Scoliosis, a curvature of the spine, is most often attributed to congenital vertebral malformations, possibly associated with malposition of the fetus in the uterus. However, researchers have linked cases of acquired scoliosis presented at Cornell University to migration of a parasite only recently reported in horses. Amy Johnson, DVM, a resident in Large Animal Internal Medicine at Cornell, discussed this issue at the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif.

The cases Johnson discussed included four affected horses and a Miniature Donkey that were stabled on different farms in New York state. These equids ranged in age from 6 months to 9 years, and each had acute onset scoliosis that developed suddenly. The C-shaped scoliosis occurred in the neck vertebrae of the horses, while the donkey's deformation occurred in the middle thoracic spine. Three of the horses were euthanized, while one horse and the donkey were discharged. The discharged horse, a 6-month-old filly, was still alive 2.5 years later, but showed no improvement.


Scoliosis of neck vertebrae

A 6-month-old Arabian filly with acute onset scoliosis. She has survived, but has not shown improvement.

None of the cases were associated with trauma, and veterinarians did not identify any evidence of trauma or fractures on radiographs. Once the scoliosis became evident, there appeared to be no further progression of the malformation. One horse that survived exhibited progressive (worsening) gait deficits related to nerve damage for more than two years. The horses also demonstrated neurologic problems consistent with stenosis (narrowing) of the cervical vertebrae (wobbler syndrome) and associated compression of the spinal cord.

Muscles on the convex side of the malformation lacked tone, while those on the concave side were tense. The absence of bony abnormalities of the vertebrae and the presence of different muscle tone on either side of the affected area of the spine pointed to a neuromuscular cause. Johnson proposed that the acute onset of the scoliosis might in part be due to weakness of the muscles on the convex side of the curve related to damage within the cord and/or near the nerve roots, with the spine deviating toward the unaffected side. Also, with advanced pathology (structural and functional deviations from the normal) in the spinal cord, reduction in spinal movement could cause remodeling and potential fibrosis of the joint capsules to a "fixed" position.

On post-mortem exam of one horse, researchers identified nematode larvae (specifically Parelaphostrongylus tenuis, often called brainworms or meningeal worms) and accompanying inflammatory changes within the spinal cord. Another five cases examined post-mortem had nearly identical lesions consistent with aberrant migration of a parasite. Such migration of P. tenuis has been recognized in small ruminants and camelids, which develop weakness in all four limbs and ataxia when infected. It is a recent discovery that horses are also susceptible to infection with this parasite.

Treatment with anti-parasitic drugs and anti-inflammatory medications did not ameliorate the scoliosis or associated clinical signs in these cases. Due to severe degenerative effects of the parasite's migration through the spinal cord, scoliosis is not likely to improve even if a treatment protocol manages to kill the parasitic larvae.

Other diseases that potentially could create scoliosis include trauma, equine protozoal myeloencephalitis, cancer, drug reactions, or an abscess. A veterinarian needs to rule these out by completing athorough history, examination, and diagnostic testing with ultrasound, radiography, cerebrospinal fluid tap, and computed tomography or MRI when necessary.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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