FAQ – Overdoses and Overeating

If your horse has been overdosed with something or has gotten into the feed room, here are some recommendations.

It is always best to call your veterinarian immediately and not wait for an HVC veterinarian to respond to your post.

Common overdoses:

– Dewormers:

• The locking mechanism on the paste dewormers can fail to lock in some cases, resulting in an inadvertent overdose of medication. We recommend dosing out the proper amount into a smaller syringe for miniature horses or foals to ensure proper dosing and to eliminate the risk of the locking mechanism failing.
• Ivermectin: ivermectin is very safe in general. It has been studied to be safe in most cases up to 10 times the label dose. However, some plants can increase the risk of the medication crossing the blood-brain barrier (https://thehorse.com/124247/ivermectin-toxicosis/). Additionally, foals less than 4 months of age have a more permeable blood-brain barrier so they are more susceptible to the effects of ivermectin. Common clinical signs of an overdose include sleepiness and drunken behavior – this can be more severe and life-threatening in miniature foals and foals less than 4 months of age. Hospitalization with supportive care is often needed for a chance at survival in these smaller patients.
  • Treatment: remove as much of the dewormer from the mouth as possible. If the patient is a miniature adult horse or adult donkey, and the dose is less than 5 times the normal dose (ie, the animal is more than 250 pounds (113 kg)), then monitoring the animal for clinical signs is warranted. If the animal is a foal less than 6 months of age, or the animal is less than 250 pounds and received the entire syringe, then an emergency veterinary visit is warranted.
• Moxidectin: Of the available OTC dewormers, this one a narrower safety range than other dewormers. Moxidectin has been demonstrated to be safe for most horses, including breeding mares, stallions and even young foals, when administered at up to 3 times the recommended dose of 0.4 mg/kg bwt (Rulli 1996). Reactions have been associated with over dosage, either by gross over-estimation of weight by owners or from slippage of a faulty syringe locking mechanism. Clinical signs include: sedation and muscle relaxation. In foals, toxicity signs include difficulty breathing, depression, ataxia (a drunken gait), weakness, seizures, and coma. Clinical signs can be seen 2–22 h after moxidectin is administered. Most affected foals survived with supportive care that includes fluid therapy and administration of drugs to control seizures and to regulate temperature. Intralipid therapy is also beneficial in these cases. This dewormer is not labelled for foals less than 6 months of age nor for thin or debilitated animals and should not be given to these animals. For more info, see: https://aaep.org/sites/default/files/issues/eve-20-10-Schumacher_EVE_20-10.pdf. Also: https://www.facebook.com/groups/277077519400687/permalink/407507773024327/
  • Treatment: If the animal is less than 6 months of age or is a debilitated horse and received any moxidectin, an emergency veterinary visit is warranted. If an overdose (greater than 2 x the weight of the horse) has been administered, please call your veterinarian for advice.
• Panacur (fenbendazole): Fenbendazole is very safe and unlikely to cause problems even in foals or debilitated horses.
• Pyrantel (Strongid): Pyrantel is very safe and is not absorbed into the bloodstream in horses

NSAIDS:

• Phenylbutazone: Bute comes in 20 g paste tubes – of which the locking mechanism can fail. Normal dosing for a 1000# horse is usually 1-2 grams one to two times daily. However, even at normal doses, some horses are sensitive to the NSAIDs and can develop right dorsal ulcerative colitis, kidney injury, and/or gastric/oral ulceration. Even one dose over 8.8 mg/kg (i.e., 4 grams for a 1000# horse) or 2-3 days of 8.8 mg/kg/day can lead to these side effects. We recommend transferring a dose to a smaller syringe before administering to the horse in order to prevent the locking mechanism failure. For more information, see here: https://thehorse.com/129108/bute-how-much-is-too-much/
  • Treatment: if an overdose has occurred OR clinical signs such as diarrhea has occurred while on treatment with Bute, stop the medication and call your veterinarian. Fluid therapy to prevent/treat kidney injury and treatment for right dorsal ulcerative colitis may be necessary.
• Flunixin meglumine (Banamine): Since Banamine comes in a 3 dose paste tube, massive overdose is unlikely unless treating a foal or miniature horse. Similar side effects would be expected in a Banamine overdose as for phenylbutazone overdose; however, Bute is more toxic in general. Dehydration will compound the toxic effects to the kidneys for both NSAIDs.
  • Treatment: if an overdose of Banamine has occurred, stop the medication and call your veterinarian. Fluid therapy and gastric ulcer treatment may be needed.

– Grain overload:

• Horses that get into the feed room and over-indulge are at risk of colic, diarrhea, and laminitis. The most serious overload is if the feed contains a lot of corn (chicken feed, for instance) – corn by-product is how laminitis is induced in study conditions.
  • Treatment: If a horse has ingested a large amount of grain, an emergency veterinarian visit is warranted. The veterinarian may do a physical examination, pass a nasogastric tube to remove as much feed from the stomach as possible, and may administer mineral oil and/or charcoal to prevent colic/colitis. If a veterinarian is not available, withhold feed for at least one day to allow the feed to move through, administer probiotics, and begin to ice the feet. Icing the feet requires full immersion of the feet in ice cold water continuously for at least 48 hours. Intermittent icing or cold hosing can be more detrimental than not icing at all – so the owner must be committed to continuous icing if this is started. Monitor the horse’s temperature and continue icing until there is no fever for 24 hours and no signs of colitis or endotoxemia (red mucous membranes, diarrhea, colic, fever). The product “Jack’s Ice Boots” or similar can be very helpful and convenient for icing feet – while they do not encompass the feet themselves, the ice cold water constantly drains over them and the legs are cold up to the carpi/hocks. Synthetic ice packs are not recommended as they can burn the limbs and actually do not transfer heat the same as ice.
• Horses that get into medicated cattle feed (those that contain monensin (Rumensin) or lasalocid), are at risk of cardiac (heart) damage.
  • Treatment: this is an emergency and requires a veterinary visit immediately for removal of as much feed as possible from the stomach and for administration of heart supporting medications.

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