Likely lingering in our equine first aid kit, barn and tack room cabinets, or in the horse trailer is an assortment of medications, in bottles, syringes, and tubs, awaiting the next "incident" that might unfold with our horses. Often we take for granted the handling of commonly used substances, with barely a thought about the adverse effects they could have on us or our horses. A number of medications can, in fact, be harmful. We will discuss information on medication risks of some commonly used products with input from Scarlet Thomas, BSPharm, RPh, director of pharmacy at Hagyard Equine Medical Institute in Lexington, Ky.
Thomas stresses, "According to the FDA (Food and Drug Administration), all drugs are dangerous. Caution and common sense should be used when handling and administering drugs to veterinary patients."
Always follow a veterinarian's advice and heed label directions and warnings. And use medications only on the specific horse for which they were prescribed.
General Safe Handling Rules
You can protect yourself and others from accidental drug exposure by following basic guidelines when handling medications:
- Lock away all medications to prevent child and animal access.
- Restrain a horse adequately during medication administration.
- Remove hazards from the treatment area that could cause you to trip, slip, or fall.
- Wash hands thoroughly after treating a horse to reduce risk of skin, eye, or oral contact with residue on fingers.
- Avoid uncapping needles with your teeth to prevent accidental ingestion of drugs.
- Administer injectable drugs slowly to avoid sprayback of medication in your face.
- Use aerosolized products (insecticides) or those with fumes (disinfectants) only in well-ventilated areas.
- Dispose of unused or outdated medications and used syringes and needles conscientiously (into a sharps container) to protect others and the environment.
- Consider the potential for environmental contamination, particularly with antibiotic and dewormer metabolites and residues.
- Discuss with your veterinarian any other substances your horse might be receiving in order to avoid adverse drug interactions.
- Advise your veterinarian if you are pregnant or allergic to medications so you can take precautions in advance of horse treatment.
- Post in the barn the number for the National Capital Poison Control Center (800/222-1222 or www.poison.org) in case of accidental human drug exposure.
Non-steroidal anti-inflammatory drugs (NSAIDs), such as phenylbutazone, flunixin meglumine, and ketoprofen, inhibit prostaglandin synthesis to control inflammation and pain. However some prostaglandins (PG-E) protect the lining of the stomach and large intestine, and they dilate kidney blood vessels. So, chronic administration of an NSAID can lead to gastric or colonic ulcers or to kidney disease.
Horse risk Phenylbutazone ("Bute") can only be given orally or intravenously, as it will cause severe tissue necrosis if injected in muscle (intramuscularly, or IM).
Human risk Researchers have documented that the drug, if ingested or inhaled in any form (paste, tablets, powder), can cause bone marrow suppression in humans, with potentially fatal consequences. Thomas reports, "Phenylbutazone is known to induce blood dyscrasias (disorder), including aplastic anemia, leukopenia, agranulocytosis, thrombocytopenia, and death." And, the National Toxicology Program considers phenylbutazone to be a carcinogen (cancer-causing agent).
Horse risk Although flunixin (trade name Banamine) is labeled for intramuscular use, it has potential to elicit an anaerobic infection (with Clostridium) at the IM injection site. This causes extensive sloughing of tissue around the infected area and can be life-threatening. Thomas clarifies, "Flunixin is not the only substance associated with clostridial infections. Origin of the bacteria is unknown, although some hypothesize that anaerobic spores present in normal muscle begin to colonize following intramuscular injection with an irritating substance."
To avoid complications it is best to give flunixin either intravenously or in oral form. In addition, the label-recommended dose of flunixin can mask clinical signs of a surgical condition in a colicky horse, thereby delaying appropriate and life-saving treatment. So, seek your veterinarian's advice before administering it.
Human risk Thomas notes, "Overexposure to flunixin may cause GI irritation and bleeding, or adverse kidney or central nervous system effects."
Other Anti-Inflammatory Drugs
Dimethyl Sulfoxide (DMSO)
Human risk While it is categorized as a solvent, DMSO is recognized for its potent anti-inflammatory properties that are useful for managing trauma or swelling in the horse. Past studies have suggested that cataracts can develop in humans from handling this medication; however, Thomas reports this data is inconclusive due to the small number of studies or discrepancies between them. In addition, DMSO is a teratogenic (it has the ability to disturb the health and development of a fetus), so it can be a risk for women in the first trimester of pregnancy. It is best to use nonporous gloves to avoid direct skin contact, and to administer DMSO in well-ventilated areas when possible.
Horse risk When given intravenously in concentrated proportions, DMSO can adversely affect platelet (blood cells involved in clotting) numbers.
Human risk It can be hazardous for a pregnant woman to handle injectable dexamethasone formulations due to the drug's potential to induce labor or abortion.
Corticosteroid-Containing Ophthalmic Medications
Horse risk Never use any ophthalmic (labeled for use in eyes) medication containing corticosteroids in an eye with an active corneal ulcer, as this can impair the corneal epithelium's ability to heal. A veterinarian should examine an inflamed or painful eye immediately to reach an accurate diagnosis and recommend appropriate therapy. Until then, use only ophthalmic preparations that do not contain any corticosteroid. If unsure, then don't put medication in a horse's eye.
Tamed Iodine (povidone-iodine)
Human risk Povidone-iodine (PVP-I; e.g., Betadine) is commonly used for wound care. The iodine component has the potential to adversely affect humans, as Thomas describes: "Increased serum iodine levels have been associated with metabolic abnormalities after topical povidine-iodine treatment in a human patient. Another case report showed a large amount of povidone-iodine absorbed through healthy human skin after disinfection prior to surgery. This absorption could result in thyroid dysfunction in susceptible people as well as having the potential to interfere with nuclear scintigraphy scans or with radioactive iodine treatment."
Tincture of Iodine
Horse risk Iodine tincture is applied to the bottom of a horse's feet to eliminate malodor, to toughen the soles, and to disinfect the frogs. It can be irritating if it contacts the skin above the hoof. The intense strength of 7% iodine should not be used in wounds or on a neonate's umbilicus, as it can cause inflammation and create necrotic tissue, both of which can be counterproductive to healing.
Human risk Tincture of iodine stains human skin and clothing, so apply it with care and a precise applicator.
Topical Wound Antiseptics
Horse risk Do not use topical wound ointments, salves, and gels in a horse's eye, as they could cause irreparable damage. Only use medications in the eye that are specifically formulated for ophthalmic use. It's best to dress deep wounds with water-soluble products.
Human risk Inadvertent contact with steroid-based topical medications could delay healing of a human skin wound, and prolonged use might reduce skin elasticity.
Horse and human risk Antibiotics are not always used judiciously in veterinary medicine, and inappropriate use can lead to bacterial resistance. This is not only a hazard for infection containment in a treated horse, but it also poses potential human health consequences as bacteria develop more strategies to elude our antibiotic armamentarium. Then infections become difficult to control, and even as researchers develop more classes of antibiotics, bacteria evade these products' killing effects.
Thomas remarks, "Antibiotic resistance not only creates more expensive treatment, but, in the worst case, treatment is ineffective."
She is circumspect about antibiotic use in general, noting, "Antibiotic resistance occurs not only with inappropriate use, but also appropriate use. Any exposure to antibiotics helps microorganisms evolve and develop ways to combat threats against them. Overuse, as seen when 'treating' viral infections against which they have no effect, or incorrect use, such as not finishing a full treatment course or using sub-therapeutic dosing, all lead to increased resistance."
Additionally, antimicrobial-associated diarrhea or colitis in horses, due to the overgrowth of pathogenic intestinal bacteria, are serious concerns and even can be life-threatening. Antibiotics such as tetracyclines, macrolides (e.g,. azithromycin or erythromycin), cephalosporins (e.g., Naxcel), clindamycin and lincomycin (although the two are rarely used), or potentiated sulfas can be responsible for these gastrointestinal problems.
Hyperthermia can be a life-threatening side effect of erythromycin use in foals.
Only use antibiotics when indicated and under the justifiable recommendation of a veterinarian. Ideally, a bacterial culture and antibiotic sensitivity will point to the exact drug to use, its route of delivery, dose, and duration of therapy.
In addition, a person with allergies or sensitivity to an antibiotic should take particular care in handling or administering that medication to a horse; preferably, he or she should have someone else do it. Some specific antibiotics (like chloramphenicol) have the potential to pose serious human side effects, like aplastic anemia (when bone marrow doesn't produce sufficient new cells to replenish blood cells).
Thomas recommends, "Avoid direct contact with skin, eyes, or inhalation and ingestion. Wear gloves, eye protection, and a mask when handling risky medications. If contact occurs, flush the affected area with water and seek medical attention."
Macrocyclic lactones (includes ivermectin)
Environmental risk One popular deworming medication, ivermectin, has potential environmental toxicity to invertebrates, such as dung beetles and earthworms. Thomas explains, "Ivermectin can be passed in feces in an unmetabolized form, leading to soil and/or water contamination. However, the range of effect is disputed. While fecal levels remain above MIC (minimum inhibitory concentration, the lowest concentration of an antimicrobial that will inhibit visible growth of a microorganism after overnight incubation), soil levels may not reach MIC. Direct contact with fecal matter poses a danger to invertebrates, as do adverse "sublethal" effects that impact the food source of invertebrates rather than the organisms themselves."
Human risk Progesterone is used for suppressing the estrous cycle in mares. Thomas explains, "Liquid altrenogest can be absorbed through human skin and also penetrates porous gloves." Nonpregnant women handling this product might experience dysmenorrhea (painful or difficult menstruation) with associated hormonal imbalances. People with bleeding disorders, coronary disease, or reproductive-oriented cancer also should avoid contact.
Wear nonporous gloves when handling progesterone, or use an oral spray applicator to administer your horse's daily dose. Another useful protective tip is to fill red top blood tubes (10 mL size) with a several months' supply of each daily amount (about 10 mL/day for a 1,000-pound horse). Then, pop the cap and pour on horse feed without directly contacting the medication.
Human risk This medication (cloprostenol or dinoprost, known as Estrumate or Lutalyse) is commonly used on a breeding farm to manipulate a mare's breeding cycle. Anyone with asthma or bronchial disease should handle this with the utmost care. "Pregnant women should not handle a prostaglandin or its container," Thomas adds, "nor should they work in an area or touch equipment where this drug has been used. If accidental exposure occurs, the exposed area should be washed and a physician contacted immediately."
Human risk The original use of isoxsuprine in humans was as a vasodilator to improve peripheral circulation in the limbs. There is controversy regarding the efficacy of isoxsuprine for treating navicular disease, yet many horses with heel pain seem to respond well to its use. The owner or manager grinds tablets into powder using a mortar and pestle or a coffee grinder, then he or she mixes the drug into the feed. Thomas reports, "Risk is through inhalation or drug absorption through skin. Adverse effects include pronounced allergic reactions, such as rash, hives, breathing difficulty, chest tightness, or swelling of the mouth, face, lips, or tongue; chest pain; fast or irregular heartbeat; and severe or persistent dizziness. Tablet grinding should be done while wearing a face mask to prevent accidental ingestion or inhalation of aerosolized powder." Follow these precautions if using pentoxyfylline in a similar manner, particularly if pregnant or affected with heart problems.
Human risk Albuterol is a bronchodilator that is used for human asthma patients, and it is available in a metered-dose inhaler or a tablet. Equine veterinarians prescribe it to open airways of horses with inflammatory airway disease. Thomas notes there is limited human risk from tablet contact due to minimal absorption. However, side effects might include dizziness, excitement, headache, hyperactivity, muscle cramps, nausea, nervousness, shakiness, sleeplessness, or tremors. And, people may become tolerant with chronic exposure, which can mean the inhaled drug could have a less therapeutic benefit.
Horse risk Clenbuterol is used to relax smooth muscle of the airways to provide symptomatic relief for recurrent airway obstruction (chronic obstructive pulmonary disease, or heaves). Reports suggest that veterinarians or managers should not use this drug (Ventipulmin) in pregnant mares near full term, as it might diminish normal uterine contractility necessary during labor.
Human risk Researchers have documented relay toxicity in humans consuming animals treated with clenbuterol, so take precautions when handling this drug, as well. Also, there have been reports of human athletes abusing this drug as an alternative to the body-building effects of anabolic steroids.
Other Adverse Drug Reactions
Any medication has the potential to elicit an allergic reaction in horses, ranging from mild hives or facial or abdominal swelling to severe anaphylaxis. Ones that are notorious for these occurrences include: procaine penicillin, oxytetracycline, NSAIDs, and occasionally immunization products.
Additionally, xylazine--a sedative, pain reliever, and muscle relaxant commonly used in horses--when given in moderate doses can slow the heartbeat and might cause a transient second degree heart block in horses, as well as mild respiratory depression. Other side effects are localized sweating and increased urination. Generally, these side effects wear off quickly and do not cause any lasting problem for the horse. However, this drug should be handled and used only under the advisement of a veterinarian.
Accidental human injection can depress myocardial function, with the potential for death. Another sedative, detomidine, might elicit the same kind of serious problems in people if accidentally injected. All such medications should be under lock and key away from children, and they should only be handled by knowledgeable and responsible individuals, particularly when dealing with fractious horses.
Finally, persistent use of atropine in the eyes to manage uveitis or corneal ulcers has the potential to slow a horse's intestinal motility.
Be familiar with associated side effects of commonly used medications and possible adverse drug interactions before administering therapeutic agents to your horse. Always exercise caution, follow safety guidelines, and remember that it's best to administer drugs to horses only according to the advice of your veterinarian.
About the Author
Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.
POLL: Managing Working Horses