The curly coat phenotype is a unique feature of the Bashkir Curly horse and is also found in some Missouri Fox Trotters. Two genes are associated with this trait, with a mutation in either one being capable of producing curly. The genes are KRT25 and SP6.  The mutation in KRT25 can also produce a condition where hair is sparse or lacking, known as hypotrichosis. This mutation exhibits incomplete dominance, meaning horses homozygous for the KRT25 mutation exhibit a more severe hypotrichosis than horse with only one copy of the mutant allele.  The SP6 mutation is a dominant mutation, so horses with one or two copies of the mutant allele exhibit the curly coat.

The cost is $25 for each test or $40 for both tests.

Reference: Thomer, A., Gottschalk, M., Christmann, A. et al. An epistatic effect of KRT25 on SP6 is involved in curly coat in horses. Sci Rep 8, 6374 (2018).
https://doi.org/10.1038/s41598-018-24865-3

Test Submission Form

Members of the Friesian Horse Association of North America should contact FHANA regarding testing procedures and submission of samples.

Dwarfism in the Friesian horse is characterized by abnormally short limbs and laxity of the flexor tendon resulting in an abnormal gait.  The rib cage may also exhibit malformations. The mode of inheritance is as a simple recessive. Carriers of the mutation appear normal, and should two carriers be bred, there is a one in four chance that a dwarf will be produced. A mutation has been discovered in the B4GALT7 gene that is causative for the disorder. The genetic test allows carriers to be identified so that production of dwarf foals can be avoided. Friesians or Friesian crossbreds should be tested prior to breeding to avoid breeding two carriers together.

The cost is $35 for each test.

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ACAN Dwarfism Mutations in the Miniature Horse

Four mutations in the ACAN gene have been found in Miniature Horses associated with a dwarf phenotype by John Eberth, MS, working in the lab of Dr. Ernie Bailey at the Gluck Center. It is important to test breeding stock for these mutations because at least one of these mutations is lethal in combination with any of the other mutations causing early pregnancy loss. Care must also be taken in breeding two horses together that are carriers for Dwarf mutations, as the presence of two mutations in any combination will lead to early abortion or live foals with a range of physical ailments associated with the dwarf phenotype. Some of these physical ailments seriously affect the health of the horse and include breathing problems, malformed mouths which lead to eating difficulties, and abnormal bone growth leading to chronic soundness issues.

Horses carrying only one of the mutations are normal in appearance, exhibiting the desired stature and proportions the Miniature Horse should possess.  There are four identified mutations, designated D1, D2, D3 and D4. The normal copy of the gene is designated as N.

The following table summarizes the effects of the various mutation combinations:

NOTE: There are additional mutations that cause dwarfism in Miniature Horses that are the focus of ongoing research. The tests offered detect the most common mutations, however cases of dwarfism have been reported from matings of horses that test clear for all four of the known mutations in the Aggrecan gene.

DWARFISM TEST SUBMISSION FORM

Dr. Udeni Balasuriya, in collaboration with Dr. Peter Timoney and Dr. Ernie Bailey at the Gluck Equine Research Center, have discovered a genetic basis for the likelihood of a stallion becoming a long term shedder/carrier of the Equine Arteritis Virus (EAV) following infection with the virus.Stallions possessing the Susceptible haplotype, consisting of four specific nucleotide changes in the CXCL16 gene, are significantly more likely to remain permanent carriers of the virus in their reproductive tract than are horses that possess the Resistant haplotype. Stallions homozygous for the Resistant CXCL16 haplotype, while initially shedding the virus in their semen following infection, were found in most cases to clear the virus from the reproductive tract within months following infection. Stallions possessing even one copy of the Susceptible haplotype (heterozygotes) are at greater risk for becoming permanent shedders of EAV.

Since the common course of action following the diagnosis of a stallion as an EAV shedder is castration, this test can identify which horses may indeed become clear of the virus, thus preventing the loss of a valuable breeding animal. In addition, the test indicates which horses have the Susceptible haplotype and therefore are at higher risk for becoming shedders if infected with EAV. In these cases management practices can be implemented such as vaccination and limiting opportunities to become infected.

This work was funded by a grant from USDA-NIFA

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Foal Immunodeficiency Syndrome (FIS)  is an autosomal recessive disorder, meaning the affected foal must receive a copy of the mutation from each parent. The mutation occurs in the SLC5A3 gene and results in a deficiency of B lymphocytes. Affected foals are unable to produce antibodies on their own and succumb to infection as the maternal antibodies supplied by the mare’s colostrums are lost at 3-6 weeks of age. Foals either die or fail to thrive to the point that they are humanely destroyed.

About 30% of Fell Ponies and 18% of Dales Ponies are carriers. The syndrome is also found in Gypsy horses, most likely due to crosses with Fell Ponies. Since carriers of the mutation show no outward signs, it is important to determine if a horse is a carrier for this mutation prior to breeding.

The cost is $45 for each test. 

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Members of the Friesian Horse Association of North America should contact FHANA regarding testing procedures and submission of samples.

Hydrocephalus in the Friesian horse is a developmental defect of the foal characterized by an accumulation of cerebrospinal fluid in the ventricular system of the brain. Foals are born with grossly enlarged foreheads and are often stillborn or require euthanasia soon after birth. A hydrocephalic foal also presents a potential dystocia (difficult birth) for the mare. While hydrocephaly may have other causes such as infection or trauma, a mutation associated with this  condition has been identified in the Friesian horse in the B3GALNT2 gene. Friesians or Friesian crossbreds should be tested prior to breeding to avoid production of an affected foal. The mutation is recessive and the genetic test allows breeders to avoid breeding two carriers together. Should two carriers be bred, the probability is that one out of four foals will be hydrocephalic.

The cost is $35 for each test.

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Hyperkalemic Periodic Paralysis was first described in American Quarter Horses tracing back to the popular sire Impressive. HYPP is a defect in a sodium channel gene (SCN4A) that alters the transport of potassium in the muscle. Signs of HYPP include muscle tremors, weakness and collapse and in some cases death. The mutation is semi-dominant, meaning that heterozygotes are less symptomatic, or even asymptomatic, in comparison to homozygotes.

WHY TEST: Impressive was a popular Quarter Horse sire whose offspring were particularly successful in halter horse competition. Breeds that include Quarter Horse bloodlines, such as Paint Horses and Appaloosas, may also include Impressive in their pedigrees. Heterozygotes can be managed with diets low in potassium and other measures. Breeding two carriers is strongly not recommended as resulting homozygotes are likely to be severely affected and die prematurely. Therefore horses of susceptible lineages should be tested before mating to avoid producing homozygous offspring.

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Junctional Epidermolysis Bullosa (JEB1 and JEB2) JEB is a lethal skin condition that occurs in American Saddlebreds (JEB2) and the Belgian draft horse and French draft breeds (JEB1). The laminin protein that anchors the epidermis is defective in both these forms of JEB, but the causative mutation in each of these breeds affects a different sub-chain of the molecule.  It has a recessive mode of inheritance, therefore the foal must inherit the mutation from both parents. Carriers with one copy of the mutation are unaffected. Carriers can be safely bred as long as they are not bred to another carrier.

Myotonia An inherited neuromuscular disorder has been identified in New Forest ponies (Wijnberg ID et al., 2011) which is caused by an autosomal recessive mutation in the CLCN1 gene. Carriers of the mutation appear normal, but when two carriers are mated there is a 25% chance that an affected foal will be produced. Affected foals have abnormal muscle activity and spend more time than usual laying down and also exhibit gait abnormalities. In cooperation with Dr. Cord Drögemüller in Switzerland, UK Genetic Testing at Gluck is offering this test. Please see the submission form for instructions on submitting a sample.

Overo Lethal White Syndrome (OLWS) This is so named because affected foals are all white or near white at birth and have improperly formed rectal tissue resulting in death or requiring euthanasia at a few days of age. It is caused by a mutation that in the heterozygous form results in a frame overo pattern. Therefore the color pattern is very desirable for Paint or Pinto horse breeders. There are no health effects for carriers. However if two carriers are bred 25% of the foals on average will be homozygous for the mutation and will be affected. Although most common in Paint horses and Quarter horses, the mutation has been found in Tennessee Walking Horses as well.

Polysaccharide Storage Myopathy 1 (PSSM1) A mutation in the glycogen synthase gene (GYS1) causes abnormal accumulation of glycogen in the muscle, leading to symptoms that include the condition commonly known as “tying up”.  The mutation can be found in a number of breeds, including Quarter Horses and draft breeds. There is another form of PSSM known as PSSM2.  The mutation for PSSM2 is not known and this condition must be diagnosed by muscle biopsy. The University of Kentucky holds a license agreement with the American Quarter Horse Association which stipulates that the PSSM1 test must be performed as part of a disease panel, therefore at least one other test must be ordered in addition to the PSSM1 test.

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Warmblood Fragile Foal Syndrome is an autosomal recessive disorder resulting in extremely fragile skin and abnormal laxity in the joints occurring, as the name suggests, primarily in Warmblood horses. The mutation occurs in the LH1 gene (also known as PLOD1). A foal must inherit the mutation from both parents to be affected.

A study of 500 Warmblood horses in Germany found that 9.5% of the horses were carriers or approximately 1 in 10 (Monthoux, et al. 2015)

The test allows the identification of carriers so that breeding two carriers together can be avoided. Animals carrying one copy of the mutation are unaffected.

The test can be performed using hair samples. Cost is $40 per sample. 

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Reference: Monthoux, C., de Brot, S., Jackson, M. et al. Skin malformations in a neonatal foal tested homozygous positive for Warmblood Fragile Foal Syndrome. BMC Vet Res 11, 12 (2015). https://doi.org/10.1186/s12917-015-0318-8

Distichiasis is an abnormality of eyelash growth that results in the potential for eyelashes to irritate and abrade the cornea of the eye.  A deletion on chromosome 13 is associated with Distichiasis in the Friesian horse breed. Horses homozygous for this mutation are at risk for having distichiasis and will pass the mutation to 100% of their offspring. Horses with only one copy of the mutation are not likely to have distichiasis.  Because aberrant eyelashes may not be present continuously, owners should note any signs of eye discomfort and consult a veterinary ophthalmologist. To avoid producing affected foals, it is recommended that at-risk individuals only be bred to horses that do not possess this marker.

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