis with Demodex: innocent observer or pathogen?

Angel Fernandez-Flores^I,III; Antonio Alija^II

^IAnatomic Pathology and

^IIDermatology, Hospital El Bierzo

^IIIService of Cellular Pathology, Clinica Ponferrada; Ponferrada, Spain

^{Address for correspondence}

We recently saw a case of an acute ulcerating folliculitis in a 75-year-old man, in which many Demodex sp. were found. Although the lesions were resistant to systemic conventional antibiotic therapy, they soon responded to topical permethrin plus oral metronidazole. This led us to consider that this mite has a pathogenic role, at least, in certain types of cutaneous pathology. After reviewing the literature, it seems that Demodex might be just an "innocent" observer in many cases, though it could play a pathogenic role under certain clinical conditions.

The patient, a 75-year-old male, came to the dermatology clinic, complaining of erythematous papules evolving into crusts, which he had suffered for eight years, with several episodes of varied intensity. The lesions were located in the scalp, the front side of the neck, the retroauricular area and the cheek. He had received oral treatment with several antibiotics (cefuroxyme, fuchsinic acid, and isothretinoin), with either only partial or no response at all. Laboratory cultures were negative for fungus, mycobacteria and common bacteria, except for Staphylococcus epidermidis, which was reported as a probable component of the normal flora.

The examination showed ulcerated lesions with an erythematous non-indurated border. The largest ulcer measured 6 mm in diameter (Figure 1). When the lesions regressed, they left whitish depressed scars.

A biopsy of one of the scalp lesions was performed, showing an intense acute folliculitis with ulceration of the skin in some areas. Many Demodex-type arthropods were evident in the inflammatory infiltrate, as well as inside the follicles that were immersed in the infiltrate, next to the infiltrate or close to the infiltrate (Figure 2). We, therefore, established a diagnosis of folliculitis by Demodex. We also used special staining, including periodic acid Schiff (PAS) reaction, Giemsa and Ziehl-Neelsen, with no evidence of fungi or microorganisms. The PAS technique contributed, nevertheless, as it allowed us to identify the Demodex organisms (Figure 2).

Treatment with topical permethrin plus oral metronidazole was established, and a good response was obtained within a few days, with clearance of the lesions. Seven months later, the patient was free of disease.

The mite Demodex (Demodecidae) [1] is commonly present in folliculosebaceous units of nearly everyone older than 10 years old [1-5]. In humans, two different species have been found: D. folliculorum, which is present in the infundibulum, and D. brevis, which inhabits the sebaceous glands as well as the meibomian glands [3,4,6,7].

Demodex is usually a saprophyte on humans, feeding from gland secretions [4], and causing no symptoms [3].

Nevertheless, D. folliculorum has also been associated with varied cutaneous pathologies [8], such as rosacea [1,9-14], granulomatous peri-oral dermatitis [15], blepharytis [1,16,17], inflammatory nodules [1], and pustular folliculitis [8,9,18,19].

Since Demodex is a saprophyte in the human skin, a causal relationship with those pathologies has always been controversial [8,20,21]. Nevertheless, some studies have demonstrated an association between Demodex and follicular inflammation [22]. This is also supported by the fact that the symptoms are clearly reduced when this parasite is correctly treated [8-10,14,17,23-27]. On the other hand, some symptoms persist when only antibacterial therapeutic agents are used [14]. In some cases, therapy with metronidazole was not successful [28], which supports an etiological role of Demodex in certain cutaneous pathologies. Although some authors have demonstrated the presence of a high density of Demodex in common rosacea [18,29], the latter usually responds to Metronidazole, with clearance of the symptoms, even if the number of Demodex persists [30]. We could then assume that in certain cutaneous pathologies, Demodex acts as an innocent observer rather than a pathogen.

There has been some speculation about the clinical differences between the two types of rosacea [10,29]; on one hand, the one which is associated with Demodex as a causal agent presents a "dry" eruption [10], with follicular scaling, vesicles and pustules. On the other hand, the common rosacea usually presents an oily skin, without scales, and the pustules and papules appear inflamed [28].

Some authors have indicated two main conditions that have to be met before Demodex can be considered as a pathogen in a biopsy: an abnormal number of mites, or their presence in an abnormal place, i.e., the dermis [22,28,30]. For instance, more than five Demodex per square centimeter has been suggested as an abnormal condition [31]. Considering this, Demodex could be in fact, pathogenic, not only in the dry variant of rosacea, but also in some cutaneous disorders that present with non-classic signs and symptoms, such as facial itch, with or without erythema, or as non-specific pitiriasiform scaly pruriginous lesions [31].

On the other hand, Demodex might be a mere "observer" in other types of more classic conditions in which many studies have failed to connect the mite with pathology [4,20,32,33].

Received on 19 September 2008; revised 18 December 2008.

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Publication Dates

• Publication in this collection
  29 Jan 2010
• Date of issue
  Apr 2009