The Modern Equine Vet
May 2024
Vol 14 Issue 5 2024
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Birth Weight, Mare Parity Influence Foals’ Risk for Skeletal Disorders

By Conni Koury

Osteochondritis dissecans (OCD) and sesamoiditis, commonly observed in Thoroughbred foals, can greatly influence the sales price and the athletic performance of a horse.

Previous research seeking to identify risk factors associated with skeletal disorders has indicated that larger foals are more likely to develop OCD compared with smaller ones. In further exploring variables that influence the development of OCD and sesamoiditis, those investigators found foals that had OCD surgery were heavier at birth and more likely to have a multiparous mare. Sesamoiditis, on the other hand, was increased in lower weight foals with primiparous mares.

The average weight of foals that had OCD surgery was 59 kg versus 56 kg for those with no OCD, Joe D. Pagan, PhD, reported at the AAEP 2023 Annual Convention.

“Really heavy foals, those in the 90th percentile according to a global database for birth weights (≥63 kg), had a 2-fold increased incidence of OCD surgery compared with the lighter birth weights. Yet, foals from primiparous mares, which tend to be lighter weight, had a higher incidence of sesamoiditis versus those from mares that already had foals,” said Dr. Pagan, the owner of Kentucky Equine Research in Versailles.

Spring Radiograph Study
The study drew from spring survey radiographs of 1,047 foals located at 11 Kentucky horse farms over a period of 6 years. Dr. Pagan and his team categorized the foals into quartiles according to height and weight and divided them into 5 age brackets from foal to yearling (1-30 days, 31-90 days, 91-240 days, 241-360 days and >360 days). Birth month was noted, and mares were categorized as primiparous or multiparous. Using percentiles of weight allows investigators to rank the relative size of an individual in a population based on its age and gender, which is useful for comparisons, he noted.

Researchers calculated the incidence and location of OCD (stifle, hock and fetlock), incidence of OCD surgery, and incidence of sesamoiditis.

“About 15% of the foals in the surveys had OCD, the hock was the most prevalent location, and 10%—more than 100 foals—had OCD surgery,” he said. “We found a 23% incidence of sesamoiditis in these survey radiographs, all cases of which were classified as mild.”

Only a small group of 23 foals had both sesamoiditis and OCD in their survey radiographs, indicating different factors are at work, he added.

Deeper Dive: Different Ages
When looking at the foals’ weight across age groups, the study found foals that had received OCD surgery were heavier during their first 240 days of life than the foals that did not have OCD surgery, and the OCD surgery foals were also taller than those with no OCD through the entire study. The greatest difference in both height and weight were during the first 30 days of age.

“Foals with no OCD tended to be fairly distributed among all four quartiles of weight,” Dr. Pagan added. “Only 4% of foals that had OCD surgery came out of the first quartile of weight—the smallest foals. By comparison, 48% came from the fourth quartile.”

Foals that had OCD surgery were significantly heavier than either the foals with sesamoiditis or the foals with no skeletal abnormalities at 1 to 30 days of age. “But things changed dramatically by the time of the surveys when the foals were 240 to 360 days of age. The smaller foals increased their body weight significantly compared with the OCD group and foals with no significant abnormalities,” Dr. Pagan reported.

The average weight of foals with OCD surgery was at the 69th percentile versus 52 for sesamoiditis; later the average weight percentile is virtually the same. “The foals with sesamoiditis caught up with the ones that had OCD surgery.”

Role of Parity
There was a 7-kg difference between the average weight for first foals at 51 kg versus those from multiparous mares at 58 kg, and researchers found that 42% of first foals were in the first quartile for weight at birth compared with 4% of OCD surgery foals.

“Primiparous foals were much smaller than multiparous at birth, but we saw that they change dramatically and almost catch up completely,” Dr. Pagan said. “There’s some real compensatory growth that occurs in the primiparous mares’ foals compared with multiparous.”

At 1 to 30 days, the primiparous foals had an average percentile weight of 34 compared to 62 for the multiparous foals. By the time they are yearlings, the primiparous group had jumped up to the 58th percentile compared with 63rd in the multiparous. Foals from primiparous mares start with a much lower body weight, but they increase over time.

These primiparous foals had a lower incidence of OCD, 6.7% versus 10.6% in the multiparous foals; however, they were more likely to have sesamoiditis, 28.5% incidence compared with 21.2%.

In Kentucky, foals born in January and February are lighter than foals born in April and May. This is largely due to maiden mares typically being bred early in the year and foaling in January, February and March. Factors that favored heavier foals—having a multiparous mare and being born in April—are associated with an increased incidence of OCD. Factors that favored compensatory gain—having a primiparous mare and being a February foal—are associated with sesamoiditis.

It has been suggested that, both in pigs and horses, osteochondritis is the result of a failure of blood supply to the epiphyseal growth cartilage.

“Could it be that these big, tall foals were somehow compromising this vascular supply early in life that led to OCD?” Dr. Pagan mused. “Heavier and taller foals have a higher incidence of surgical OCD with the difference most apparent early during the first 30 days, indicating that OCD may begin earlier in the foal’s life than we previously believed,” Dr. Pagan said. MeV

For more information:

Pagan J. Relationship between foal growth, survey radiographic findings and sales and racing performance in thoroughbred racehorses: a global research initiative. Accessed April 1, 2024.

Ytrehus B, Carlson CS, Ekman E. Etiology and pathogenesis of osteochondrosis. Vet Pathol. 2007;44:429-448.


Birth Weights of foals out of primiparous and multiparous mares.

Body weights of Thoroughbred colts.
Source: Dr. Joe D. Pagan