Demodex in horses

Amanda Mansz                      

Animal Health Laboratory, University of Guelph, Guelph, ON

 AHL Newsletter 2023;27(4):20.

Due to concern for eyelid squamous cell carcinoma, surgical biopsies of three 2-3 mm nodules at the eyelid margin of a 15-year-old female Welsh pony were submitted to AHL for histological diagnosis (Fig. 1).  The nodules were situated on non-pigmented skin and had been present for one year with very slow growth.

Microscopic review of the nodules revealed a hyperplastic conjunctival and epithelial surface with hyperplastic sebaceous/meibomian glands forming expansile nodules interspersed with ectatic ducts.  Within these ducts, and occasionally in acini, were numerous Demodex mites (Fig. 2).  Surrounding inflammation was relatively mild.

Demodectic mange in horses is caused by infestation with Demodex equi or Demodex caballi.  D. caballi is often restricted to the eyelids and muzzle, whereas D. equi infestation occurs over the body.  Demodex mites infest hair follicles and sebaceous glands (as seen in this case).

Equine demodicosis can manifest as nodules or patchy alopecia and scaling.  It has been reported in association with pituitary pars intermedia dysfunction or chronic corticosteroid treatment (neither of which were associated with this case).  Demodectic mange is rare in horses.  This is the only case of equine Demodex that has been diagnosed at the AHL in a data search spanning back to 2010.

Figure 1. Equine Demodex. Multiple small, raised, tan/pink nodules on the eyelid margins.

Figure 2. Equine Demodex. H&E stain. Demodex mite in cross-section within a duct of the sebaceous (meibomian) glands of the eyelid. 40x.

Reference

1. Mauldin EA, Peters-Kennedy J.  Integumentary System. In: Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals, 6^th ed. Maxie, ed. Elsevier, 2016;vol 1:682.