Wednesday, March 30, 2011

Poisonous Plants Webinar Notes

My notes from tonight's webinar on Toxic Plants. Please be aware I have not edited them and that some plants were spelled phonetically. 

Poisonous Plants
Hosted by
March 30, 2011
Anthony P. Knight, BVSc, MS, Dipl. ACVIM, is a professor of large animal medicine in the College of Veterinary Medicine and Biomedical Sciences at Colorado State University. He received his veterinary degree from the University of Nairobi, Kenya, in 1968. After completing a master’s degree at Colorado State University, he joined the faculty in 1974. His current professional interests include livestock heath, foreign animal diseases, emergency management, and plant toxicology. He has written two books on poisonous plants of animals in North America, and maintains a poisonous plants website for use by anyone wanting poisonous plant information.
Karyn Bischoff, DVM, MS, Dipl. ABVT, is a veterinary toxicologist at the New York State Animal Health Diagnostic Center and an assistant professor at Cornell University. She graduated with her bachelor's degree in Animal Science from the University of Wisconsin (Platteville campus), and she obtained her DVM from the University of Illinois. She earned her master’s degree at Oklahoma State University while completing a residency in toxicology, and she went on to complete a pathology residency at the University of Florida before ending up in the lovely rolling hills of upstate New York.

Can a horse tell a toxic plant from a non toxic plant?
They do naturally avoid toxic plants. Bitter tastes tend to avoid them if they have something else to eat. Some plants are very palatable and even addictive. Some plants are also more toxic under certain conditions than others.

What would increase a horse’s risk of eating a toxic plant?
Seasons can make a plant more toxic. Climate conditions can be more toxic. And some plants are more tasty after sprayed with herbicides because they produce more sugars.

Are poisonous plans still poisonous if they are cut and baled in hay?
Yes. They may not be as toxic as they are when they are green but remain toxic when dry. A good example is milkweed.

Are there any general symptoms of harmful plant ingestion?
There are not. Plants do all types of different things: sudden death (you plants); chronic disease (ragwort); colic (gimsun weed, walnut); kidney failure (oaks), etc…

What should be a horse owner’s first response when plant ingestion is suspected?
Take them off of the suspected source (hay, pasture, etc…). Symptomatic therapy is the best.

Are there any medications you should keep on hand?

Banamine if they are showing colic to relieve those symptoms. Laxative to remove them from the GI system.
Activated Charcoal (not BBQ coal) can be very healthy as an absorbent – if you can get a horse to eat it!

What is the most common toxic plant?

West Coast – Hounds Tongue. Causes irreversible liver disease. Loco Weed. Milk Weed.
East Coast – Rag Wort causes liver disease. Red Maple Poisoning.
Florida – Crotolaria

Environmental conditions and their affect on plant toxins:

Drought changes the compounds of the plans. Also affects the plants if they are in the shade versus the sun. Rainfall also makes plants grow more vigorously.
A stressed plant generally has more potential for poisoning than a healthy plant.

Is there a horse by horse basis for toxicity that could cause some horses to react and not others?

Sometimes plants need to be ingested in HIGH quantities to have an effect. “It’s the dose that makes the poison.”
Some of these plants don’t produce clinical signs for months.

What is a good resource online for identifying poisonous plants with good photos?
Cornell’s poisonous plants website
USDA Plant Database
Wikipedia – if you know what you are looking for

Is there a website where you can send in photos of suspicious plants?
Dr. Anthony Knight is very willing to identify the plant for you:
Local plant stores can also identify plants so you can research further.
Plant biologists and taxonomists at universities.

Poisonous Plants:

Japanese and English Yew plant  --- Sudden Death
Water Hemlock  --  The root is the most poisonous. 4-6 oz. kills a horse.

Can weeds cause hives and edema?

Yes there are possibilities. Poison Ivy doesn’t affect horses.
Recent evidence to show that tall fescue (especially of the Mediterranean type) that is infected with the fungal entophyte will cause swelling of tissues.

Can poisonous plants cause laminits?

Yes. Some are associated.
Black Walnut Shavings. 20% of the bedding or more can cause it. Ingestion of bark and twigs can also do this.
Sarsaparilla variation.
Usually signs would be within a day.

Can dandelions cause issues?

False dandelion causing string halt causing them to lift their feet very high. It is most common in Virginia.
This is actually a very old disease and not all string halt is plant related.
False Dandelion = Cat’s Ear = Flat Weed

Can plants cause irritated gums with blisters?

Butter cup can be quite irritating to the mouth when eaten regularly.
Could be that the hay has weeds with sharp horns in it.
Could be a chemical on the grass.
Could also be a viral disease.

What plants cause liver failure?

Ragwort. Cineceo. Rattle Box. Amsinkia (fiddle neck). Cow’s tongue.
Finding the source of the plant is very difficult because it is a chronic disease, not a sudden disease. Sometimes you can only identify in certain seasons which makes it more difficult.

What plants can cause choke?

Plants with a hard pit in them. Apples and pears. Persimmons (can also cause GI obstructions further down).
Treatment for minor cases can actually be CocaCola!

Are acorns safe for horses?

The oak tree is not usually good for horses, including live oak. It can cause Colic, Kidney Failure, etc.
But a handful of acorns is not going to hurt the horse.

Are cherry trees safe?

Cherry trees have the potential to contain cyanide compounds. Cattle, Sheep and Goats are much more susceptible than horses. A horse isn’t going to be able to convert the glycosides to cyanide as quickly.
That said in a frost or stress conditions for the tree, and the horse eats large quantities, it could cause cyanide poisoning or sudden death.

Is there any danger from pine needles and pine bark?
Pine trees are toxic to pregnant cows. Pine needles and pine bark have no nutrition  but there are not the same reactions in horses as in cattle.

Are maple trees toxic?

The most toxic one is the Red Maple. The poisonings vary by location, but usually in late summer or autumn when the leaves are wilted or drying up.
Damage is caused to the red blood cells causing them to become anemic.

Trees to avoid:

Black Walnut
Red Maple and its hybrids
Black Locust
Golden Chain tree
Horse Chestnut, Buckeye
Choke Cherry and other cherry trees
Kentucky Coffee Tree
Russian Olive
Chinese Tallow Tree
China Berry

Toxic Shrubs to Avoid:

Yellow Oleander
Rhododentron (azalea)
Japanese Peiris
Black Laurel
Burning Bush
Angels Trumpet
Day or night blooming Jasmine
Scotch Broom

Are nightshades poisonous to horses?

The contain glycol-alkaloids that can block motility in the intestine and can cause colic. If they eat enough it could actually stop the heart and lungs.
Potato and Tomato vines are issues, as are green potatoes. These are also nightshades.

Is wild carrot the same thing as queen anne’s lace and is it poisonous?
It is the same and there is no reported toxicity.

Horse Nettle?

They can increase the neurotoxicity of ivermectin.
Horse nettle is different than stinging nettle and is not poisonous.

Indian Paintbrush?
Not poisonous. It is an indicator of high selenium content in soils so the surrounding grasses could cause chronic selenium poisoning with long term exposure.

Butter Cups?
They cause irritation in the oral cavity and are bitter. They are usually the last plant standing in pastures!


The only one that is toxic is Russian napweed that causes chewing disease and can damage the brain and inhibit their ability to bite off and chew food.
Other napweeds are not poisonous.

More of a problem for ruminants but horses are fine with them.

Tends to like wet areas and is not a good food – indigestible. It has been associated with blindness and colic.

Skunk Cabbage?
Not very palatable and can cause damage to the mucosa of the oral cavity.

No problem!

Toxic and can develop a neurotoxicity. But there are no reports in North America.

Contains high caffeine content.

Non-toxic but could be injurious.

Non-toxic. Can make your horse smell nice though.

Fruits and Vegetables to not give as treats?
Potatoes. Green potatoes. Green tomatoes. Avocados.

How can you safely get rid of noxious plants when horses are on pasture?

Mowing can greatly impact the pasture. Let horses eat half and leave half. If there is viable grass the grass will crowd out the weeds.
Herbicides can increase the toxicity and how attractive plants are as food for horses.

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