Relevant medications and dosages are provided in the chapters for specific toxicants, but several are reviewed here.
Detoxication
Activated charcoal (AC):Many toxicants are adsorbed by AC, thereby reducing the amount of toxicant absorbed from the GI tract.Mix the AC (1–4 g/kg) with warm water to form a slurry.AC binds many organic compounds but is relatively ineffective against inorganic compounds (e.g., heavy metals), mineral acids, and alkali.Follow the AC with a laxative (cathartic) to hasten removal of the toxicant from the intestinal tract.
Cathartics can result in significant diarrhea; therefore, ensure the horse is adequately hydrated:Magnesium sulfate (Epsom salts) is an osmotic laxative that draws water into the intestines. The recommended dose is 250–500 mg/kg mixed in several liters of water.Sorbitol 70% (3 mL/kg) or sodium sulfate (250–500 mg/kg mixed in several liters of water) are alternative cathartics.The efficacy of mineral oil for treating intoxicated patients has not been established and its use is discouraged for routine GI decontamination.
Cholestyramine (cholestyramine) is useful in some intoxications.Some toxicants are eliminated primarily by the fecal route or undergo enterohepatic recirculation. AC/cholestyramine followed by a laxative is the most effective means for increasing elimination of these toxicants.
Many toxicants are eliminated by the kidneys, and, in some cases, renal excretion can be enhanced by increasing urine output via fluid administration. Diuretics can be used to increase urine flow – furosemide (1 mg/kg IV); mannitol (0.25–1.0 g/kg as 20% solution by slow IV infusion).
Manipulating the urine pH can increase the excretion of some toxicants in the urine via ion trapping. Weak acids are ionized in alkaline urine, whereas weak bases are ionized in acidic urine. The normal range of pH for urine in adult herbivores is alkaline (pH 7–9).
Medications
Toxicants typically are metabolized and/or excreted by the liver, GI tract, and/or kidneys. Select medications that will minimally affect these systems.
Before administering oral products, determine that the horse is not exhibiting gastric reflux.
Alpha2‐adrenergic antagonists
Atipamezole (Antisedan):Used off‐label for reversal of xylazine, other alpha2‐adrenergic antagonists, and potentially amitraz.Dose: 0.1 mg/kg slow IV.
Tolazoline (Tolazine):Used primarily to reverse xylazine. The effects may be partial and transient.Dose: 0.5–2 mg/kg IM or very slow IV; labeled dose is 4 mg/kg slow IV.
Yohimbine (Yoban):Used off‐label to reverse the effects of xylazine, other alpha2‐adrenergic agonists, and potentially amitraz.The half‐life is short (1.5–2 h) and the drug will likely need to be repeated if used to reverse longer‐acting agonists. Yohimbine has more side‐effects including CNS excitation, tremors, and hypersalivation.Dose: 0.05–0.2 mg/kg IM or very slow IV.
Atropine (generic)
Antimuscarinic agent used for treatment of SLUDGE (salivation, lacrimation, urination, defecation, and gastroenteritis) that accompanies OP and carbamate insecticide toxicity.
Competes with acetylcholine at the postganglionic parasympathetic sites.
Dose 0.2–2 mg/kg. One‐quarter of the dose should be given IV and the remainder IM or SC. The dose will likely need to be repeated; heart rate and secretions should be used to guide redosing.
It is important that enough atropine be provided, especially in large overdoses of OP or carbamates. Atropine should be given despite initial tachycardia, in order to adequately compete with acetylcholine. Without adequate therapy for OP toxicosis, patients may drown in their own secretions.
Skeletal muscle relaxants
Methocarbamol (Robaxin, Generic):Used for the treatment of tremors associated with pyrethrins and pyrethroids, tremorgenic mycotoxins, strychnine, and CNS stimulant toxicosis.Centrally acting skeletal muscle relaxant.Dose: 4.4–22 mg/kg IV to effect (moderate conditions); 22–55 mg/kg IV (severe conditions). Do not exceed 330mg/kg/day.
Dantrolene (Dantrium):Used for the treatment of malignant hyperthermia reactions, acute rhabdomyolysis, or as an adjunct therapy for black widow spider bites.Direct‐acting skeletal muscle relaxant.Dose: 2–4 mg/kg PO q 24 hours × 3–5 daysHepatotoxic so limit use in horses ingesting known hepatotoxins or those with preexisting liver disease.
Others
Intravenous fat emulsion/intravenous lipid emulsion (Intralipid, others):Used in human and small animal medicine to reverse the signs associated with several toxins such as lidocaine, mepivacaine, ivermectin, moxidectin, beta blockers, calcium channel blockers and others.Has been used successfully in a miniature horse and foals with ivermectin toxicosis.There is no equine dose available. Information and dosage have been extrapolated from small animal and human data.Dosage: using a 20% solution, inject 1.5 mL/kg through a jugular catheter over 5–15 minutes; follow with a CRI of 0.25 mL/kg/min over 1–2 h. Repeat in several hours if signs of toxicosis return. Check serum for lipemia prior to giving additional doses and do not administer if lipemia is present.
Methylene blue (generic, various):Infrequently used to treat methemoglobinemia formed secondary to oxidative agents such as hydroxyurea, nitrates, and phenols.Dose: 1% solution, 4.4 mg/kg IV; repeat dose in 15–30 minutes if no response.
Neostigmine:Neostigmine (AChE inhibitor) has been used in animals and humans to successfully reverse Micrurus venom‐induced cholinesterase actions.Dose: 0.02 mg/kg SC or IV.
Client Education
This is an important component of management of intoxications. Client education is essential to prevent recurrent exposure, prevent new exposures, and affect the outcome of case management.
Prevention/Avoidance
Minimize the risk of intoxication by using medications and pesticides according to the label directions, storing all chemicals safely, and identifying and removing all potentially toxic plants in the animal’s environment.
Possible Complications
The type and severity of complications as well as potential sequelae will vary with each toxic incident.
Laminitis is a possible secondary condition to any severe disease in horses.
Abortion or teratogenic disease may be a concern in pregnant mares depending on the toxicant and stage of pregnancy at the time of exposure
Some poisons can affect the fertility of mares or stallions.
Abbreviations
See Appendix 1 for a complete list.
Suggested Reading
1 Bright SJ, Murphy MJ, Steinscheider JC, et al. Treatment of animal toxicosis: a regulatory perspective. Vet Clin NA: Food Anim Pract 2011; 27(2):481–512.
2 Bruenisholz H, Kupper J, Muentener CR, et. al. Treatment of ivermectin overdose in a miniature Shetland