Myotonia Discovered in New Forest Ponies

Researchers at the Utrecht University Equine Clinic, in the Netherlands, have discovered a mutation in the DNA of the New Forest pony that can cause the muscular disease myotonia. Congenital myotonia has never before been diagnosed in New Forest ponies.

Myotonia is a hereditary muscular disease that had only been known to occur in goats and canines until this discovery. The disorder is characterized by increased muscular irritability and contraction with decreased power of relaxation. In goats and some dog breeds, a disruption in the development of the ion channels results in involuntary muscular contractions that can become aggravated by excitement (essentially, when myotonic animals are startled their muscles lock up suddenly, and they fall over and lie stiff for a few seconds). A specialized research laboratory at the University of Bern, Switzerland, has been asked to conduct exhaustive genetic research into this phenomena. In New Forest ponies, a DNA defect seems to disturb the development of a protein, resulting in the symptoms of myotonia in foals.

The disorder was first seen in New Forest ponies in 2009 when Inge Wijnberg, DVM, PhD, Dipl. ECEIM, a specialist in equine internal diseases at the University Equine Clinic, noticed a foal that fell down after exercising in the pasture.

"With the help of the foal's breeder and other respected breeders, we were able to conduct a detailed genetic diagnosis, including studies of the foal's parents," she explained. "We were able to determine that the disease only manifests itself through clinical symptoms if both parents pass on the defective gene to the foal."

Researchers are currently studying which stallion lines carry the defective gene in order to develop an approach to prevent the spread of the disease among New Forest ponies.

"Ideally, we would like to set up an extensive program to prevent the breeding of foals with congenital myotonia," said Wijnberg. Foals with myotonia cannot be used as athletic or pleasure horses, and even at rest they can display clinical signs. At the moment, there are no European guidelines and studbooks are free to decide whether or not to breed animals with congenital defects.

The study, "A missense mutation in the skeletal muscle chloride channel 1 (CLCN1) as candidate causal mutation for congenital myotonia in a New Forest pony," was published in the April issue of Neuromuscular Disorders. The abstract is available online.

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Utrecht University

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