Poisoning of Horses by
Plants
Table of Contents
- Poison Hemlock
- Tansy Ragwort
- Field Horsetail
- Buttercups
- Yew
- Oleander
- Bracken Fern
- St. John's-Wort
- Nightshade
- Reference
- Go To
Poisonous Plants Quiz
Horses will usually avoid eating poisonous plants (they don't taste
very good) as long as there is an abundant supply of good quality
hay or pasture available. However, faced with no pasture or hay,
a horse might decide to sample one of the poisonous weeds still
left standing in the field.
The best medicine for dealing with poisonous plants is ... PREVENTION.
- Ensure that horses on pasture have adequate hay and/or pasture
so that they won't have to resort to eating poisonous weeds.
- Avoid overgrazing, if no supplemental hay is provided.
- Learn to recognize poisonous weeds and control them by pulling
or by use of commercially registered herbicides.
- Examine your hay for unwanted plants.
Poison Hemlock
Poison Hemlock is found throughout North America in water areas,
roadsides and dry ditches. It is distinguished from other members
of the carrot or parsley family by its smooth stems with reddish
purple spots and finely divided leaves.
Poison Hemlock contains assorted piperidine alkaloids that cause
respiratory failure in less than three hours.
After ingestion, the following symptoms may be observed:
- frothing at the mouth
- uneasiness
- dilated pupils
- weak, rapid pulse
- convulsions
- clamping of jaws
Stimulants administered immediately and supportive therapies may
help to counteract the effects of the poison conine found in this
weed.
Death may occur within 15 minutes.
A veterinarian should be conuslted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Tansy Ragwort
The yellow flowers of the Tansy Ragwort flower from July to October;
the flowers give off an unpleasant odour. Tansy Ragwort is found
throughout North America in pastures, hayfields, waste areas and
roadsides.
Tansy Ragwort contains liver-damaging alkaloids which cause liver
cells to expand, then die.
Symptoms include:
- weakness
- liver failure
- high temperature
- incoordination
- yellow mucous membranes
No known anti-dote for this alkaloid based toxin. Efforts should
be concentrated on prevention through pasture management.
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Field Horsetail
Field Horsetail is found in poorly drained soils, as well as low,
sandy or gravel soils with good drainage. It can severely suppress
field crops and other plants. It is especially poisonous in young
horses. Hay containing this weed may be more poisonous than fresh
plants in the field. Symptoms are slow to develop.
Symptoms:
- jaundice
- loss of appetitie
- weakness
- staggering gait
- excitability
- paralysis
There is no known anti-dote. Toxic substance is thiaminase plus
an unknown factor which wipes out Vitamin B.
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Buttercups
This yellow flowered weed is very common throughout North America,
particularly in wet areas. Buttercups include tall (2.5 feet), creeping,
meadow and celery leafed varieties.
Buttercups contain irritant juices that severely injure the digestive
system. Sap from stems can cause inflammation and blistering on
skin or mucous membranes and even around the hooves of horses used
to harvest. Note: poison is inactive when buttercup is dried and
included in hay.
Symptoms from ingestion include:
- mouth blisters cause drooling and loss of appetite
- colic
- bloody urine
- diarrhea
- colic
- twitching of the eyelids
- loud breathing
- weak pulse
There is no known antidote for the poison ranunculin found in the
buttercup varieties. Efforts should be concentrated on prevention
through pasture management.
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Yew
Yew trees are extremely toxic to horses and all grazing animals.
Yew poisoning is the commonest form of animal poisoning. All parts
(dead or living) are poisonous, especially the leaves. Yew contains
an alkyloid that depresses the action of the heart.
Yew is sometimes used in hedges or as an individual ornamental
plant, e.g. be careful what you put in the show ring! Yew needles
are greenish-yellow on the under surface and have no white stripes.
Symptoms:
- moderate amounts - mild to severe digestive upsets that may
result in death
- sudden death, without warning or symptoms
A handful of Japanese yew is enough to kill a horse!
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Oleander
Oleander is a common outdoor woody shrub in warmer regions and
is grown occasionally as a large potted plant. Its flowers are show
and very fragrant, but the belief that their perfume is dangerous
is unfounded.
Oleander ranks with yew in toxicity. Oleander contains a poisonous
principle similar to digitalis in its effect on the heart, causing
arrhythmia and cardiac arrest. The effects are reversible and the
horse may recover if he ingests a less-than-lethal dose (a single
ounce of oleander leaves can kill a 1,000 lb. horse).
Symptoms:
- colic
- sweting
- bloody diarrhea
- difficult breathing
- arrhythmia
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Bracken Fern
Also referred to as Eastern bracken, this weed is found in open
fields and woodlands. Leaves of bracken fern are poisonous to horses
both when fresh and when dry in hay. Cattle are far more likely
to be poisoned by bracken than horses.
Bracken fern contains thiaminse, which causes a deficiency in Vitamin
B1 (thiamine) and which is important for proper nerve function.
Signs of thiamine deficiency occur when hay containing bracken fern
at 10%-20%, or more, of the horse's dry-matter intake is fed for
approximately 4 weeks (1). For the average 454-kg (1,000-lb)
horse, 1-2 kg (2.2-4.5 lb) of bracken fern would have to be consumed
each day for a minimum of a month. Symptoms are slow to develop.
Symptoms:
- loss of flesh
- jaundice
- loss of appetite
- weakness
- staggering gait
- excitability
- paralysis
Well-timed injections of thiamine can successfully reverse the
damage caused by ingestion of bracken fern.
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
St. John's-Wort
St. John's-Wort occurs throughout Ontario in pastures, edges of
woodlots, roadsides, abandoned fields, water areas and occasionally
in lawns and lower borders.
If eaten by horses, St. John's-Wort may cause photosensitization,
since this weed contains black dots composed by hypercin, a pigment
that is absorbed by the body and activated by exposure to sunlight.
Ingestion can result in a condition in which patches of white or
light-coloured skin become seriously sunburned under normal exposure
to sunlight.
Symptoms:
- photosensitivity
- inflammation of the unpigmented portion of the skin
- affected area becomes sore and reddened and may peel
- tongue and mouth may be affected
Treatment: Avoid pasturing horses where this weed is abundant or
keep animals under shade during normal sunny days.
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Nightshade
Climbing nightshade occurs throughout Ontario in open woods, edges
of fields, fence lines, roadsides and occasionally in hedges and
gardens.
Stems and leaves are poisonous to livestock. Nightshade contains
alkaloids that interfere with digestion by inhibiting the autonomic
and parasympathetic nervous systems and by directly irritating the
digestive system.
Symptoms:
- abdominal pain
- stupidity
- dilation of pupils
- loss of appetite
- diarrhea
- loss of muscular coordination
- unconsciousness
- death
A veterinarian should be consulted immediately for supportive treatment
to reduce the damage caused by ingestion of poisonous plants.
Reference
- Burrows GE, Tyrl RJ. Toxic Plants of North America. Ames, Iowa:
Iowa State Press, 2001:415-422.
