Swayback can occur in old horses as soft-tissue attachments slacken and muscle loses tone.
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.
Spinal Designs and Misalignments
Appearances are deceiving even with normal horse backbones. The familiar rise and fall and rise again of the equine topline does not describe the path followed by the spine itself. Instead, the vertebrae from chest to tailhead follow one another in a nearly horizontal straight line.
The undulations on the exterior of the horse are created by finlike projections, called dorsal spinal processes, on the vertebrae's upper surfaces. The processes are longest at the withers, shortest under the saddle and longer again toward the hips. The processes that produce the withers grow taller as horses pass from infancy to maturity, so deepening of the back contour is a normal conformational development. The five immature horses in Gallagher's study who were remeasured after the initial assessment showed an average increase of one-third inch in their back contours in a one-year interval.
The most significant characteristic of the torso portion of the normal equine spine is its rigidity. That barely flexible row of vertebrae is the fixed point around which horse gaits are made and upon which riders are supported. It also provides an essentially immobile, highly protective conduit for the spinal cord within. The vertebral bodies are strapped in place by ligaments and other soft tissues, and the horse's spinal column is held under tension by the belly muscles. Swayback associated with old age occurs with slackening of the soft-tissue attachments and loss of muscle tone in the belly and back that allow the spine to sag somewhat. Even with these age-related changes, which are far from universal, old horses' backs are still considerably more rigid than other species'.
Early-onset lordosis studied in other animals involves deformities in the vertebrae themselves, not just problems with the supporting structures. To establish a similar structural defect in lordotic horses, Gallagher had an extremely swaybacked 12-year-old Saddlebred gelding radiographed from chest to loins. As expected, the spinal column was far from straight. Three of the vertebrae early in the sequence were wedge shaped. These hemivertebrae, so-called because of their "half" development, were shorter on the underside, thus causing the spinal column to bow at that point. The following vertebrae showed blunting of the upper front surfaces as they adjusted to the arched segment preceding them. Additionally, there were indications that impingement, or contact, of the dorsal processes toward the end of the line may have occurred.
With all its abnormalities, however, the deformed spine exhibited one key characteristic: It followed a smooth curved path, which is not the case with species that experience neurologic effects with lordosis. The "kinks" in other lordotic backbones put pressure on the spinal cord, producing paralysis and other dire results. With horses, the smooth flow of the deformed vertebrae, fixed firmly in place by the strong vertebral attachments, appears to protect the spinal cord from harm and swaybacked horses from serious consequences.
"The comparatively extreme rigidity of the horses' spines might protect them from the major disabilities and life-threatening conditions experienced by species with more flexible spines," says Gallagher.
Radiographs of one deformed horse spine are not proof positive that incompletely developed vertebrae are the structural basis for, rather than an effect of, equine lordosis. But Gallagher cites supporting evidence from other species to make the case for their causative role in horses. Hemivertebrae have been shown as the cause of spinal deformity in humans, dogs, goats and cows. In human infants, the wedge-shaped vertebrae radiographed before the back deformity became apparent produced rapid spinal curvature during the youngsters' skeletal growth spurts.
One of the 32 remeasured horses in the Kentucky study appeared to follow the same course: Normal as a youngster with only a three-quarter inch (2 cm) contour, the horse dropped into the swaybacked category with a measurement differential of 2 3/4 inches in just a year's time. Already-lordotic horses remeasured in the study maintained the same contour from year to year.
"Only in severe cases does a newborn foal show signs of lordosis," says Gallagher. "The back abnormality usually progresses with skeletal growth and becomes apparent in the first 18 months of life. Then it stabilizes and remains at that final contour."
Gallagher used pedigree studies to determine the influence of inheritance on the incidence of lordosis in Saddlebreds. Of the 14 sires represented by three or more offspring in the study group, five appeared in the lordotic pedigrees. The fact that the deformity did not appear uniformly in all 14 families but did appear at the same statistical frequency on all 12 farms indicates that genes, not environment, are the determining influence.
"We can definitely say that there is a major genetic component to lordosis," says Gallagher. "And given the observation of the one case radiographed in a horse, plus the studies of human and canine lordosis, likely a hemivertebra in the thoracic spine is the cause. The puzzle is how a single gene or a combination of two or more genes can affect individual vertebrae."
Gallagher posits that the deformity arises from a single recessive gene or a small cluster of genes in the species' 30,000- to 40,000-gene package. For the trait to surface in an individual, both sire and dam would have to contribute the lordotic gene(s). If only one parent contributed the recessive trait, the dominant gene in the pair would prevail and the spine would develop normally. The researcher also suspects that the reason for the uncommon occurrence of lordosis in Saddlebreds evolved because breeders have been selecting for the positive, dominant characteristic that's linked with the recessive trait. The breed's fine bones and streamlined body build are possible linked characteristics.
"It appears that there may have been some type of selection for the gene or genes that affect lordosis," says Gallagher. "I don't believe breeders have been selecting for lordosis itself but for an alternate positive effect of the genes. If lordosis is a recessive trait, the selection has been for the heterozygous [dominant] effect paired with it, but with the increased presence of the characteristic in Saddlebred breeding stock, there's more opportunity for offspring to receive two recessive genes and thus be affected by lordosis."
The specific gene(s) involved in lordosis have not been identified, so there's not currently a DNA-based test available to screen for the recessive characteristic. Nor has an exhaustive pedigree analysis of breeding stock been done to identify carriers of the trait. Yet with the genetic basis for the deformity indicated, breeders can be
especially cautious in selecting sires to mate with mares who have already produced foals with early-onset lordosis. Stallions without known offspring affected by the deformity are safe choices.
Swaybacks under Saddle
Lordosis affects horses of all breeds, and neither the early-onset variety nor the late-arriving form has any particular influence on horses' health or soundness. Swaybacked broodmares have no additional problems in conceiving, carrying or delivering their foals. Performance horses are not impaired to any significant degree by lordosis, though swaybacked racehorses aren't among the fleetest and affected show stock may have slightly altered gaits.
"Lordosis has been somewhat of a controversial issue because affected horses are able to perform and with a saddle on aren't particularly noticeable," says Gallagher. "Horsepeople have differing opinions about the effect on gait, which is minor and might be dealt with by shoeing and training."
The one area of concern for horses with abnormal spinal curvature is in saddle fit and back injury. A saddle tree that bridges across the deep back contour, thus putting extreme pressure on four small areas, or concentrates the rider's weight behind the withers can cause the horse much discomfort. The abnormal pitch of the dorsal processes of a swaybacked horse can also cause painful impingement of bone on bone.
Gallagher doesn't recall seeing a lordotic horse with obvious use-caused back injury, but he has observed affected horses who displayed some subtle sourness under saddle that may have been associated with impingement. Careful saddle selection and appropriate padding are the primary considerations in outfitting lordotic horses for riding. Swaybacked horses should be able to perform in harness without any particular accommodations to their abnormality.
This article originally appeared in the February 2004 issue of EQUUS magazine