Pasture-Associated Stringhalt: Nutritional Considerations

Pasture-associated stringhalt is thought to be caused by a phytotoxin contained in Hypochoeris radicata, known commonly as flatweed, false dandelion, or cat’s ear, a low-lying plant with a yellow, dandelion-like flower at the end of a long stem. The plant is found throughout Australia, North America, and other continents. In part because of its long taproot, it is incredibly hardy, thriving in conditions that other flora find too challenging, including drought-affected or fire-ravaged pastures.

Under the right conditions, flatweed swiftly overruns freshly disturbed environments, as evidenced by its profusion following the eruption of Mount St. Helens in 1980, and can crowd out more palatable and productive forage species when it invades overgrazed pasture.^1,2

Once ingested, the toxin predominantly affects the nervous system. The primary clinical sign of horses with pasture-associated stringhalt is bilateral, exaggerated, and prolonged flexion of the hind limbs, often associated with peripheral neuropathy. Affected horses show varying degrees of hind limb hyperflexion; some fold their hind limbs so tightly that the fetlocks touch the abdomen. The severity of hyperflexion might decrease after the initial few steps. Some horses refuse to back. Gait abnormalities are also common, though are individual-dependent, with some horses exhibiting a hopping action whereby both hind limbs are elevated from the ground simultaneously. In extreme cases, horses are unable to rise.

Affected horses nearly always display muscle atrophy, especially of the long and lateral digital extensor muscles. These muscles are situated in the gaskin and, in normal horses, are used to flex the hock and extend the hoof.³ Atrophy of other muscles has been documented, including the infraspinatus, supraspinatus, biceps, and triceps of the forequarters, and the quadriceps of the hindquarters.⁴

Laryngeal hemiplegia has been reported regularly in horses diagnosed with pasture-associated stringhalt. This respiratory condition limits airflow into the lungs, causing horses to make a characteristic roaring noise during exertion. Horses with this condition are often called “roarers.”

Veterinary advice should be sought immediately if pasture-associated stringhalt is suspected. Nutritional help for horses affected with pasture-associated stringhalt begins by removing them from pastures that contain flatweed. Most horses recover over a period of 6-18 months, though mildly affected horses may recover more quickly. On the opposite end of the spectrum, severely affected horses may have protracted convalescence lasting more than two years. While almost all horses recover fully, regaining normal use of their hind limbs, a few horses do not, possibly as a result of prolonged atrophy or fibrosis. Some horses may have to be euthanized.⁵

Nutritional support during convalescence is important. Antioxidants are often prescribed for horses with pasture-associated stringhalt, presumably to ameliorate oxidative injury to the distal axons of the long nerves.^1,4 Among the antioxidants mentioned is natural vitamin E, with a suggested dose of 6,000 IU for at least seven days based on those levels recommended for other neurological disorders.⁶

A review of vitamin E requirements and supplementation of the horse found natural vitamin E supplementation improves immune function, provides antioxidant effects, and assists in maintenance of normal nerve tissue, thereby a potentially important adjunct therapy for horses with neuromuscular disease. Studies confirm National Research Council (NRC) recommendations for vitamin E intake at 5,000-7,000 IU per day for equine motor neuron disease. A previous study found 40% of horses treated with vitamin E improved within six weeks of supplementation. NRC guidelines also recommend feeding no more than 10,000 IU of vitamin E daily, as there is no additional benefit with increasing dosage.⁷

When choosing a vitamin E supplement for horses with pasture-associated stringhalt, select a natural-source, water-soluble product. One study measured plasma levels of alpha-tocopherol following oral supplementation of 5,000 IU of vitamin E in the form of synthetic vitamin E (dl-α-tocopherol acetate), natural-source vitamin E (d-α-tocopherol acetate), or natural vitamin E (d-α-tocopherol) in a micellized water-soluble form for 56 days. Blood samples were collected prior to supplementation and at weekly intervals. Horses given natural-source vitamin E had significantly higher levels of plasma tocopherol compared to the synthetic group. Those supplemented with micellized natural vitamin E had the highest tocopherol plasma levels compared to the other two groups.⁸

One research-proven form of natural vitamin E is KER Nano-E, a water-soluble supplement superior to other forms of vitamin E.

Supplementation with thiamine might also provide some relief of symptoms. Thiamine is an essential cofactor in neuronal cell metabolism, neurotransmitter production, and myelin synthesis, and has been used in the treatment of human peripheral neuropathies.

Reports of thiamine use in PSH treatment are common, but its success has been difficult to measure. Timelines of improvement differ among patients, and thiamine is usually administered with another medication. One case report from Virginia, for example, described that thiamine supplementation in combination with the anticonvulsant phenytoin helped horses with pasture-associated stringhalt.⁴ In another instance, eight Argentine horses diagnosed with pasture-associated stringhalt were treated with thiamine and the nonsteroidal anti-inflammatory drug phenylbutazone. While six of the horses survived after hospitalization, the authors concluded that there was no way to gauge absolutely the effectiveness of thiamine because these survivors, like most affected horses, began to improve once removed from the pasture.⁹

For horses diagnosed with pasture-associated stringhalt, a high-quality thiamine supplement is recommended. Because horses receive the majority of their daily thiamine requirement from pasture, it is essential that horses removed from pasture for recovery be provided free-choice hay and thiamine supplementation. While no dose recommendation has been established for thiamine supplementation to affected horses, KER B-Quiet offers high levels of thiamine (1,000 mg of thiamine) per 30-g serving.

Literature Cited

 ¹Schoenfelder, A.C., J.G. Bishop, H.M. Martinson, and W.F. Fagan. 2010. Resource use efficiency and community effects of invasive Hypochaeris radicata (Asteraceae) during primary succession. American Journal of Botany 97:1772-1779.

 ²DiTomaso, J.M., G.B. Kyser, S.R. Oneto, R.G. Wilson, S.B. Orloff, L.W. Anderson, S.D. Wright, J.A. Roncoroni, T.L. Miller, T.S. Prather, C. Ransom, K.G. Beck, C. Duncan, K.A. Wilson, and J.J. Mann. 2013. Weed control in natural areas in the Western United States. Davis, California, USA: Weed Research and Information Center, University of California.

 ³Sisson, S., and J.D. Grossman. 1938. The Anatomy of Domestic Animals. 3rd rev. ed. W.B. Saunders Co., Philadelphia, p. 335-337.

 ⁴Gardner, S.Y., A.G. Cook, B.S. Jortner, B.V. Troan, N.J.H. Sharp, N.B. Campbell, and C.F. Brownie. 2005. Stringhalt associated with a pasture infested with Hypochoeris radicata. Equine Veterinary Education 17:118-122.

 ⁵El-Hage, C.M., P.J. Huntington, I.G. Mayhew, R.F. Slocombe, and B.S. Tennett-Brown. 2017. Pasture-associated stringhalt: Contemporary appraisal of an enigmatic syndrome. Equine Veterinary Education.

 ⁶Armengou, L., S. Añor, F. Climent, G.D. Shelton, and L. Monreal. 2010.  Antemortem diagnosis of a distal axonopathy causing severe stringhalt in a horse. Journal of Veterinary Internal Medicine 24:220-223.

 ⁷Valberg, S., and C. Finno. 2018. Vitamin E nutrition in healthy and neurologic horses. In: Proc. Kentucky Equine Research Conference, p. 93-103.

 ⁸Pagan, J.D., D. Nash, E. Kane, and P.J. Huntington. 2005. Form and source of tocopherol affects vitamin E status in Thoroughbred horses. Pferdheikunde 21:101.

 ⁹Oliver, O.J., and J.C. Suárez. 2016. An outbreak of Australian stringhalt associated with skin lesions in Colombia. Revista Colombiana de Ciencias Pecuaries 29:226-234.