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What is Plant Neurotoxicity?

In normal nerve function, a nerve impulse travels along nerve fibers. When it reaches the end of the fiber, a small amount of acetylcholine is released and broken down into pieces to enable the cells on the adjacent fiber to receive the nerve impulse. Neurotoxins stop this transmission of the impulse in a variety of ways. Some toxins can prevent the breakdown of acetylcholine, which can disrupt the transmission of the impulse and result in overstimulated nerves, muscles and glands. Alkaloids such as atropine can take the place of acetylcholine on the nerve endings at the end of fibers, stopping the signal from being carried on.

A plant neurotoxicity can occur when a horse eats a poisonous plant that negatively affects the brain and nervous system. There are many plants which can cause symptoms related to movement, behavior and paralysis. Some of these can cause mild symptoms that may resolve, while others can result in life threatening conditions and need to be treated immediately.

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Symptoms of Plant Neurotoxicity in Horses

Symptoms of a plant poisoning can occur within minutes, hours, days, or even months after the initial ingestion has taken place. For some plants, only one dose is needed to be deadly, while others may need to be eaten many times before they affect your horse. Plants that affect the nervous system will result in varying degrees of abnormal behavior, loss of motor functions, and eventually loss of organ function. Some of them can also affect digestive and urinary health, or can result in abortions or fetal abnormalities. Signs generally begin as mild changes that can progress into severe paralysis, troubles with breathing, eating and elimination, and eventually death. Symptoms include:

  • Incoordination
  • Muscle tremors
  • Muscle weakness 
  • Reluctance to move
  • Stance with an arched back and wide base 
  • Inability to rise 
  • Tongue that protrudes or lolls 
  • Head tossing
  • Stumbling 
  • Falling
  • Abnormal gait 
  • Abnormal or violent behavior
  • Recumbence 
  • Loss of body control
  • Paralysis
  • Convulsions 
  • Dilated pupils
  • Poor performance
  • Difficulty swallowing or eating
  • Lethargy
  • Severe depression
  • Loss appetite
  • Weight loss
  • Dehydration
  • Malnutrition
  • Difficulty breathing
  • Hypersensitivity
  • Excessive thirst
  • Excessive salivation
  • Excessive defecation or urination
  • Urinary issues, such as scalding, dribbling, cystitis and incontinence
  • Sweating
  • Allergic reactions
  • Digestive problems 
  • Mouth ulcers 
  • Skin issues due to photosensitization 
  • Leg swelling 
  • Abdominal pain
  • Increased respiratory and heart rates
  • Death
  • Abortions
  • Fetal deformities 

Types

There are many plants that can cause a neurotoxicity in horses when ingested. Some of them include:

  • Yellow star-thistle, Centaurea solstitialis 
  • Locoweeds, Astragalus and Oxytropis spp. 
  • Moldy corn
  • Poison hemlock, Conium maculatum 
  • Jimsonweed, Datura spp. 
  • Bracken fern, Pteridium aquilinum
  • Horsetail, Equisetum spp. 
  • Sudan grass and sorghum, Sorghum spp. 
  • Singletary pea, Lathyrus hirsutus 
  • Flatweed, or false dandelion, Hypochaeris radicata

Causes of Plant Neurotoxicity in Horses

The cause of a plant neurotoxicity in your horse is the ingestion of a neurotoxic plant. Alkaloids and other chemicals in these plants have a negative effect on the nervous system, causing varying types of dysfunctions through different processes. Some of these toxins include:

  • Swainsonine from locoweeds, which impairs cell metabolism
  • Fumonisin from moldy corn, which alters the integrity of cell-membranes and can result in a liquefaction of cerebral tissue
  • Atropine and scopolamine from Jimsonweed
  • Thiaminase from bracken ferns and horsetails breaks down vitamin B1
  • Cyanogenic glycosides and nitrates are accumulated by plants such as Sudan grass and sorghum

Diagnosis of Plant Neurotoxicity in Horses

Diagnosis of a plant poisoning can be easy if you have seen your horse eating a potentially toxic plant. Be sure to bring a sample of the plant your horse may have ingested for identification to your veterinarian. This identification, along with symptoms, can lead to a diagnosis. If you have not seen any evidence of a plant ingestion, identifying the reason for your horse’s symptoms can be more difficult. Due to the severity of some poisonings, your veterinarian may begin treatment before a diagnosis is made.

Tests will look for signs of a poisoning or infectious disease, and can include blood, serum, and urine testing. These can often reveal the presence of particular toxins. Imaging techniques may be employed to look for any masses or physical reasons for the neurological symptoms. As these tests come back negative for other causes, your veterinarian will discuss plants that may be in your horse’s environment that could have caused a poisoning.

Your horse’s feed can also be tested, as contamination with poisonous plants or seeds is common.

Treatment of Plant Neurotoxicity in Horses

Once a plant has been identified, removal of this plant from your horse’s environment is often the first step in treatment. This may include replacing the current feed, or removing plants from pastures and other areas in your horse’s reach.

Some plants may have specific antidotes, while for others such as locoweed, there are no treatments or antidotes available. Most often, treatment consists of reducing the absorption of toxins in your horse’s body and treating symptoms. 

Activated charcoal or mineral oil can be used to reduce toxin absorption in the digestive tract. Fluids and electrolytes can be given to help support your horse, and glucose injections can be administered for low blood sugar. Muscle relaxants may help reduce muscle spasms and tremors. Thiamine can be added to the diet in the case of a bracken fern poisoning. Any other treatments will be specific to the type of plant that has poisoned your horse, as well as your horse’s condition, and will be discussed with you by your veterinarian.

Recovery of Plant Neurotoxicity in Horses

Recovery is entirely dependent on the plant that has caused the toxicity, the amount ingested, and your horse’s condition when treatment began. In many cases, recovery is good if treatment is prompt. Your horse may completely recover after treatment. However, in some poisonings,  a relapse of symptoms can occur once treatment has ended is possible. Other poisonings are irreversible and can be lethal.

Horse do not normally seek out poisonous plants to eat, due to their bitter and often unpalatable nature. There are situations when a horse will eat such a plant. Knowing how this could occur can allow you to take preventive measures to ensure your horse does not ingest a potentially fatal plant. Identifying lethal plants on your property, pastures, and trails can alert you to potential problems, and allow you to remove any plants that may be ingested. Conditions an ingestion could occur in include:

  • Contaminated or moldy feed
  • Lack of adequate green forage available
  • Overgrazed pastures
  • Drought 
  • Inadequate diet
  • Accidental ingestion
  • Boredom