Any use associated with non‐veterinary‐approved antidotes is considered extra‐label. The dosages used are often extrapolated from the human literature with very little scientific animal data available. Veterinarians are allowed to legally use medications in an extra‐label manner, but when doing so assume all the responsibility associated with their use.
Little effort has been made by manufacturers to produce antidotes approved for use in veterinary medicine. There is little financial incentive for them to do this as the use of antidotes is limited, making research and manufacturing cost‐prohibitive.
Antivenom
Intravenous antivenom can be used in horses to prevent paralysis, coagulopathies, and thrombocytopenia from snake and black widow spider bites. They have little to no effect on tissue necrosis. Early administration is preferred as it not only lessens the severity of signs but attenuates the need for a large number of doses at a later date.
Anaphylactoid, anaphylactic, and serum sickness reactions can occur, especially if the animal has received antivenom at a prior time.
Sensitivity testing for antivenom reactions is frequently unreliable with false‐positive and ‐negative results.
Elapid antivenin (coral snakes):North American Coral Snake Antivenin (LLC [subsidiary of Pfizer Inc.] has a new product, Lot Y 03625 [3‐year shelf life]) available. Product is prescription only. Contact for new in‐date antivenin: +1 (844) 646‐4398. However, cost is considerable, and as such veterinary use may be difficult.Mexican, Costa Rican, and Australian antivenoms have been shown to be effective in neutralizing North American coral snake venoms (Coralmyn™, Instituto Bioclon, Mexico; Anticoral antivenom, Instituto Clodomiro Picado, Costa Rica; Australian ANG Polyvalent Snake Antivenom, CSL Limited, Australia). These antivenoms may be available through some zoos or by contacting a regional poison center with Antivenom Index access.Neostigmine (AChE inhibitor) has been used in animals and humans to successfully reverse Micrurus venom‐induced cholinesterase actions (Vital Brazil, 1996). In the absence of antivenom this may be considered for rescue and short‐term use.
Crotalid antivenin (pit vipers; rattlesnakes, copperheads, and cottonmouths):Antivenin (Crotalidae) polyvalent (equine origin) (ACP); lyophilized IgG preparation; (Antivenin™; Boehringer Ingelheim Vetmedica, Inc., St Joseph, MO – +1 866‐638‐2226); canine indication. Administration of one vial (reconstituted in 10–50 mL normal saline) diluted to 100–250 mL (volume depends on animal size) administered IV may be adequate for many cases, but three or more vials may be required in more severe envenomations.Antivenin Crotalidae polyvalent equine origin; liquid plasma preparation (Rattler Antivenin™; Mg Biologics, Ames, IA – +1 877‐769‐2340); canine and equine indication. Product is a whole IgG preparation. The product comes in 50 mL bags for use with an IV filtration administration set. A unique feature is that it contains all the clotting factors in addition to venom antigen antibodies. Typically, one or two 50 mL doses are adequate.Antivenin polyvalent Crotalidae (F(ab′)2 equine origin; liquid injectable preparation; (VenomVet™; MT Venom, LLC, Canoga Park, CA – +1 800‐385‐6914). Product is an F(ab′)2 preparation (two‐thirds molecular size of whole IgG). Slow IV administration over first 10 minutes. If no adverse effects entire dose should be given with 60 minutes.Additionally, there are two human FDA‐approved antivenin products for treating North American pit viper envenomation; however, implementation of use must be carefully considered as they may be difficult to obtain and are significantly more costly:Antivenin (Crotalidae) Polyvalent Immune Fab (ovine origin); lyophilized preparation; (CroFab®; BTG International, West Conshohocken, PA).Crotalidae Immune F(ab′)2 (equine); lyophilized preparation; (Anavip®; Rare Disease Therapeutics, Inc., Franklin, TN).
Black widow (Latrodectus spp.) spider:Antivenin (Latrodectus mactans) (Black Widow Spider Antivenin, Equine Origin) (Merck & Co., Inc., Whitehouse Station, NJ 08889 – +1 800‐444‐2080) – this is a biologic drug, whole IgG antibodies, for use in humans; there is no scientific information on the use in horses. A single 2.5‐mL vial diluted to 100 mL administered IV may be successful in reversing signs (in humans, signs begin to subside in 1–2 h).
Chelating agents
These antidotes are generally used to remove heavy metals from the body. The chelating agent combines with a metal ion to form a complex that is then excreted.
Calcium disodium ethylenediaminetetraacetic acid or CaNa2EDTA (Calcium Disodium Versenate, Medicis):Labeled for use in pediatric and adult humans with acute and chronic lead poisoning. The use has declined over the years due to side‐effects and the decreased incidence of lead toxicosis in human beings.Still widely used in veterinary medicine to chelate lead, zinc, inorganic mercury, and perhaps cadmium, particularly in birds.Calcium disodium EDTA and not disodium EDTA must be used. These two should not be confused.Should not be used while lead remains in the GI tract as it may enhance the systemic absorption of lead.Use may result in GI signs.Nephrotoxic, so caution needs to be taken to ensure hydration; should not be used in animals with chronic renal failure.Dose: 73 mg/kg/day divided into two or three doses. Dilute product in 5% dextrose or 0.9% sodium chloride and give by slow IV over the course of the day. Administer for 3–5 days. Rest for 2 days. Repeat as needed.Alternate dose: 110 mg/kg IV BID × 2 days. Rest 2 days. Repeat as needed.IM injection is painful and not recommended.
Deferoxamine (Desferal, Novartis Pharmaceutical Corporation):Labeled in human beings for the treatment of acute iron intoxication and chronic iron overload due to transfusion‐dependent anemia. Veterinary experience is limited.Deferoxamine is most effective if used within the first 24 hours while iron is still in circulation and has not been distributed to tissues.Contraindicated in patients with severe renal disease or anuria and those with high circulating levels of aluminum.Complexes with iron; deferoxamine chelated complex is water‐soluble and excreted primarily in urine.A recommended dose of deferoxamine is 40 mg/kg IM every 4–8 h; can be given as a continuous infusion at the rate of 15 mg/kg/h. Chelation should continue until serum iron concentrations fall below 300 μg/dL (3 ppm) or below the TIBC.The excreted complex turns the urine pink or salmon‐colored and is sometimes referred to as the “vin rose” of iron poisoning. Continue treatment until the urine is clear or serum iron levels are within normal limits.
Dimercaprol (BAL in oil, Taylor):Labeled in human beings for use in the treatment of arsenic, gold, and mercury poisoning. Can also be used in acute lead poisoning concomitantly with CaNa2EDTA.Dimercaprol (British anti‐lewisite) is the classic arsenic chelator.Complex is water‐soluble and excreted in urine.A loading dose of 4–5 mg/kg is given by deep muscular injection, followed by 2–3 mg/kg q4h for 24 hours and then 1 mg/kg q4h for 2 days.IM injections are painful (peanut oil carrier) and should only be given deep IM.Adverse reactions include tremors, convulsions, and coma.Dimercaprol is nephrotoxic so limit use and monitor BUN and creatinine. Be sure that patients are adequately hydrated while product is used.
Dimercaptosuccinic acid (also referred to as DMSA or Succimer; Chemet, Schwartz):Labeled for use in pediatric human beings for lead poisoning when the blood levels are > 45 μg/mL. Unlabeled use includes mercury and arsenic toxicosis.Used in equines for arsenic toxicosis.DMSA is a less toxic chelator (equine dose not established, but 10 mg/kg PO q8h is suggested).Advantages over other chelators:Can be given PO or rectally if GI signs are severe.Incidence of adverse GI signs is much lower.Can be used while lead is still present in the GI tract.Disadvantages:Cost – expensive.Availability – often difficult to find.May have a transient increase in AST and ALT.
Digoxin immune Fab fragments – ovine (Digibind, Glaxo Smith Kline; Digifab, BTG Specialty Solutions):Specific antidote used for digoxin toxicosis. It may also be useful for cardiac glycoside containing plant intoxications.Fab fragments should be reserved for the treatment of life‐threatening cardiac arrhythmias that do not respond
