Spinal problems identified in skeletons of three important historical sires

Researchers in Germany have found spinal malformations in the skeletons of three historic stallions considered important in several modern breeds.

Elisa Zimmermann and her fellow researchers, writing in the journal Animals, noted that malformations in the area of the cervicothoracic junction have been described in modern horses as well as in skeletons from museums.

The cervicothoracic junction refers to the area of the spine where the cervical (neck) vertebrae transition to the thoracic (chest) vertebrae. Broadly, it covers the lower neck, shoulder and withers region.

Malformations of the junction affect the C6 and C7 cervical vertebrae, the T1 thoracic vertebra and to a variable extent the first and second sternal ribs.

To date, the clinical impact of this malformation, its prevalence and mode of inheritance in equine populations are not yet determined.

Researchers in the University of Veterinary Medicine Hannover (Foundation) study examined five historical skeletons for signs of malformations of the cervicothoracic junction. They were able to detect malformations of C6/C7 in three of the five historical horses.

These affected horses were the Thoroughbred stallions Dark Ronald (1905-1928), and two of his great-grandsons, Der Loewe (1944-1973) and Birkhahn (1945-1965).

Malformations of C6 and C7 showed significant variation between the three stallions, the study team reported. Dark Ronald, Der Loewe and Birkhahn were affected uni-laterally at C6 and C7, uni-laterally at C6 and bi-laterally at C6 and C7, respectively, with varying grades.

In order to evaluate whether these malformations were incidental, they took a random sample of 20 living German Warmblood horses, who are distant descendants of these stallions.

The sample consisted of ten controls and ten horses with known malformations of C6/C7.

Blood proportions of the historical sires in the modern Warmblood horses ranged from 0.10 to 6.25%.

The contribution to inbreeding in each individual horse of the selected horse group by those sires was expressed as a percentage of the total inbreeding coefficient and ranged from 0.01 to 17.96%, demonstrating their influence on the modern Warmblood.

The study team was able to describe the highly variable nature of the malformation of C6/C7 within this horse family, including historic and modern horses.

Additionally, they detected variations appearing in connection with malformations of the cervicothoracic junction that have not been described in the literature yet.

“This is the first time that the malformations of C6 and C7 have been described within a familial context, providing hints on inheritance in Thoroughbreds and Warmbloods,” they said.

It would be worthwhile to carry out further studies in a larger setting to gain more comprehensive insights into the inheritance of the malformation and the role of important ancestors. they said.

Studies with larger groups, a standardised and published radiographic and clinical examination protocol, and possibly genome-based analyses are needed to further elucidate the genetic background of the malformations and their significance for the modern horse population, especially when it comes to the effects on the wellness of the horse and their usability in sports.

Discussing their findings, the authors noted that all three stallions were heavily used in German horse breeding, with Der Loewe in the Hanoverian Warmblood and Birkhahn in the Thoroughbred, while Dark Ronald was used extensively in both Thoroughbred and German Warmbloods.

The study team noted from historical sources that Birkhahn was very successful in racing sports for about three years. Dark Ronald and Der Loewe had only very short racing histories, and were soon exclusively used for breeding purposes.

“Based on the scant information on performance data on all three stallions, we are not able to draw conclusions as to whether or not they were restricted in health and performance due to the malformation of C6 and C7.”

Dark Ronald was born in 1905 in Ireland and is considered one of the most important imported horses in German horse breeding. He is responsible for famous sire lines in Thoroughbred and Warmblood breeding.

The influence of Dark Ronald on the modern sport horse is also considerable.

Interestingly, his blood proportions are almost as high as the blood proportions from Der Loewe, even though he exerted his genetic influence on the German horse population three ancestral generations earlier.

“This again points to his extensive use in breeding and the fact that many of the ancestral lines of our modern sport horses can be traced back to Dark Ronald,” they said.

“In our modern horses, we could not find a difference between blood proportions of Dark Ronald in dams or sires. Dark Ronald caused a small part of the inbreeding seen in the Warmblood study group with a rather high number of pedigree paths.

Born in 1944, Der Loewe was a frequently used Thoroughbred stallion in Hanoverian Warmblood breeding after World War 2. His offspring were excellent performance horses. However, individual animals were difficult in terms of handling.

In a reference population of Hanoverian Warmblood horses born between 1980 and 2000, Der Loewe could be identified as a significant sire, having the greatest influence as a Thoroughbred with 2% blood proportions, and being the founder of a new sire line.

In the study group, Der Loewe presented with the highest average blood proportions among the three examined stallions.

“His blood proportions are significantly higher in horses with C6/C7 malformation, which should be interpreted with caution due to the small sample size.”

The share of inbreeding in the individual horses that is caused by Der Loewe (mean 1.78%) is markedly higher than the share from Dark Ronald (mean 0.74%) and Birkhahn (mean 0.10%).

Birkhahn was considered an exceptional talent in German racing during the period around World War 2. Poor management reportedly ended his career, before he was used for breeding in large German studs from 1951 to 1965.

In the Hanoverian reference population from 1980 to 2000, English Thoroughbreds were detected to contribute 35% of the genes.

“With Birkhahn being a popular Thoroughbred sire in Germany around that time, we can assume that he also influenced the German Warmblood population.

“The blood proportions in our modern horses calculated for Birkhahn were lower than those for the other two stallions. However, this proves that sires predominantly well known for their influence in Thoroughbred breeding can be traced in the German Warmblood population due to the frequent cross-breeding with Thoroughbred horses over many years.”

Birkhahn was responsible for only a small amount of inbreeding in one modern horse in the study, which classifies his influence as comparably small in the current data.

The authors noted that the Thoroughbred breed is a very large population with a closed studbook and a selection practice based on valuable pedigrees, with an increasing loss of global genetic diversity over the last five decades.

“The increasing mutational load that comes along with a high amount of inbreeding is one of the major factors causing inbreeding depression,” they said. Inbreeding can result in an increase in harmful variations within populations.

“We find only relatively small amounts of inbreeding caused by Dark Ronald and Der Loewe, and only one horse that is affected by inbreeding by Birkhahn,” they said.

“We do not believe that inbreeding caused by these historical sires is significant for the occurrence of a malformation of the cervicothoracic junction in the modern horse studied here.”

The authors noted that while Warmblood and Thoroughbred horses account for the largest proportion of horses examined and affected, the malformation occurs in other breeds as well.

“As the malformation of C7 was already described in historic articles hundreds of years ago, we assume that the malformation of the cervicothoracic junction is not exclusively a relevant condition in the modern horse.

“Further information about the occurrence of cervicothoracic malformations in other ancestral sires of our modern horse population could give further indications of the genetic background of the condition.”

The authors said their study is the first to provide hints on the inheritance of the trait, with much larger studies required to find stronger evidence and to identify the role of important ancestors.

The study team comprised Zimmermann, Christiane Pfarrer and Ottmar Distl, all with the University of Veterinary Medicine Hannover (Foundation); and Katharina Ros, with the Veterinary Clinic PZZ Döhle in Germany.

Zimmermann, E.; Ros, K.B.; Pfarrer, C.; Distl, O. Historic Horse Family Displaying Malformations of the Cervicothoracic Junction and Their Connection to Modern German Warmblood Horses. Animals 2023, 13, 3415. https://doi.org/10.3390/ani13213415

The study, published under a Creative Commons License, can be read here.