A team of University of Kentucky (UK) researchers recently evaluated whether horses consuming endophytic alkaloids experienced vasoconstriction (narrowing of the blood vessels), a phenomenon previously reported in cattle. As pard of the study, researchers also tested a method by which to detect the condition.

Tall fescue is a cool-season perennial grass prominent in the eastern portions of the United States that can be infected with an endophytic fungus known to produce chemicals called alkaloids.

“It is the alkaloid chemicals that have the detrimental health effects on cattle and horses, and it is the alkaloids such as ergovaline that we are interested in,” explained study author Karen McDowell, MS, PhD, a reproductive biology specialist at UK’s Gluck Equine Research Center.

These alkaloids, when consumed by mares in late pregnancy, can cause dystocia (difficult birth), thickened placenta, and reduced milk production. Previous studies have shown that endophyte alkaloid consumption causes vasoconstriction in cattle, but this phenomenon had not been examined in horses. To that end, the research team tested the hypothesis that if horses consumed fescue seed containing the endophytes, the horses would experience vasoconstriction measurable by Doppler ultrasonography. Doppler ultrasonography illustrates velocity (speed) and direction of blood flow, and during vasoconstriction, both are negatively affected.

The team employed 11 horses housed in drylots and randomly assigned them to one of three fescue seed treatments: ground endophyte-free seed (E-G), whole endophyte-infected seed (E+W), or ground endophyte-infected seed (E+G). The horses were adapted to the treatment, which the team mixed with a commercially available concentrate, and offered free-choice alfalfa cubes.

The team determined that the palmer artery (located on the back of the horse’s front leg near the fetlock joint) was the best vessel for detecting vasoconstriction in the horses.

They scanned each horse with Doppler ultrasound on four separate days both prior to and during the time the animals consumed the treatment diets. Based on the Doppler ultrasound results, the researchers observed that:

  • The horses fed E+G had a significant reduction in artery lumen diameter, circumference, and area—meaning the artery showed a greater amount of vasoconstriction, resulting in reduced blood flow—compared to those fed E-G. The researchers made this observation at the first scanning, which took place approximately 40 hours after the horses consumed the fescue seed, and continued for at least 15 hours after the final fescue seed feeding.
  • Horses fed the E+W diet tended to closely resemble the scans from the horses fed E-G diet.
  • Blood flow volume in the palmer artery was significantly reduced in the E+G compared to E-G during the treatment period.

The team concluded that Doppler ultrasonography can be a useful tool for assessing vasoconstriction in the horse’s palmar artery and that blood flow volume decreased significantly in horses consuming ground fescue seed containing the endophytes.

Future research is needed to determine the minimum amount of endophyte infected fescue that will cause vasoconstriction in horses, the team noted.

The study, "Vasoconstriction in horses caused by endophyte-infected tall fescue seed is detected with Doppler ultrasonography," was published in April in the Journal of Animal Science.

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More information on Gluck Equine Research Center and UK Ag Equine Programs.