The dry conditions this past summer in Ontario have been perfect for the spread of a toxic weed that is especially dangerous to horses.

Hoary Alyssum (Berteroa incana) is a plant from the mustard family and in many areas, it's considered a weed and, in some locations, it's prohibited and outlawed.

Dr. Huw Llewellyn told Trot Insider that he's recently been involved in an unusual situation where horses at pasture have developed swollen legs and fevers. The horses are usually depressed and occasionally develop laminitis, with no signs of respiratory disease.

"These horses have been grazing pasture that contains the noxious weed Hoary Alyssum," stated Dr. Llewellyn. "The weed thrives in dry conditions as we have had this summer and it is still toxic if made into hay."

The weed is still toxic in hay for as many as nine months after baling, possibly longer. Interestingly, other herbivores like cattle and sheep are not affected by Hoary Alyssum; but it is extremely toxic to horses and could cause a pregnant mare to abort.

Dr. Llewellyn notes that affected horses respond quickly to Banamine, as long as they are removed from the affected pasture.