Equine Hyperelastosis Cutis Update

Hyperelastosis cutis (HC) is an autosomal recessive connective tissue disorder of economic importance in Quarter horses and horses of Quarter horse lineage. It occurs most frequently in popular cutting horse bloodlines. Affected horses have extremely fragile skin that tears easily and exhibits impaired healing. There is no curative treatment. A DNA test for carrier identification is not yet available, and therefore, managed breeding strategy is currently the only option for reducing the incidence of HC.

1. Introduction

Equine hyperelastosis cutis (HC), also known as dermatosparaxis, Ehlers-Danlos syndrome, or hereditary equine regional dermal asthenia (HERDA), is a rapidly emerging disease within the Quarter horse industry. Similar conditions also occur in cattle, cats, dogs, rabbits, humans, and other animals [1-9]. Within the equine species, cutting horses are most commonly affected, but reining, working cow horses, and pleasure horses may also exhibit HC. Horses with Quarter horse lineage, such as Paints and Appaloosas, have also been affected. In horses with HC, the skin separates between the deep and superficial dermis. This results in "mushy" or "stretchy" skin that is not securely attached in multiple areas ranging from the size of a dime to nearly the entire horse (Fig. 1 and Fig. 2). Pain may occur when traction is applied to affected areas, and defects may have a depressed surface compared with surrounding skin [1,2,10,11]. Affected horses frequently have painful skin sloughing with disfiguring scars (Fig. 3 and Fig. 4). The condition may be apparent at birth; however, HC is not usually noticed until training begins around 2 yr of age and after considerable financial and emotional investments have been made. Skin lesions are associated with even mild trauma such as the saddle imprint. Aside from minimizing trauma, no treatment is known. The objectives of this study were to further characterize the clinical progression of HC and to determine whether there was familial basis to the condition.

Figure 1. A horse with HC. Note the dorsal lesions.

Figure 2. Easily manipulated skin on the horse’s dorsum consistent with HC.

Figure 3. "Stretchy" skin on the horse’s dorsum consistent with HC.

Figure 4. Acute wounds associated with HC.

2. Materials and Methods

Twenty-four horses with HC were available for this study. Twelve horses became part of the HC breeding program at Mississippi State University and were followed for more than 1 yr. The remaining 12 horses were evaluated and discharged to their owners or euthanized after samples [blood, hair, histopathology, transmission electron microscopy (TEM), and scanning electron microscopy (SEM)] were collected. An additional 56 pedigrees of confirmed cases were available for evaluation. These cases were referred for evaluation by veterinarians familiar with the disorder. In six of these additional cases, biopsies were also submitted for histopathologic evaluation (Fig. 5).

Figure 5. SEM of an HC lesion. Note that the superficial and deep dermis layers are separated.

Diagnosis was based on history, clinical signs, and pedigree analysis. Although histopathology is currently controversial [3,10,11], proper biopsy technique can provide an informative sample. Because the skin splits between the superficial and deep dermis (Fig. 6), biopsy samples are difficult to obtain from the deeper, critical tissues. In addition, samples are friable and can separate during processing. Ideally, a biopsy includes a sample of skin at full thickness that is skewered on a 25-gauge needle. The entire specimen and needle should be placed in formalin (Fig. 7). Evaluation of the biopsy site before closure should show the presence of muscle tissue in the deepest part of the incision and confirm that an adequate depth of the biopsy was obtained (Fig. 8).

Figure 6. Punch biopsy of horse with HC.

Figure 7. Skewering the biopsy sample to ensure proper alignment and sample preparation.

Figure 8. Post-biopsy site. Note that the muscle at the deep part of the incision insures adequate depth of biopsy.

The registered names of affected horses were obtained from owners or referring veterinarians. Complete pedigrees for all horses were obtained from the American Quarter Horse Association (AQHA), American Paint Horse Association (APHA), or Appaloosa Horse Club (ApHC). The inter-relatedness of affected horses was evaluated based on these records. The statistical significance of consanguinity to one of two prominent, closely related sires within six generations was evaluated in 75 randomly chosen AQHA or APHA registered horses with HC (compared with control horses without HC) using Pearson’s χ2 (2-sided) test. Pedigrees of systematically selected age- and breed-matched controls were obtained from the appropriate breed registry. Controls were chosen by taking the tenth consecutive registration entry above those of affected horses with even registration numbers, and the tenth consecutive registration entry below those of affected horses with odd registration numbers. This was done to avoid bias in timing of registrations and to avoid "close" registration numbers that might be from the same breeding operation. Additionally, the association was described by the odds ratio (odds of being affected if related to either sire compared with odds of being affected if not related to either sire).

3. Results

There is no known cure for HC; however, palliative therapy can improve the quality of the affected horse’s life. Horses are less apt to develop lesions during the winter, suggesting that solar- or heat-related stress to the skin may contribute to the development or progression of lesions. The skin of affected horses appears to improve when they are kept indoors and away from other horses. Reproductive problems have not been observed in conjunction with this disease, and affected mares have been successfully used as embryo-transfer recipients. Because affected horses are essentially unusable for performance and breeding (because of the heritable nature of the disorder), they are frequently euthanized.

Pedigree evaluation of HC-affected horses supports an autosomal recessive mode of inheritance. With the exception of planned matings of affected horses, which occur rarely in the industry, affected animals result from matings between clinically normal stallions and mares. HC affects horses of both sexes and is transmitted by both sexes. All 90 pedigrees of affected horses examined show consanguinity (in both the sire’s and dam’s lineage) to the prominent Quarter horse stallion Poco Bueno (AQHA 3044) or one of his immediate ancestors. This observation suggested a familial basis for the disorder. Analysis of 75 randomly chosen pedigrees of affected horses showed that, in 72 cases, both the sire’s and dam’s lineages were consanguineous to Poco Bueno or his sire, King (AQHA 234), within six generations. In the remaining three cases, Poco Bueno was not in one of the two lineages until the seventh generation. In pedigrees of presumed normal control horses, consanguinity to either sire within six generations was observed in 22 of 75 horses. The association between HC and consanguinity to either Poco Bueno or King was highly significant (p < 0.0001) with an odds ratio of 58 (95% confidence interval: 16,203). These results strongly support a familial basis for the disorder with one particular bloodline of Quarter horses being at risk for developing HC. Although the specific biochemical defect causing HC in this bloodline has not been identified, we have used PCR-based cloning techniques to rule out mutations in genes coding for several of the major structural proteins of the dermal extracellular matrix.

4. Discussion

The results of our studies show that there is a strong familial basis to HC. We have identified the Poco Bueno/King bloodline of Quarter horses as the population in which HC is segregating. We have also shown that horses consanguineous to either sire have a 58-fold higher odds of developing HC than horses without this pattern of consanguinity. These findings are consistent with those of other investigators.[8] During the course of these studies, we have confirmed that fourteen of the top cutting sires (Equistat 2004 top lifetime cutting sires, all ages, all divisions), with gross earnings in excess of $70.3 million, are HC carriers. Between 1998 and 2002, these 14 sires had 1241 offspring sold at auction for $26.8 million. One-half of these offspring (value of approximately $13 million) will also be carriers. The 5-yr leading sires of cutting horses include 13 known carriers with gross offspring earnings of $27 million (the top 100 sires had offspring earnings of $118 million). The large number and significant economic value of equine carriers of HC suggest that the disease will be with us for many years to come. Even with the advent of DNA testing, it is likely that affected horses will still be produced. Recognition of the more subtly affected animals will be an important aspect of equine practice in the future. Managed breeding strategy is the only method currently available to minimize the production of affected horses. When performed correctly, biopsy for histopathologic assessment can be valuable, particularly for veterinarians previously unfamiliar with the disease.