During one spring a couple of years back, four horses on one farm in Colorado started to lose weight and also became oversensitive to the sun’s rays (a condition termed as photosensitization). In addition, they showed signs of having neurological disorders. After a thorough examination and analysis by vets, they were found to be suffering from severe chronic liver disease due to poisoning caused by pyrrolizidine alkaloids. These alkaloids are normally found in plants such as rattle pod, tansy ragwort, nd fiddle neck. The pasture that the horses grazed in did not have these plants.
However, after carefully examining the hay that had been fed to the horses over the course of winter, the horse owner found dried leaves of a plant called hound’s tongue. This is a highly poisonous weed that is endemic to many parts of the country-and it contains significant amounts of pyrrolizidine alkaloids that remain poisonous even when it is dry .The cumulative effect of consuming these alkaloids over the winter resulted in the horses getting chronic liver disease and liver failure, and they had to be euthanized since it had reached a stage where it was irreversible.
Hounds tongue is among the countless plants that are poisonous to horses and can even kill them. As such, horse owners need to be on the lookout and remove such plants when they are observed growing in the horse’s pasture or in the vicinity.
Carey Williams, a horse expert from Rutgers University, New Jersey notes that horses are normally able to avoid consuming toxic plants, particularly if they have other better choices of food available. In addition, many poisonous plants are equipped with appropriate defense mechanism like thorns or bitter sap which discourages animals from eating them. However, some toxic plants may appeal to horses at specific times during the year. For instance, wilted red maple leaves, irresistible to horses owing to their high sugar content, can be blown to the horses’ pastures during fall. Just eating one and half pounds of red maple leaves can be toxic to a horse, and eating three pounds is deadly.
According to Anthony P. Knight, a veterinary professor at Colorado University and author of a plant poison guide, red maple destroys the red blood cells, making the affected horses look sickly. They may also show signs of anemia and jaundice. Their urine will have a dark brown coloration as the body system tries to eliminate the dead cells.
Other toxic plants found in areas outside the pasture zones include ornamental plants like azalea, rhododendron and yew. Evergreen plants remain poisonous throughout since they do not shed their leaves. Just 10 to 15 ounces of the evergreen yew plant can kill a full-grown horse weighing one thousand pounds.
Oleander, a hardy flowering plant usually used for landscaping in the southern states including Florida and California, contains compounds called cardiac glycosides. When ingested, these compounds can cause body coordination problems, breathing difficulties, colic, excessive sweating, muscle tremors and heart complications leading to heart failure. Knight cautions that a couple of mouthfuls of oleander leaves will affect the normal heart rhythm and can be deadly to a horse. It contains toxins with similar characteristics to digitalis - a drug that is derived from the foxglove plant and which is used to treat certain heart conditions.
A lot of times these toxic plants can enter a horse’s environment accidentally. A well-intentioned neighbor can, for example, clip their rhododendron, Japanese yew or azalea plants and place the clippings into the horse’s pasture- thinking that they are safe horse feed.To curtail this, a horse owner should ask the neighbors to completely desist from feeding the horses.
Knight warns that some wildflowers and weeds can also be deadly to horses. In just about one month or so, the weed known as Senecio (or groundsel )can slowly and irreversibly destroy a horse’s liver, leading to jaundiced eyes, weight loss and photosensitization that is manifested as a serious sunburn in the white areas of a horse skin . Fortunately, signs of poisoning due to Senecio are normally easily identifiable.
Consuming water hemlock can turn fatal for a horse very quickly. Found in water bodies across the United States, this is one of the most toxic indigenous plants. All parts of this plant, and particularly the stems and roots, contain cicutoxin -an unsaturated alcohol that causes serious stimulation and nervous system paralysis. Water hemlock is so poisonous that just two or three ounces can be fatal.
There are other plants that can create problems to horses if they feed on them due to lack of hay or the right pasture. For instance locoweed, which is found in western United States can cause severe neurological disorders in horses. Others such as yellow star thistle and Russian knapweed will cause irreversible brain damage.
Detecting and Preventing Plant Poisoning
It is important for horse owners to be on the lookout for changes in the behavior and appearance of their horses so as to detect plant poisoning. Williams advises owners to call their vet immediately they suspect that the horse has consumed anything toxic. An extension agent can also be contacted so that they can coordinate with the vet to examine the pasture area and see if they can identify the culprit. Knight and Williams also highlight the importance of walking across the pastures to see if there are toxic plants growing. If you find an unfamiliar plant, take a photo and email it to the local extension agent. The extension agent can even come to your farm and identify the poisonous plants that are prevalent in your area.
Williams notes that creating buffer zones between the properties of neighbors can reduce the risk of poisonous plants growing in the pastures of horse owners. Such zones also enable the owners to manage plant growth without encroaching on other people’s properties. Moving the fence line for up to six feet inside one’s property line can help when controlling the growth of toxic plants in the pasture area of a horse owner.
If a horse owner uses herbicides to control the invasion of toxic plants, they should ensure that they are made specifically for the targeted plants. Ensure that you read and strictly follow the usage instructions provided and wear gloves that are waterproof before handling and removing toxic plants. All toxic plants should be safely disposed in a place where the horse cannot reach.
The barn can also contain plant toxins. The shavings of black walnut wood are harmful and may cause the laminae (the tissue that joins the hoof to the bone) to become inflamed, a condition known as laminitis. Williams advises horse owners to check and ensure that the beddings are not made of black walnut shavings.
According to Knight, owners should also examine the bales of hay to see if they contain toxic plants like hounds tongue. Buying certified hay is the best way of minimizing the risk of poisonous weeds. In addition, horse owners should regularly check and clean the bottoms of the feed buckets. This will help to avoid the risk of the horse consuming poisonous seeds such as jimsonweed seeds.
Both William and Knight advise horse owners to reduce the chances of toxin exposure in their horses by buying products from reputable local dealers. Where supplements are used, they should be bought from established manufacturers and the horse owner should strictly adhere to the feeding instructions provided. Herbal supplements use small dosages and are unlikely to be harmful if the directions provided are followed.
In spite of the best efforts of horse owners, their animals can still consume toxic plants. In the event that this happens, you need to try and identify the ingested plant and contact your vet immediately. According to Knight, the horse should be removed from the suspected source of the poison-whether it is pasture or hay. Both Williams and Knight emphasize that in the end, feeding your horse with a quality diet helps a lot in reducing the risk of plant poisoning. Even if some horses may sample some potentially toxic plants as they are grazing in their pastures, they will generally not consume large amounts of these plants as long as good quality hay and rich grass are available.
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