From time-to-time unusual syndromes or cases are reported from abattoirs or slaughterhouses by astute staff on the lookout for possible exotic or new diseases. In this case, in a mob of 50 lambs submitted by a producer in the Adelaide Plains area, one lamb had unusual skin lesions that were collected and sent for examination. Exotic diseases were ruled out but a diagnosis of follicular (epidermal inclusion) cysts was made.
50 lambs of about 11 months of age were presented for slaughter in August 2022. All lambs were healthy and in good condition. One lamb however had unusual lumps in the skin. It was assumed that these could be grass seed abscesses, but they were not typical abscesses and August is earlier than expected for grass seeds.
Report excerpt: “From four sections of haired skin, the epidermis and follicular infundibulum is hyperplastic with orthokeratotic hyperkeratosis. Multifocally, there are intraepidermal or intracorneal abscesses and ballooning degeneration. No infectious agents are noted. In the dermis there is fibroadnexal dysplasia and a single or double follicular cysts lined by well-differentiated keratinising stratified squamous epithelium surrounded by a dense collagen. The lumen of the cyst is filled with lamellar keratin and scattered pale eosinophilic ghost cells. The epithelial keratinisation contains a sparse granular layer with patchy areas of scant pigmentation. A few small sebaceous gland lobules are situated at the superficial aspect of the cysts.
The diagnosis is follicular cysts; with epidermal hyperplasia and hyperkeratosis, epidermal and intracorneal abscesses.”
Gram and Periodic Acid-Schiff (PAS) histochemical stains applied to areas of inflammation were negative for bacterial and fungal organisms.
Histomorphologic features were compatible with follicular cysts (syn. epidermal inclusion cyst). There was no evidence of viral cytopathic effects or neoplastic transformation in these tissue sections. In general, follicular cysts represent a non-neoplastic follicular anomaly that is idiopathic (congenital) or acquired (as in this case) from occlusion of follicular orifices due to local inflammation or scarring. Provided there was no prior antibiotic treatment, the negative culture results excluded a bacterial infection in this instance. Nevertheless, congenital cysts from developmental anomaly of follicular orifice could not be completely excluded; these are most commonly identified in Merino and Suffolk sheep.
Follicular cysts are often solitary, but may be multiple either synchronously or sequentially in some animals. They typically present as intradermal or subcutaneous nodules, occasionally with a pore from which keratin/grumous content may be expelled onto the skin surface. Gradual enlargement and eventual rupture releases keratin into the dermis which stimulates a granulomatous foreign body reaction with rapid increase in mass size.
Sheep pox and Sheep scab (both exotic to Australia) were ruled out in this case and no further cases have been reported from this property. PIRSA appreciates submissions of unusual cases, particularly where an exotic disease is a possibility. There appears to be very few reported cases of follicular cysts in the literature.1