Genetic Test: Squamous Cell Carcinoma
Squamous cell carcinoma is the second most common type of equine tumor and the most frequent tumor of the horse’s eye, according to a report on research led by Rebecca R. Bellone, PhD, from the University of California Davis School of Veterinary Medicine and the Veterinary Genetics Laboratory. SCC can lead to vision impairment and the loss of an eye and can spread into the orbital bone and the brain, potentially turning fatal.
Genetics seem to be a risk factor for the disease since several breeds, including Haflingers, tend to experience a higher occurrence. Dr. Bellone and her team, including UC Davis veterinary ophthalmologist Mary Lassaline, DVM, PhD, MA, DACVO, set out to investigate this genetic link. Their study included 67 Haflingers—43 identified as affected by SCC and the others unaffected. The research team examined five-generation pedigrees for all 67 horses, looking for a common ancestor who would support the theory of a genetic link. They found one. Examining the relationship between affected and unaffected horses showed that the genetic link was most likely inherited as a recessive trait, says Dr. Bellone.
Researchers used this information to guide their next step: a genome-wide study where they investigated approximately 65,000 genetic markers. They found that risk for SCC was associated with markers on chromosome 12 and, specifically, a risk variant of gene DDB2, says Dr. Bellone. DDB2 is a protein needed to repair UV damage to DNA. The team’s working hypothesis (currently being tested) is that the risk variant they identified “impairs the ability of this protein to recognize DNA that is damaged, thus the damage can’t be repaired and that leads to cancer,” says Dr. Bellone.
The researchers further determined that horses who are homozygous for the risk factor (they’ve inherited it from both sire and dam) are 5½ times more likely to develop SCC than those with one copy or no copies of the risk factor.
The Veterinary Genetics Laboratory used these findings to develop a genetic test for the SCC risk factor in Haflingers. (To learn more, visit https://www.vgl.ucdavis.edu/services/HaflingerSCC.php.)
While not all cases of SCC can be linked to genetics, this study shows they play a significant role, particularly in the Haflinger breed. This knowledge, along with the new genetic test, can help owners assess the risk their own horses face and help them make more informed breeding decisions.
The variant was also found in Belgian and Percheron horses, and the researchers are now working to determine if it’s a risk factor for these breeds, too.
Interestingly, a similar disease in humans has also been linked to DDB2. So insights from the equine study may lead to new information for human health as well.
A New Understanding Of Saddle Fit
You know that saddle fit influences your horse’s comfort and performance. But new research shows that even established industry fitting standards may need some tweaking to better alleviate back pressure and improve gaits.
That’s the bottom-line finding of research led by Rachel Murray, VetMB, MS, PhD, MRCVS, Dip ACVS, associate of European College of Veterinary Diagnostic Imaging, who works at England’s Animal Health Trust. Specifically, Dr. Murray and colleagues compared pressure, limb protraction, knee and hock flexion, and back width between saddles fitted to industry guidelines and one specially designed to reduce pressure along either side of the thoracic vertebrae known as T10 to T13. (Previous research had shown that a horse’s back, or thoracolumbar, width increases after exercise when his saddle fits properly as well as when horses are ridden more correctly with a more skilled rider.)
The study included 13 international-level dressage horses, aged 8 to 16 years, with no existing lameness or performance problems. They were each ridden in the study by their usual professional riders. For the control portion of the test, they were ridden in their normal saddles, which were evaluated and fitted by four qualified saddle fitters.
The test saddles had modifications over typical saddles. The tree shape, alignment of the girth billets and shape of the panels were adjusted to accommodate the horse’s musculature (thoracolumbar expansion)
during exercise versus while standing. The solid arm of the panel was shortened to reduce the area of potential restriction at the front of the saddle and the stirrup bars were attached to the exterior of the tree. The panels were lined with pressureabsorbing material.
Researchers used a pressure mat under the saddle panels as well as highspeed motion capture to evaluate the effect of the two saddles on the horses while being ridden at sitting trot. In addition, the thoracolumbar width was measured before and after exercise.
The researchers found that peak pressures were significantly less with the test saddles than the control saddles. In addition, the test saddles allowed greater limb protraction as well as greater knee and hock flexion. The post-exercise thoracolumbar width was also significantly greater with the test saddles.
Dr. Murray explains that the most significant takeaway from the study is the understanding that when a horse works correctly, his back expands under the saddle. “If the saddle does not allow this expansion/lift, the horse will be discouraged from working correctly and encouraged to work with a hollow, potentially dipped back, which can affect the movement of the forelimbs and hind limbs as well as restrict back movement,” she says.
Standard saddle-fitting guidelines don’t necessarily take this into account. Therefore, she adds, it’s important to assess saddle fit not simply when the horse is standing still, but also during exercise. — Sushil Dulai Wenholz
Researchers have discovered that genetics play a key role in the risk factor for squamous cell carcinoma in several breeds, including Haflingers.
A recent study showed the importance of assessing your horse’s saddle fit when he’s moving, not just while he’s standing still.