It’s rare for horses to eat toxic vegetation, but the ingestion of certain plants, seeds and leaves can prove potentially catastrophic for horse health. Keith Chandler MRCVS explains

While ragwort has long been regarded as an equine enemy, the fruit and seeds of some common British trees, such as the sycamore, have recently reminded us of their deadly potential.

In general, horses will avoid plants that are likely to cause illness. But if hungry or on bare pastures, they are more likely to eat harmful vegetation.

Plants that have toxic effects fall into 2 groups — those that cause minor irritation or illness, which is easily treatable, and those that can bring about serious illness or fatality.

It isn’t always easy to pinpoint the cause of the toxicity, so we have to treat the horse for the clinical disease he is showing. This is partly because the toxins are often rapidly metabolised and are no longer detectable. In other cases, the disease can occur long after exposure occurred.

What’s your poison?

Common dangers include: 

Yew. Often found in ornamental gardens or churchyards, yew trees are renowned for living for many hundreds of years. The leaves and bark are highly poisonous to animals, even when wilted or dried up. Horses appear to be most sensitive to their toxic effects.

Because these effects are so rapid, there are sometimes no signs of disease other than sudden death. Affected horses develop a high heart rate, however, along with muscle tremors and convulsions, before collapsing with heart failure.

Care should be taken when horses are grazed adjacent to old churchyards if yew is present. If in any doubt, fence off a gap between the churchyard wall and the pasture in order to prevent accidental ingestion. 

Bog Asphodel and St John’s Wort. These plants can be found growing wild in many pastures and cause a skin disease called photosensitisation if ingested. This results in what can look like severe sunburn on the white areas of the face and occasionally on the limbs and body.

The skin becomes very sore, inflamed and scabby, can ooze serum and even start to bleed. Affected areas, particularly those on the limbs, can become swollen.

Because photosensitisation can also be caused by liver disease, a liver screen is often recommended to rule this out.

Photosensitisation is usually treated successfully with a combination of oral and topical corticosteroids.

The horse should be removed from the offending grazing and direct sunlight until the condition settles. 

Buttercups. Horses will eat buttercups, despite their bitter taste, particularly if there is little else on the pasture. Buttercups contain a powerful irritant that can cause mouth ulcers and increased salivation. The ulcers are usually found just inside the lips or along the rim of the tongue.

While a horse might resent examination, particularly if his mouth is sore, the ulcers are usually easy to spot and heal quickly. Treatment is not necessary, other than to remove the horse from the pasture for a short while. 

Acorns. Last year was a so-called “mast year”, when woodland trees such as the oak produced a bumper crop of fruit. Although climatic conditions are often blamed, it is not fully understood why these random mast years happen. Press reports highlighted the increase in the number of acorns on the ground and the multiple deaths of feral New Forest ponies.

Acorns are highly toxic to horses. Once ingested, they can cause colic, diarrhoea and even kidney failure.

These types of poisonings can occur as outbreaks, affecting many numbers of horses grazing the same pasture. A post-mortem examination of a fatality is often the best way of confirming a diagnosis. However, the time of year, access to oak trees and the symptoms — particularly if there are partly digested acorns in the dung — provide a strong suspicion of the cause. 

Ragwort. This well-known toxic plant can cause fatal liver disease, although a one-off exposure to a small amount is unlikely to have any significant effect.

Being able to define the exact cause of liver disease is a veterinary challenge. Even on post-mortem examination, pathologists are often at a loss to explain exactly why liver failure occurred.

What we do know is that horses have to eat substantial quantities of ragwort to become ill and are most likely to do this by ingesting small amounts in forage over time. While the toxin within ragwort does not build up in the body, its effect is cumulative and the liver fails in its normal capacity to heal itself.

The safest route is not to allow your horse any exposure to ragwort. The plant is also poisonous to humans and its toxin can be absorbed through the skin, so wear gloves when pulling it from the ground.

Sycamore. The Equine Veterinary Journal has recently published work on toxins found in Acer trees, which include sycamores. This is now recognised as the likely cause of atypical myopathy (AM) — a serious and often fatal disease.

The toxin, hypoglycin A, is not always present in every seed, or in seeds from every tree. This makes it difficult to predict whether a particular horse will become ill when exposed.

AM usually affects young horses and those in poorer body condition, causing widespread muscle destruction and pain that leads to collapse and kidney failure in many cases.

Managing affected horses can be very challenging. We only expect around a quarter to survive, even with intensive treatment and the best veterinary care.

Preventative measures include fencing off areas where sycamore seeds fall and providing hard feed and hay when pasture is bare. Always avoid feeding hay from the ground near sycamores and reduce access to affected pasture when grazing is poor or during the critical autumn and spring.

Emergency treatment

Horses, unlike humans, cannot vomit. When we know that a horse has very recently eaten a toxic substance, we use a combination of intestinal adsorbents and lavage — removal of the stomach contents with copious amounts of water and a stomach tube.

Intestinal adsorbents such as psyllium husks or activated charcoal can help prevent the body digest and absorb the toxins.

Once the toxic effects start to cause some signs of disease, we work hard to stabilise the horse with a combination of symptomatic therapy (to relieve the signs) and treatment for the toxic effects, where possible. Many horses need to be hospitalised for the best chance of recovery, so they can receive intravenous fluids through a drip and regular monitoring the night.

‘His field was surrounded by sycamore trees’

When Matt Legg found his 21-year-old gelding Freckles in distress, he suspected colic.

“Freckles was nodding his head violently, whinnying and was very uncomfortable” said Matt, joint-head veterinary nurse at Bell Equine.

“He was dull and depressed, and reluctant to move. As he was out in the field we couldn’t tell if he had passed faeces, but he was eating well.

“A colic examination didn’t reveal much, so the vet took some bloods. Freckles perked up a little after intravenous painkillers.”

Blood test results rapidly revealed that Freckles’ muscle enzymes were “through the roof”, indicating atypical myopathy (AM).

“He also had an increased heart rate and a murmur, so he came into the hospital for fluid therapy,” added Matt. “After two days he was back to normal and has suffered no long-lasting effects.

“His field was surrounded by sycamore trees, but we have since moved him elsewhere. A lot of horses don’t recover from AM — intensive care helped Freckles survive.”

What’s the risk

While there are no definitive figures on the numbers of plant poisonings in horses per year, it is evident that this remains a surprisingly unusual occurrence.

It cannot be overstated that horses are at more risk from the chronic overconsumption of grass, or the rapid ingestion of grain, than the occasional exposure to a noxious plant.