Alan Copson/Getty Images
Maple (Acer spp.)
Where it's found: Thirteen species of maple trees are found throughout the Northern Hemisphere, with a larger distribution in the eastern United States and Canada. The red maple (Acer rubrum) is among the most common, as are the sugar maple (Acer saccharum), silver maple (Acer saccharinum), and box elder (Acer negundo). Only a few species have been associated with the development of clinical signs.
The toxin and how it works: The toxin is unknown but it damages the red blood cells, making them unable to carry oxygen.
Threat to horses:
Signs: These can occur as early as a few hours after ingestion or be delayed for four to five days. Depression, lethargy, and anorexia usually occur first and are followed by reddish-brown urine and pale yellowish gums and mucous membranes. Later signs include dark-brown muddy gums and mucous membranes, difficulty breathing, inability to rise, and death.
Treatment: Use activated charcoal and mineral oil to decontaminate. Aggressive IV fluids to correct dehydration and protect the kidneys as well as blood transfusions, ascorbic acid (Vitamin C), nonsteroidal anti-inflammatory drugs (NSAIDs), and corticosteroids may all be necessary.
Prognosis: Good if animals are treated before signs begin. Once evidence of red blood cell damage occurs, aggressive in-hospital treatment will be needed for survival.
Darlyne A. Murawsk/Michele Constantini/Pete Saloutos/Getty Images
Foxglove (Digitalis spp.), oleander (Nerium oleander), rhododendron (Rhododendron spp.)
Where they're found: Each of these plants is found to some extent throughout the United States. Different cultivars of foxglove and rhododendron grow and overwinter in just about every state. Oleander is not hardy enough to overwinter in northern climates, but it's often found as a houseplant or ornamental container-grown plant.
The toxin and how it works: Foxglove, oleander, and rhododendron contain toxins known as cardenolides or cardiac glycosides. Cardenolides interfere with the electrical conductivity of the heart, resulting in irregularities in heart rate and rhythm.
Threat to horses:
Signs: Signs generally begin just a few hours after ingestion, and most horses are simply found dead. Other early signs include weakness; edema of the head, neck, and eyes; and a slow heart rate that progresses to irregularity. Seizures and inability to rise often occur before death.
Treatment: Rapid development of illness and signs generally make treatment impossible. Veterinarians can use activated charcoal and mineral oil to decontaminate if done so early after ingestion. Other drugs such as atropine and lidocaine that focus on specific cardiac conduction abnormalities may be useful in hospitalized cases. Digoxin-specific Fab fragments have been used successfully in small animals but are cost-prohibitive in horses.
Prognosis: Very poor once signs have developed. Early and aggressive therapy before the appearance of clinical signs improves the prognosis.
Tony Sweet/Getty Images
Bracken fern (Pteridium aquilinum)
Where it's found: Bracken fern is found throughout the United States in open pastures and woodlands. It prefers moist, acidic soils.
The toxin and how it works: Bracken fern contains a type I thiaminase enzyme. It works by degrading or destroying thiamine (Vitamin B1) and creating a thiamine analog (fake thiamine) that interferes with nerve function and other bodily processes.
Threat to horses:
Signs: Signs are related to neurological dysfunction and include depression, blindness, gait abnormalities, muscle twitching, and seizures.
Treatment: Administer IV or IM thiamine for days to weeks. Other treatment is primarily supportive and includes NSAIDs, IV fluids, and drugs to prevent seizures.
Prognosis: Generally very good if treatment is begun before neurological problems develop. The onset of seizures and blindness is associated with a poor prognosis.
Matthew Ward/Getty Images
Black walnut (Juglans nigra)
Where it's found: Black walnut trees have been cultivated in the United States since 1868. They are commonly found in the eastern half of the United States except the northernmost border.
The toxin and how it works: The toxin is unknown. Many believe that juglone, present in black walnut roots and leaves, is the culprit, but scientists are unable to reproduce toxicosis by oral or dermal exposure to juglone.
Threat to horses:
Treatment: Removal of the horses from the shavings as soon as signs are noticed often stops the progression of laminitis. Wash the horse’s feet and limbs with cold water to remove any remaining shavings and help decrease signs of laminitis. Further treatment is based on the signs and generally includes an NSAID—such as flunixin or phenylbutazone—as well as mineral oil and good farrier care.
Prognosis: Generally very good if horse is removed within a few hours of exposure. Once laminitis develops, the prognosis for a full recovery decreases.
Robert and Jean Polluck/Getty Images
Tansy ragwort (Senecio spp.)
Where it's found: More than 70 different species of Senecio are present in the United States. This daisy-like weed is found in hay fields, pastures, ditches, and other unimproved areas.
What is the toxin and how does it work? Senecio species plants contain pyrrolizidine alkaloids, which are metabolized to pyrroles in the liver. Pyrroles inhibit cellular division, resulting in production of abnormal liver cells (megalocytes). As the megalocytes die, they are replaced with fibrotic tissue. Not all Senecio species have the same amount of toxin but all contain at least some concentration of pyrrolizidine alkaloids and all are considered harmful.
Threat to horses:
Signs: Generally aren’t present until the liver has failed. Once the liver fails, anorexia, weight loss, photosensitization, depression, blindness, unusual behaviors, and jaundice swiftly follow.
Treatment: There is no treatment once signs are present. If ingestion is suspected and complete liver failure has not developed, supportive care is recommended, but the horse may never return to its previous healthy state. Electrolytes, IV fluids, glucose, and B vitamins are useful, as is protecting the horse from the sun.
Prognosis: Very poor to fatal once liver failure has occurred. Poor for cases of suspected ingestion that are caught earlier.
Dr. Hovda is director of veterinary services at SafetyCall International and Pet Poison Helpline in Bloomington, Minn.
Pet Poison Helpline is a service available 24 hours, seven days a week for pet owners and veterinary team members who require assistance treating a potentially poisoned pet and can provide treatment advice for poisoning cases of all species, including dogs, cats, birds, small mammals, large animals, and exotic species. As the most cost-effective option for animal poison control care, Pet Poison Helpline’s fee of $35 per incident includes follow-up consultation for the duration of the poisoning case. It is available in North America by calling 800-213-6680. Click here for additional information.