Watch Out For Poisonous Plants  

 Robert Perce, DVM

November 2004

In spring, some toxic plants are the first things to green up.  In the fall, the pastures may be grazed down to the point that mostly weeds are left.  Some plants are only toxic in some situations, such as red maple leaves when the leaves are dried or wilted.  Others are toxic in hay as well as growing in pastures.  Plants also taste different when they are dried, so check out the hay you buy.   

According to the State Diagnostic Lab’s veterinary toxicologist, the most common plant toxicities currently in Minnesota are white snakeroot and hoary alyssum.  Red maple poisoning is another problem that can occur at this time.   

WHITE SNAKEROOT

These plants are found in low, moist areas bordering streams or in open woodlands.  The plants are usually 3 to 4.5 feet tall with sharp, pointed leaves that are 3 to 6 inches long and have three distinct veins.  The flowers are snow white and showy.  They are small and in open clumps of 10 to 30 flowers.   

The toxin, tremetone, is highest in green plants but remains toxic when dried in hay.  It can be secreted in mare’s milk and affect a foal.  The disease in animals is known as “trembles” because of muscle tremors induced by the toxin.  Poisoning develops after an animal eats between 0.5 and 1.5 percent of its body weight in green plants.  There are other plants and some fungi that produce tremetone toxins also. 

Horses are initially listless, depressed and reluctant to move.  They may develop tremors and may show colic, signs of choking, patchy sweating and dark brown urine.  They nay have a rapid, irregular heartbeat and difficulty breathing.  The prognosis is poor for horses with these symptoms.  Many of these symptoms are due to skeletal and heart muscle damage from the toxin.  Kidney failure may occur because of the breakdown material from the muscle damaging the kidneys.   

There is no specific antidote for the toxin.  Treatment is symptomatic and supportive.  Early treatment is essential if recovery is to occur.  Blood tests will show severe muscle damage. 

Prevention: Identify and remove the plant. 

RED MAPLE

Red maples are a relatively common tree in this area.  They grow wild or are planted as ornamentals because of their striking color in the fall.  The leaves are a typical maple leaf shape with a shiny green top and a gray/white underside.  They turn bright red in the fall.   

The dry or wilted leaves and the bark are toxic to horses.  Consuming as little as 1.5 pounds of dried leaves may be fatal within one to five days.  Eating relatively small amounts of dried leaves will cause horses to act sick within one to two days.  Often this happens when a branch breaks off where horses can sample it. 

The toxin causes rapid breakdown of red blood cells.  This results in rapid anemia with weakness, rapid breathing and heartbeat, yellow to purple membranes and red-brown urine.  The prognosis is guarded to poor.  Pregnant mares may abort without showing other clinical signs.  Treatment is blood transfusion and fluid therapy. 

Prevention: Keep horses away from red maple leaves that are wilted or dried and don’t plant these trees in or close to the pasture.   

Wilted leaves of other trees in this area that should be avoided are in the plum family – wild plum, chokecherries, domestic plum and cherries.  These trees cause cyanide poisoning.  Also regarding trees, uncontrolled consumption of acorns can result in laminitis.  The prognosis is guarded.  Some horses and especially ponies eat these as if they were candy. 

HOARY ALYSSUM

This is a common weed in Minnesota.  It grows up to 3 feet high and has a gray appearance.  The flowers are clusters of small white flowers with four deeply divided petals located at the end of the branches.  The toxin is in both green and dried plants.  Hay containing hoary alyssum may remain toxic for up to nine months.   

Most common clinical signs are stocking up in the legs and laminitis.  Horses may also show stiffness, fever, diarrhea, shock and death.  Pregnant mares may abort.    Remove hoary alyssum from pastures and inspect hay for the weed.  This plant is a common part of wildflower mixes and grows well in sandy soil.   

With any toxic plants, the best treatment is prevention.  Identify and remove all poisonous plants.