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Concept: Icelandic horse


Many genes are known to have an influence on conformation and performance traits; however, the role of one gene, Myostatin (MSTN), has been highlighted in recent studies on horses. Myostatin acts as a repressor in the development and regulation of differentiation and proliferative growth of skeletal muscle. Several studies have examined the link between MSTN, conformation and performance in racing breeds, but no studies have investigated the relationship in Icelandic horses. Icelandic horses, a highly unique breed, are known both for their robust and compact conformation as well as their additional gaits tölt and pace. Three SNPs (g.65868604G>T [PR8604], g.66493737C>T [PR3737] and g.66495826A>G [PR5826]) flanking or within equine MSTN were genotyped in 195 Icelandic horses. The SNPs and haplotypes were analyzed for association with official estimated breeding values (EBV) for conformation traits (n=11) and gaits (n=5). The EBV for neck, withers and shoulders was significantly associated with both PR8604 and PR3737 (p<0.05). PR8604 was also associated with EBV for total conformation (p=0.05). These associations were all supported by the haplotype analysis. However, while SNP PR5826 showed a significant association with EBVs for leg stance and hooves (p<0.05), haplotype analyses for these traits failed to fully support these associations. This study demonstrates the possible role of MSTN on both the form and function of horses from non-racing breeds. Further analysis of Icelandic horses as well as other non-racing breeds would be beneficial and likely help to completely understand the influence of MSTN on conformation and performance in horses.

Concepts: DNA, Gene, Gene expression, Horse, Haplotype, Icelandic horse, Ambling, Horse gait


Horseback riding is the most fundamental use of domestic horses and has had a huge influence on the development of human societies for millennia. Over time, riding techniques and the style of riding improved. Therefore, horses with the ability to perform comfortable gaits (e.g. ambling or pacing), so-called ‘gaited’ horses, have been highly valued by humans, especially for long distance travel. Recently, the causative mutation for gaitedness in horses has been linked to a substitution causing a premature stop codon in the DMRT3 gene (DMRT3_Ser301STOP) [1]. In mice, Dmrt3 is expressed in spinal cord interneurons and plays an important role in the development of limb movement coordination [1]. Genotyping the position in 4396 modern horses from 141 breeds revealed that nowadays the mutated allele is distributed worldwide with an especially high frequency in gaited horses and breeds used for harness racing [2]. Here, we examine historic horse remains for the DMRT3 SNP, tracking the origin of gaitedness to Medieval England between 850 and 900 AD. The presence of the corresponding allele in Icelandic horses (9(th)-11(th) century) strongly suggests that ambling horses were brought from the British Isles to Iceland by Norse people. Considering the high frequency of the ambling allele in early Icelandic horses, we believe that Norse settlers selected for this comfortable mode of horse riding soon after arrival. The absence of the allele in samples from continental Europe (including Scandinavia) at this time implies that ambling horses may have spread from Iceland and maybe also the British Isles across the continent at a later date.

Concepts: United Kingdom, Europe, Horse, Icelandic horse, Iceland, Ambling, Trot, Horse gait


Insect bite hypersensitivity (IBH) is an allergic dermatitis of horses caused by bites of Culicoides spp. IBH does not occur in Iceland because of the absence of Culicoides, but the prevalence is high in horses imported from Iceland to environments where Culicoides are present.

Concepts: Immune system, Asthma, Hypersensitivity, Immunoglobulin E, Allergy, Contact dermatitis, Bite, Icelandic horse


The Icelandic horse is a breed known mainly for its ability to perform the ambling four-beat gait ‘tölt’ and the lateral two-beat gait pace. The natural ability of the breed to perform these alternative gaits is highly desired by breeders. Therefore, the discovery that a nonsense mutation (C>A) in the DMRT3 gene was the main genetic factor for horses' ability to perform gaits in addition to walk, trot and canter was of great interest. Although several studies have demonstrated that homozygosity for the DMRT3 mutation is important for the ability to pace, only about 70% of the homozygous mutant (AA) Icelandic horses are reported to pace. The aim of the study was to genetically compare four- and five-gaited (i.e. horses with and without the ability to pace) AA Icelandic horses by performing a genome-wide association (GWA) analysis. All horses (n = 55) were genotyped on the 670K Axiom Equine Genotyping Array, and a GWA analysis was performed using the genabel package in r. No SNP demonstrated genome-wide significance, implying that the ability to pace goes beyond the presence of a single gene variant. Despite its limitations, the current study provides additional information regarding the genetic complexity of pacing ability in horses. However, to fully understand the genetic differences between four- and five-gaited AA horses, additional studies with larger sample materials and consistent phenotyping are needed.

Concepts: Gene, Genetics, Mutation, Evolution, Horse, Icelandic horse, Ambling, Horse gait


This study investigated the effects of rider weight in the BW ratio (BWR) range common for Icelandic horses (20% to 35%), on stride parameters in tölt in Icelandic horses. The kinematics of eight experienced Icelandic school horses were measured during an incremental exercise test using a high-speed camera (300 frames/s). Each horse performed five phases (642 m each) in tölt at a BWR between rider (including saddle) and horse starting at 20% (BWR20) and increasing to 25% (BWR25), 30% (BWR30), 35% (BWR35) and finally 20% (BWR20b) was repeated. One professional rider rode all horses and weight (lead) was added to saddle and rider as needed. For each phase, eight strides at speed of 5.5 m/s were analyzed for stride duration, stride frequency, stride length, duty factor (DF), lateral advanced placement, lateral advanced liftoff, unipedal support (UPS), bipedal support (BPS) and height of front leg action. Stride length became shorter (Y=2.73-0.004x; P0.05). In conclusion, increased BWR decreased stride length and increased DF proportionally to the same extent in all limbs, whereas BPS increased at the expense of decreased UPS. These changes can be expected to decrease tölt quality when subjectively evaluated according to the breeding goals for the Icelandic horse. However, beat, symmetry and height of front leg lifting were not affected by BWR.

Concepts: Effect, Horse, Height, Icelandic horse, Back, Horse racing, Ambling, Horse gait


This study examined the effect of increasing BW ratio (BWR) between rider and horse, in the BWR range common for Icelandic horses (20% to 35%), on heart rate (HR), plasma lactate concentration (Lac), BWR at Lac 4 mmol/l (W4), breathing frequency (BF), rectal temperature (RT) and hematocrit (Hct) in Icelandic horses. In total, eight experienced school-horses were used in an incremental exercise test performed outdoors on an oval riding track and one rider rode all horses. The exercise test consisted of five phases (each 642 m) in tölt, a four-beat symmetrical gait, at a speed of 5.4±0.1 m/s (mean±SD), where BWR between rider (including saddle) and horse started at 20% (BWR20), was increased to 25% (BWR25), 30% (BWR30), and 35% (BWR35) and finally decreased to 20% (BWR20b). Between phases, the horses were stopped (~5.5 min) to add lead weights to specially adjusted saddle bags and a vest on the rider. Heart rate was measured during warm-up, the exercise test and after 5, 15 and 30 min of recovery and blood samples were taken and BF recorded at rest, and at end of each of these aforementioned occasions. Rectal temperature was measured at rest, at end of the exercise test and after a 30-min recovery period. Body size and body condition score (BCS) were registered and a clinical examination performed on the day before the test and for 2 days after. Heart rate and BF increased linearly (P0.05), but negative correlations (P<0.05) existed between body size measurements and Hct. While HR, Hct and BF recovered to values at rest within 30 min, Lac and RT did not. All horses had no clinical remarks on palpation and at walk 1 and 2 days after the test. In conclusion, increasing BWR from 20% to 35% resulted in increased HR, Lac, RT and BF responses in the test group of experienced adult Icelandic riding horses. The horses mainly worked aerobically until BWR reached 22.7%, but considerable individual differences (17.0% to 27.5%) existed that were not linked to horse size, but to back BCS.

Concepts: Physiology, Cardiology, Measurement, Pulse, Horse, Rectum, Icelandic horse, Ambling


Insect bite hypersensitivity is an IgE-mediated dermatitis of horses initiated by bites of midges of the genus Culicoides. Culicoides spp. are not indigenous to Iceland and the prevalence of insect bite hypersensitivity is much higher in horses born in Iceland and exported as compared to Icelandic horses born in a Culicoides rich environment. Immunotherapy is therefore needed.

Concepts: Immune system, Immunology, Scandinavia, Bite, Icelandic horse


This study investigated the variation in plasma insulin concentration (PIC) in a group of Icelandic horses in training, considered to be healthy and examined possible relationships between PIC and gender, age, body size, body condition score and management factors such as feed allowance and subjective level of fitness.

Concepts: Evolution, Icelandic horse, Norwegian language


The role of equid gammaherpesviruses on ocular surface diseases has been disputed, since the diagnosis is usually based on clinical symptoms and detection of viral DNA from samples obtained from live animals.

Concepts: Disease, Medical terms, Symptom, Horse, Equidae, Icelandic horse


OBJECTIVE To compare gait mechanics and limb loading in Icelandic horses tölting and trotting at equal speeds and estimate their impact on orthopedic health. ANIMALS 12 orthopedically normal Icelandic horses. PROCEDURES Kinetic and kinematic gait variables were simultaneously recorded as each horse was ridden at a tölt and trot on an instrumented treadmill at 3.4 m/s and 3.9 m/s. Differences between gaits were tested via 1-factor repeated-measures ANOVA. RESULTS Horses had a higher stride rate and lower stride impulses at a tölt than at a trot. For forelimbs at a tölt, shorter relative stance duration resulted in higher peak vertical force (Fzpeak). Conversely, for hind limbs, longer relative stance duration resulted in lower Fzpeak. The higher head-neck position at a tölt versus trot caused no weight shift to the hind limbs, but a higher forehoof flight arc and lower proretraction movement were identified. Stance durations for forelimbs were briefer than for hind limbs at a tölt, and the inverse was observed at a trot. Minimal height of the horse’s trunk at the point of Fzpeak of the respective limb suggested a spring-like mechanism for all limbs at a tölt. Hind limb measurements revealed no evidence of increased collection. Stride-to-stride limb timing varied more at a tölt than at a trot. At a trot, horses had brief or no suspension phases and a slightly 4-beated footfall rhythm was common. Post hoc energetic estimations revealed that tölting at the measured speeds was less advantageous than trotting. CONCLUSIONS AND CLINICAL RELEVANCE High forelimb action in Icelandic horses and higher head-neck position at a tölt were associated with more restricted limb proretraction, higher Fzpeak, and faster force onset than at a trot. The impact of these differences on orthopedic health needs to be investigated more in detail.

Concepts: Horse, Classical mechanics, Velocity, Icelandic horse, Ambling, Trot, Combined driving, Horse gait