When I saw more than one horse getting sick on a ranch where I was boarding my beloved old gelding, I was naturally concerned. He had been visiting over the fence with this other herd and I was afraid there might be some sort of communicable disease afoot.
Can you pick out the toxic plant?
After several days it became apparent that there were only a couple affected horses and the problem did not seem to be spreading at all. My natural curiosity and tendency toward risk aversion prompted me to investigate what had made the horses sick in order to protect my aged gelding.
I researched the signs I had observed: isolation from the rest of the herd, depressed stance with head lowered to within an inch of the ground, labored breathing, weakness, long periods of laying prone and not rising even when approached by myself and my dog, wobbly gait which included tremors of the legs, shoulders, neck, and head.
It seemed to be a fairly obvious case of plant poisoning. What was not so obvious was the precise source. The herd in question was occupying a pasture my own horse and friends’ horses had occupied for several years prior. So what new evil botanic infiltration had occurred?
Upon searching the Internet I found a lot of information about Rayless Goldenrod and White Snake Root which caused problems with livestock in Arizona. But upon walking the pasture, I was unable to locate these plants, so it was back to the computer. I had picked a variety of the “weeds” that were growing in abundance out there—the same weeds I’d been seeing for several years—and brought them home with me so I could try to discover which, if any, might be toxic to horses.
Silverleaf Nightshade aka Purple Nightshade aka Horse Nettle
What I found, was that which we so often admired—”Oh, look at all the pretty purple flowers!” Those pretty purple flowers turned out to be Silverleaf Nightshade. What had never caused a problem before when we had four horses in the pasture, had caused serious problems in a herd of twenty. The plants had always been there. The equine population size is what had changed. The pasture was at that time being overgrazed and at least some of the horses were eating plants they would normally have avoided.
The Silverleaf Nightshade has beautifully scallop-edged elongated oval leaves in a sort of dusty-green color. The gorgeous purple flower opens to reveal bright yellow tubules (stamens). I found mention of some plants producing white flowers, but in this pasture, they were all purple.
The toxic agent is called solanine and is contained in both the leaves and the fruit (tiny round yellow berries) at all stages of maturity, but the ripe fruits are the most toxic. (The plants were not in fruit at the time I observed the signs of plant poisoning in the horses.)
The toxin affects horses, sheep, cattle and humans. Goats seem to be resistant. In controlled experiments performed by Texas A&M University, goats were not poisoned by the plants. These results would suggest that mixing goats in with other livestock may be beneficial to your overall pasture maintenance. There are herbicides that can be used on these plants, but this particular ranch was associated with an organic orchard so there were no chemicals used anywhere on the property.
Besides the signs that I observed, this toxin is also reported to produce excessive salivation, nasal discharge, nausea, abdominal pain, vomiting (obviously, not in horses), and diarrhea–sometimes with blood. I did not observe any of these symptoms in the horses I encountered. I am told the herd owner had checked the temperatures of the affected horses and found them to be normal.
The information I found about treatment through the Texas A&M University Agricultural extension was not very encouraging, Vets reported only “some success” administering pilocarpine or physostigmine once the animals were removed from the source of the toxins. It seems the main course of action is to remove the animals from the source, but not to allow the animals too much movement (best to retrieve them with a trailer), and provide good quality hay and fresh water.
The horses involved were pastured in a fairly isolated, remote area. This resulted in signs not being noticed until the poisoning was well advanced. Two horses died and one recovered after being moved to a different pasture. As far as I know, no other intervention was made, other than removing the animals from the source of the toxin. At the time the horses were moved, I do not believe the owner knew what had caused the illness, but simply moved them to a new pasture that offered more forage as he did routinely. A slow growing season onset that year probably contributed to the delay in moving the herd to better forage and caused the overgrazing and resulting poisoning.
The moral of this story is twofold. Know what is in your pasture, and take necessary steps to maintain adequate quality pasture forage. In cases of insufficient forage and no alternate pasturage, supplemental feeding of hay is necessary to maintain the health of the grazing herd.
Supplemental Note: Ivermectin Connection
In researching Silverleaf Nightshade’s toxic properties, I found information on studies conducted by Texas A&M University regarding a dangerous interaction between the dewormer ivermectin and Silverleaf Nightshade. While ivermectin is generally considered a safe and effective tool against equine intestinal parasites, researchers found a link between ingestion of Silverleaf Nightshade and ivermectin toxicity in horses.
According to the Texas Veterinary Medical Diagnostic Laboratory, “Silverleaf Nightshade (Solanum elaeagnifolium) appears to have the ability to alter the characteristics of the blood brain barrier and allow more ivermectin into the brain, where high levels can be fatal. It should be remembered that not all horses are affected and many can and do recover. Some do not, however.”
It may be advisable to pay closer attention to your pastures and your hay quality (as some exposures occur when Silverleaf Nightshade is baled in with cut hay) when using ivermectin in your regular deworming rotation.
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