That 2-year-old with what appears to be a huge notch cut out of his topline just behind the withers must belong on the disabled list for life, and that old pensioner with her bowed back and pendulous belly has earned her deformity from long years of hard riding and frequent pregnancies. Or so you'd think from looking at these abnormally conformed horses, but with swayback--or, to put it clinically, lordosis--the cause and consequence of what you see are not at all what you'd expect. Even when the spinal deformity is truly startling to behold, the affected horse functions as though he or she were normally conformed.
"One of the unique characteristics of lordosis in horses is that the spinal deviation does not have a disabling effect," observes Patrick Gallagher, PhD, who researched equine lordosis while a graduate student at the University of Kentucky. "Even the most severely affected individuals can be trained and ridden and can participate in horse shows. Reports of spinal deviations in other species, such as people and dogs, identify drastic neurologic impairment in the most severe cases. These include incoordination, paralysis and risk of death."
For as noticeable as it is, equine lordosis has received little scientific study into its origins, effects and unique inconsequence in the equine species. With less than 1 percent of the general horse population affected by the functionally benign condition, the pressure for research hasn't been strong. Gallagher's investigative interest was driven by his personal involvement with American Saddlebred show horses. As a child accompanying his father, a Saddlebred owner and exhibitor, to shows and as a youth working summers as a groom for a show stable, Gallagher observed the breed's above-average incidence of lordosis.
Later, while doing graduate study under Kentucky geneticist Ernest Bailey, PhD, Gallagher focused on the structural and genetic basis for early-onset lordosis that affects young horses during their skeletal development. His findings help clarify the cause, effects and implications for real-life management of swaybacked horses.
The Distances between Two Points
Gallagher's research started at square one with the development of a measure for determining back curvature so topline variations could be quantified in a large group of horses. The technique, which proved to be highly reliable and repeatable, is quite simple. The high points of the horse's withers and the rump are marked with adhesive tape, and the straight-line distance and the back-surface distance between these two points are measured and compared. The difference between the two lengths serves as the back-contour measure.
A total of 394 horse backs--305 Saddlebreds, 40 Arabians and 49 first-generation Saddlebred-Arabian crosses--provided the data Gallagher used to define the line between normal and abnormal in topline curvature. The differences between the two measurements in this population ranged from less than a half inch to nearly five inches, but the single most common contour in all the horses, regardless of breed, was 1 1/2 inches (four centimeters, or cm). All told, 75 percent of the horses had back contours in the three- to five-cm range, the equivalent of 1 1/8 to two inches.
Gallagher and his fellow researchers originally identified 24 of the horses as being swaybacked simply by "eyeballing" the horses' toplines. With the back measurements in hand, they found that, statistically, the abnormality started with contours greater than 2 1/2 to 2 3/4 inches (6.5 to seven cm). Four horses with slightly lesser measurements were reclassified in the normal range, leaving 10 males and 10 females in the swayback group. Of the 14 horses under age 20, nine were geldings. Of the six aged 20 and over, five were broodmares, who may or may not have developed the spinal deformity as juveniles. All of the swaybacks were Saddlebreds. Seven percent of the 305 measured Saddlebreds were affected, and at least 5 percent of the group had developed the deformity while young.