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Gene or Region: SLC5A3
Reference Variant: C
Mutant Variant: T
Affected Breeds: Dales Pony and Fell Pony
Research Confidence: High confidence - Findings reproduced in multiple studies
Explanation of Results: fis/fis = homozygous for Foal Immunodeficiency Syndrome, trait expressed fis/n = heterozygous for Foal Immunodeficiency Syndrome, carrier n/n = no variant detected
Foal Immunodeficiency Syndrome (FIS) is a failure in the development of the adaptive immune system. At 3-6 weeks of age, once the maternal antibodies begin to degrade, foals exhibit signs of anemia, diarrhea, and pneumonia. As these foals fail to respond to treatment for infections, they are humanely euthanized at a young age. FIS is an autosomal recessive condition, thus a horse must inherit two copies of the allele (fis/fis) to show any signs. Horses with only one copy of the allele (fis/n) are known as carriers due to their ability to produce affected foals.
SLC5A3 is a transporter involved in the response to osmotic stress. There are no known natural variations associated with diseases in any species, though “knock out” mice (which have had the gene experimentally removed) die shortly after birth from hypoventilation. This mutation is a single base substitution that alters an amino acid, likely changing the function of the encoded protein.
Fox-Clipsham LY et al., “Identification of a mutation associated with fatal Foal Immunodeficiency Syndrome in the Fell and Dales pony.” (2011) PLoS Genet. 7: e1002133. PMID: 21750681
Fox-Clipsham LY et al., “Population screening of endangered horse breeds for the foal immunodeficiency syndrome mutation.” (2011) Vet Rec. 169: 655. PMID: 22016514
Tallmadge RL et al., “Fell Pony syndrome: characterization of developmental hematopoiesis failure and associated gene expression profiles.” (2012) Clin Vaccine Immunol. 19: 1054-64. PMID: 22593239
Carter SD et al., “Foal immunodeficiency syndrome: carrier testing has markedly reduced disease incidence.” (2013) Vet Rec. 172: 398. PMID: 23486505
Lordosis (L1, L2, L3, L4), also known as "Swayback", is a curvature or dip in the spine that is often seen in older horses. However, in the American Saddlebred, this condition also affects younger horses. These animals do not appear to experience pain from their condition and are still able to be used under saddle.
Malignant hyperthermia (MH) is a muscle disorder in which anesthesia, stress, or extreme exercise trigger a hyperthermic state. Symptoms include high temperature, increased heart rate, high blood pressure, sweating, acidosis, and muscle rigidity. If symptoms are not immediately resolved, death is likely to occur.
Myosin-Heavy Chain Myopathy (MY; previously "IMM") is a genetic muscle disease found most commonly in stock-type horses that can result in two different presentations, Immune-Mediated Myositis (IMM) and Nonexertional Rhabdomyolysis “tying up,” which are both characterized by muscle damage or loss. Both presentations are associated with the same genetic variant. Horses with MYHM may exhibit one or both presentations at different times in their lives, although some horses carrying the variant might not display any symptoms at all.
Myotonia (MYT) is a rare disorder involving a slowed relaxation of muscles after contraction. The most well-known example of myotonia is “fainting goats,” a breed that is characterized by sudden rigidy and/or falling over when startled. In the single documented horse, this resulted in a protruding third eyelid when excited, as well as problems with muscle stiffness.
