While most of us fight to keep our horses from killing the trees on our properties, the fact is that some trees can kill horses.

Pistachio. While more research needs to be performed to isolate the toxin involved, UC Davis School of Veterinary Medicine has advised that neither ornamental or fruiting varieties of pistachio be planted within reach of horses. Any fallen leaves of trimmed limbs with leaves should also be kept out of the reach of equines. Wilted leaves, such as from cut down trees, trimmed limbs, from trees stressed by lack of water or during fall leaf drop are of primary concern, as are the nuts from the trees.

Several years ago a part of a group of pastured horses became ill, with five dying within 48 hours of symptom onset. The horses had been noted to be lethargic, jaundiced, colicking, uncoordinated, have discolored urine, pale or yellow mucous membranes and decreased appetite. Two horses were brought to the UC Davis Veterinary Hospital, which sent researchers to the horse farm. Most of the horses were fine. The researchers took samples looking for toxins in the hay fed, in the pasture grass, of the water and any trees in the pasture. No sources of toxins were found. No recent changes as to how the horses were kept had occurred.

However, alongside the pasture a pistachio orchard growing three varieties had just been cut down. A few of the horses had been seen by the farm owner to be nibbling at the downed pistachio trees, which they could reach. Being recently cut, the leaves of the trees were wilted. Other horses on the farm that did not have access to the pistachio trees did not become ill.

The sick horses were determined to be suffering from hemolytic anemia, a condition in which the immune system of the body attacks and kills its own red blood cells. While linked to the ingestion of the pistachio leaves, a specific toxin was not isolated. An earlier study UC Davis study had found a link between oxidants and ingestion of pistachio trees and nuts (seeds) in horses.

Red maple. The wilted leaves of red maples and relatives cause red blood cells to burst, causing severe anemia which can lead to death. Eating as little as three pounds of leaves has killed horses. While red maples and its relatives such as box elders are not common locally, just be aware.

Other Acer species trees such as European sycamore have been found to cause muscle fiber breakdown that can produce death in as little as 72 hours when horses eat their mature seeds. Young horses and horses new to the pasture are those most likely to eat the seeds. Blood in the urine and lethargy are usually the first symptoms noticed.

Prunus species. Prunus species trees and shrubs are very common locally, and these contain cyanide. The level of cyanide is highest when their leaves wilt, are damaged by frost, in the fruit and seeds. Young, quickly growing trees also can have higher levels of cyanide. Cherry trees were researched in Kentucky as a possible cause of Mare Reproductive Loss Syndrome with inconclusive results.

Horses which eat the toxic leaves or seed pits of Prunus species will be seen breathing heavily, often with elevated respiratory and heart rates, because cyanide impedes the ability of cellular uptake of oxygen.

Members of the Prunus species include: prunes, plums, cherries, apricot, peach, nectarine, almond, cherry laurel, chokecherry and flowering cherries. In the case of wild cherries, the bark is also toxic.

Oak trees. I did not know that oak trees are toxic to livestock. After all, didn’t Native Americans eat acorns? But the acorns, buds, leaves and flowers are, in fact, toxic to all livestock. Problems are seen most frequently in cattle and sheep, which are not as picky about their food as are horses. Locally, the majority of oak seen is scrub oak in canyon or mountain areas. Most of my friends cut back scrub oak, because it’s a dry bush full of litter that is highly flammable. So, when you trim scrub oak, do not compost it near any livestock.

Many of the large, native oaks in California cannot be trimmed or removed without permission. The Santa Clarita Valley and the far western Antelope Valley and Tehachapi has some of these native oaks. If they can’t be removed, try to fence off the trees or relocate your horses away from the oaks. If this isn’t feasible, try to vigilantly rake up acorns as they fall after windstorms, or when ripe, in the fall.

Horses rarely eat a sufficient amount of acorns for the tannins present to create problems. Symptoms include colic and bloody diarrhea, death is usually caused by severe kidney damage.

Black locust. A large deciduous tree often grown locally for shade, as well as the fragrant, white flowers in spring is the Black locust. Its seeds, leaves, bark, and twigs, whether fresh or dried, contain toxic proteins. As little as eight ounces of the leaves or bark can kill a horse. Symptoms can start within an hour of eating back locust, and include paralysis, colic, diarrhea, and abnormal heart rate and/or rhythm. Death can occur within a few days from neurological or abdominal tract damage.

Holly. An evergreen shrub or small tree, American holly with its bright red berries are often used for Christmas decoration. The berries are the most toxic part, and can cause colic or seizures. This should not be confused with the Holly Oak, a slow growing ornamental evergreen tree with holly shaped leaves that produces acorns.


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