Psychogenic alopecia in rhesus macaques presenting as focally extensive alopecia of the distal limb

Abstract

Focally extensive alopecia affecting the distal limbs is a common clinical finding in rhesus macaque (Macaca mulatta) colonies and is both a regulatory and colony-health concern. We performed diagnostic examinations including physical exams, bloodwork, skin scrapes, surface cytology, and surface bacterial-fungal cultures on 17 rhesus macaques with this presentation of alopecia. Skin biopsies from alopecic skin obtained from each macaque were compared with those of normal skin from the same animal. Immunohistochemistry and metachromatic staining for inflammatory cells were performed to compare alopecic and normal skin. In addition, we compared these biopsies with those previously obtained from macaques with generalized alopecia and dermal inflammatory infiltrates consistent with cutaneous hypersensitivity disorders and with those from animals with normal haircoats. Bacterial and fungal cultures, skin scrapes, surface cytology, and bloodwork were unremarkable. Affected skin showed only mild histologic alteration, with rare evidence of trichomalacia and follicular loss. Numbers of mast cells and CD3+ lymphocytes did not differ between alopecic and normally haired skin from the same animal. The number of mast cells in alopecic skin from animals in the current cohort was significantly lower than that in skin of animals previously diagnosed with a cutaneous hypersensitivity disorder. Numbers of both mast cells and CD3+ lymphocytes in alopecic skin from the current cohort were similar to those from biopsies of animals with normal haircoats. Together, the clinical findings and pathology are consistent with a psychogenic origin for this pattern of alopecia in rhesus macaques.

Figure 1.

Clinical appearance of distal limb alopecia, with a well circumscribed area of hair loss without underlying skin inflammation.

Figure 2.

(A) Unaffected skin had a thin epidermis, parallel hair follicles, and little inflammation. Hematoxylin and eosin stain; magnification, ×4. (B) Affected forearm skin generally looked very similar, with no notable differences or obvious epidermal or hair follicle pathology. Hematoxylin and eosin stain; magnification, ×4. (C) Rarely, there was generalized loss of follicular structures but still no inflammation or other pathology. Hematoxylin and eosin stain; magnification, ×4. Trichrome staining (inset) showed no fibrosis or other changes. Several sections had (D) damaged hair follicles or (E) intrafollicular hemorrhage, consistent with trichotillomania. Hematoxylin and eosin stain; magnification, ×40. There were no notable differences in mast cells (stained purple with toluidine blue; magnification, ×20) between (F) unaffected and (G) affected skin or in the numbers of CD3+ T cells (stained brown with CD3 immunostain with DAB chromogen; magnification, ×20) between (H) unaffected and (I) affected skin.