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Summer’s here, everything’s green and in bloom – what a great time to be outside with your horses! As you survey the landscaping around your barn, or the new weeds that have cropped up in the pasture this year, do you know which plants could be harmful to your equine friends? We get a lot of questions about poisonous plants, so we thought we’d outline some of the most common and the most dangerous plants here on our blog. Since there are so many to choose from, we’re going to break this up into three sections:

1. Common poisonous ornamental plants
2. Very harmful pasture weeds
3. Less dangerous pasture weeds

So, we begin today with our discussion of a few common ornamental plants that are extremely toxic. You may be planning some landscaping around your barn this summer, after all – and you might want to steer clear of these (very pretty) toxic plants. We see these plants around barns all the time – occasionally even in pastures. They can be extremely toxic to equines as well as other species – if removal is not an option, then protective fencing may be a way to help prevent animal access.

Rhododendron and Azalea: These beautiful, colorful flowering bushes are extremely common in New England. They are related, and easy to confuse – Rhododendron (left) have larger leaves, large balls of flowers, and tend to grow bigger than Azaleas (above). Some old Rhododendron bushes can be almost as tall as a one-story house! Both are evergreen, with oval-shaped leathery leaves. Flowers can be white, pink or purple, and form in clusters.

The leaves of these plants are highly toxic. (Incidentally, so is the nectar from the flowers – making honey made from these flowers toxic as well.) As little as 3 ml nectar/kg body weight or 0.2% of the body weight of the leaves may be toxic or lethal. For a 1000 lb horse, that’s less than 1 lb of leaves. For a 300 lb mini, that’s less than 1/3 of a pound of leaves. So, it doesn’t take long for a “munching” horse to take in a toxic dose. Signs of toxicity include: salivation and a burning sensation in the mouth, (vomiting in ruminants), colic, diarrhea, muscular weakness and impaired vision. A slow heart rate, low blood pressure, and arrhythmia are potential cardiac effects. Difficulty breathing, depression, and lying prostrate can be seen. Death can occur within 1-2 days. Treatment for ingestion includes giving activated charcoal (to absorb the toxin) and supportive care. If you suspect your horse, donkey, sheep or goat has gotten into this plant, call your veterinarian immediately.

Yew: Evergreen shrubs which have flat, needle-like 1″ long leaves, and a red fleshy berry which forms a cup around a black seed (see below). These are commonly used as landscaping shrubs around houses, and horses are sometimes exposed when a well-meaning gardner tosses clippings into a paddock. The whole plant (except for the berry) is highly toxic. 6-8 oz is enough to kill an adult horse or cow.

The most common clinical sign of yew toxicity is acute death; trembling, difficulty breathing, and collapse are sometimes seen in mild cases. Death is often attributed to severe cardiac arrhythmia.
Again, if you have concerns that your horse may have eaten this plant, you should call your veterinarian immediately. The best bet is to not have this shrub planted anywhere near where herbivores are kept.

Red Maple: This pretty tree is commonly used as an ornamental due to the vibrant red color it turns in the fall. It’s often the first to turn to fall foliage. Remember, it’s not red all year long! The best way to identify a red maple is the leaves (see below). There are three prominent points on the leaves, instead of the five prominent points on the leaf of a sugar maple. Other types of maple have not been proven to be toxic – but it’s a good idea to consider them a potential problem.

The leaves of the red maple are toxic to horses, and the toxic principle is most concentrated when they are wilted. The toxin slowly degrades as the leaves dry. This is why it is important to check your fields after storms to make sure no branches have fallen into the pasture. Fresh leaves are apparently not toxic (so if your horse takes a bite of one on a trail ride, don’t panic!) A horse must ingest 0.3% of their body weight in wilted leaves for toxic levels to be reached – that translates to 3 lbs for a 1000lb horse.

Once ingested, the toxin causes hemolysis (destruction of the red blood cells). The clinical signs associated with red maple toxicity include depression, anorexia, jaundice, anemia, and dark red urine (from hemoglobin being released through the kidneys). Sudden death is sometimes seen.
There is no antidote to red maple toxicity – the only treatment is supportive care, including blood transfusions and intravenous fluid therapy.
The best prevention is to remove red maples from horse properties, especially from the paddocks. Fencing around these trees is often not sufficient, since the wind will blow leaves or branches onto grazing areas, allowing horses to ingest them.
As always, if you ever have concerns that your horse may have ingested a toxic plant, it’s best to call your veterinarian for advice. It’s always a good idea to take a survey of the landscaping and pasture plants, and identify the types of plants you find. If you’re unsure of a plant, and want a second opinion, send your veterinarian a picture of it, or show him or her at the next appointment. We’re always happy to help make sure your horses stay safe from things that might harm them.

Stay tuned for the next installment of our poison plant series next week!