Case report A DILATED PORE OF WINER IN A YOUNG IBERIAN LYNX (LYNX PARDINUS)

A 1.5-year-old male Iberian lynx born in captivity presented with a 2.2 cm exophytic nodular alopecic mass at the left tarsus medial aspect, before being released into the wild. The lesion had keratinised material protruding through a 1.2 cm pore, forming a cutaneous horn. Histopathology revealed a cystic dermo-epidermal lesion lined by psoriasiform hyperplasia of the epithelium, thicker at the base of the cyst, together with abrupt to progressive keratinization and keratin pearls. The central cystic cavity was filled with fibrillary keratin and ghost cells. Immunohistochemistry for pan-cytokeratin showed strong positive immunolabeling in the hyperplastic epithelium and a weaker immunolabeling at the content of the cyst. A final diagnosis of a Dilated Pore of Winer was made. This paper is the first reported case of a DPW in a lynx and a wild felid.


INTRODUCTION
The dilated pore of Winer (DPW) is a benign, keratin-filled, hyperplastic and pyriform-shaped dermo-epidermal cyst that is considered a proliferative variant of the infundibular cyst [1,2]. The aetiology of DPW is unknown, but it has been suggested to be the result from follicle obstruction and increased intrafollicular pressure, with consequent hyperplastic changes [2,3]. DPW has usually been described on the face and neck of middle-aged or old cats, without breed or sex predilection [1,3]. DPW has been rarely described in other species, such as dogs and horses [4], and it is also relatively common in the head, neck and upper trunk of human beings [5]. The pathology of DPW corresponds to a nodular and alopecic infundibulocystic mass containing compact and laminated keratin with a dilated patent follicular ostium acting as a pore on the surface. Keratin can sometimes protrude to the exterior, forming a cutaneous horn [1,[3][4][5][6][7]. Microscopically, the wall of the cyst resembles infundibular epithelium, presenting with prominent psoriasiform hyperplasia at the base of the lesion and a thinner epithelium in the proximity of the pore. The cyst epithelium invades the underlying dermis forming scalloped, regular rete ridges at the base [1,3,5,[8][9][10]. The present report describes the first case of DPW in an Iberian lynx (Lynx pardinus), an endangered felid rescued from extinction based on intensive ongoing conservation actions [11].

CASE PRESENTATION
A 1.5-year-old male Iberian lynx born in captivity in 2013 in the Centro Nacional de Reprodução do Lince Ibérico (Silves, Portugal) was presented for clinical examination with a cutaneous alopecic mass located on the left tarsus medial aspect ( Figure 1) that was surgically removed before releasing into the wild. The lynx was negative, either serologically or by PCR, for feline immunodeficiency virus, feline coronavirus, feline calicivirus, feline leukemia virus, feline herpesvirus, feline parvovirus, canine distemper virus and Leptospira spp. A comprehensive clinical study showed no other abnormalities. The entire mass was fixed in 10% neutral buffered formalin and sections routinely embedded in paraffin wax and 4 µm sections were stained with haematoxylin and eosin (HE). Pan-cytokeratin immunohistochemistry was performed using a mouse anti-human cytokeratin (CK) monoclonal (AE1/AE3; M3515, Dako) at 1:50 dilution. Antigen retrieval with a 10 mM citric acid solution, an avidin-biotin complex peroxidase kit, and 3′3′-diaminobenzidine (both from Vector, Burlingame, USA) was performed.
Grossly, the mass was a solid, round, 2.2 cm, exophytic, partially alopecic, cutaneous nodule containing whitish keratinized, cornified material and a 1.2 cm dilated pore on the surface, through which the cornified material protruded to the exterior, forming a cutaneous horn; the capsule of the lesion presented tears ( Figure 1A). The cut section of the mass demonstrated central whitish discoloration, whereas a darker colour was observed at the cutaneous horn ( Figure 1B).
Microscopically, the dermo-epidermal mass corresponded to a round cystic structure lined by a layer of exaggerated regular hyperplastic (psoriasiform) and keratinizing squamous epithelium of infundibular origin that was thicker at the base of the cyst and thinner at the vicinity of the pore (Figure 2A). One or two layers of basal cells and numerous polygonal keratinocyte layers that showed moderate eosinophilic cytoplasm, prominent intercellular bridges, rounded uniform nuclei and inconspicuous nucleoli ( Figure 2B) formed the epithelium. Hyperplastic rows of epithelial cells rested on the prominent basal membrane. Variable areas of abrupt to progressive keratinization were observed along the hyperplastic epithelium ( Figure 2C). The central cystic cavity was filled by a large amount of amorphous and fibrillary keratin and abundant lightly stained squamous cells (ghost cells). At the periphery of the wall, the epithelium showed occasional keratin pearl formation ( Figure 2D). A thin layer of connective tissue that displaced the rest of the pilosebaceous structures laterally and ventrally (Figure 2A) delimited the epithelium. A peripheral lymphoplasmacytic infiltrate was observed together with occasional neutrophilic foci. IHC for pan-cytokeratin showed strong positive immunolabeling in the hyperplastic epithelium and a weaker signal at the content of the cyst (Figures 3A and B).
This paper is the first report of a DPW in a non-domestic felid, and some of its characteristics differ from previous reports in cats [1,3,12,13]. Whereas DPW affects middle-aged or older cats, the present case was observed in a 1.5-year-old Iberian lynx. Interestingly, only a few cases of DPW have been recorded in cats 1-5 years-old [3,12]. A similar situation is found in humans, in which DPW is more common between 60-70 years old, but cases in young individuals have been reported [5,14]. The location of the DPW in this lynx differs from the most frequently affected areas in domestic cats. In this case, DPW was found on a limb, whereas in cats, DPW is more frequently reported on the face and neck [3]. To the best of the authors' knowledge, this lesion has not yet been reported in a felid limb. Histopathologic features are similar to those described in cats, but the present case showed the formation of keratin pearls, a feature not commonly described for DPW [1,9,13].
Differential diagnoses for DPW in cats are limited [13], although some can be considered for the present case in a lynx. The most important is the infundibular keratinizing acanthoma (IKA), a non-recurrent benign follicular tumour with a keratin-filled central cavity arising from the squamous epithelium of the isthmus and infundibulum. IKA has only been documented in the scientific literature in dogs [13,15]. Some of the DPW characteristics are reminiscent of those of canine IKA, including: i) formation of a partially alopecic solitary nodule, ii) age range that includes animals under 5 years old, iii) location in the limbs, iv) eventual extrusion of keratin through a pore, forming a cutaneous horn and v) formation of variable numbers of keratin pearls [1,3,15,16]. Indeed, some early forms of IKA showing evident epithelial trabecular projections and scarce keratin pearls [1] may be evocative of this case of DPW in an Iberian lynx. Interestingly, in human beings, early arising pillar sheath acanthoma (the IKA counterpart in humans) could also be indistinguishable from DPW [7,8]. However, the characteristic rete ridge formation of the hyperplastic epithelium is a hallmark of DPW [15]. Well-differentiated squamous cell carcinoma (SCC) is another lesion to consider among the differential diagnoses. SCC has been described in the cervical region of a Canada lynx (Lynx canadensis) [17], inducing lytic lesions in the right mandible of an elderly bobcat (Lynx rufus) [18] and, recently, on the tongue of a European lynx (Lynx lynx) [19]. However, the location of SCC and its microscopic characteristics, such as the presence of cellular atypia together with a high number of mitoses and keratin pearls, differ from features observed in the present case of DPW.
Surgical removal of the whole DPW is curative in veterinary and human medicine [3,5,7,20]. After recovering from the surgery, this Iberian lynx was fitted with a GPS-GSM collar and released into the wild, near the city of Toledo (Spain). The individual travelled a total of 1605 km in 321 days. It was eventually a victim of roadkill, but no recurrences of the DPW were observed on post-mortem examination. In conclusion, we have described a DPW in an Iberian lynx and any other wild feline for the first time.
We have also discussed its main differences with similar lesions reported in domestic cats and other felines. Regardless of out-of-control anthropological factors, this work highlights the importance of pathologic studies in wildlife conservation to ensure a suitable health condition at releasing of captive-bred animals to ensure the widest possible life span.