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Ocular findings in Quarter Horses with hereditary equine regional dermal asthenia

Cathleen A. Mochal DVM1, William W. Miller DVM, MS, DACVO2, A. James Cooley DVM, DACVP3, Robert L. Linford DVM, PhD, DACVS4, Peter L. Ryan PhD5, and Ann M. Rashmir-Raven DVM, MS, DACVS6
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  • 1 Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762.
  • | 2 Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762.
  • | 3 Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762.
  • | 4 Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762.
  • | 5 Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762.
  • | 6 Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762.

Abstract

Objective—To compare ocular structures of Quarter Horses homozygous for hereditary equine regional dermal asthenia (HERDA) with those of Quarter Horses not affected by HERDA (control horses) and to determine the frequency of new corneal ulcers for horses with and without HERDA during a 4-year period.

Design—Cohort study of ocular structures and retrospective case series of horses with and without HERDA.

Animals—The cohort portion of the study involved 10 Quarter Horses with HERDA and 10 Quarter Horses without HERDA; the retrospective case series involved 28 horses with HERDA and 291 horses without HERDA.

Procedures—Ophthalmic examinations, Schirmer tear tests, tonometry, corneal pachymetry, histologic examinations, and scanning electron microscopy (SEM) were performed in cohorts of Quarter Horses with and without HERDA. Records were reviewed to determine the incidence of corneal ulcers in horses with and without HERDA during a 4-year period.

Results—Corneal thickness of horses with HERDA was significantly less than that of control horses, but tear production of horses with HERDA was significantly greater than that of control horses. Results of SEM revealed zones of disorganized, haphazardly arranged collagen fibrils in corneas of horses with HERDA that were not evident in corneas of control horses. The incidence of corneal ulcers was significantly greater for horses with HERDA than for horses without HERDA during the 4-year period.

Conclusions and Clinical Relevance—Alterations in corneal thickness, arrangement of collagen fibers, and incidence of corneal ulcers indicated that abnormalities in horses with HERDA were not limited to the skin.

Abstract

Objective—To compare ocular structures of Quarter Horses homozygous for hereditary equine regional dermal asthenia (HERDA) with those of Quarter Horses not affected by HERDA (control horses) and to determine the frequency of new corneal ulcers for horses with and without HERDA during a 4-year period.

Design—Cohort study of ocular structures and retrospective case series of horses with and without HERDA.

Animals—The cohort portion of the study involved 10 Quarter Horses with HERDA and 10 Quarter Horses without HERDA; the retrospective case series involved 28 horses with HERDA and 291 horses without HERDA.

Procedures—Ophthalmic examinations, Schirmer tear tests, tonometry, corneal pachymetry, histologic examinations, and scanning electron microscopy (SEM) were performed in cohorts of Quarter Horses with and without HERDA. Records were reviewed to determine the incidence of corneal ulcers in horses with and without HERDA during a 4-year period.

Results—Corneal thickness of horses with HERDA was significantly less than that of control horses, but tear production of horses with HERDA was significantly greater than that of control horses. Results of SEM revealed zones of disorganized, haphazardly arranged collagen fibrils in corneas of horses with HERDA that were not evident in corneas of control horses. The incidence of corneal ulcers was significantly greater for horses with HERDA than for horses without HERDA during the 4-year period.

Conclusions and Clinical Relevance—Alterations in corneal thickness, arrangement of collagen fibers, and incidence of corneal ulcers indicated that abnormalities in horses with HERDA were not limited to the skin.

Contributor Notes

Dr. Rashmir-Raven's present address is the Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

Supported by the Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University; the American Quarter Horse Foundation (grant No. 07070633); the National Institutes of Health (NIH-NCRR RR070710); and private donors.

Presented in abstract form at the 55th Annual Convention of the American Association of Equine Practitioners, Las Vegas, December 2009.

The authors thank Amanda Lawrence for assistance with preparation of samples for scanning electron microscopy.

Address correspondence to Dr. Rashmir-Raven (rashmir@cvm.msu.edu).