When medicating a horse, always understand the function of the chemicals you plan to use, how each drug works, its intended use, and what it will and won't do.
Photo: Anne M. Eberhardt/The Horse
Horse owners, in the everyday care of their animals, routinely need to administer medications in one form or another. The most common range from liniments and leg braces, to diverse anthelmintics for controlling internal parasites, to regular application of fly repellents. In addition to this familiar routine, the horse owner often finds it necessary to administer more complicated medicines when faced with preventing disease or treating injuries, infections, or illnesses. Drugs such as tranquilizers or stimulants sometimes are needed under veterinary orders.
Just what are drugs, and what substances are considered drugs? Paul Schaumberg, DVM, says a drug is "any chemical or compound administered to provide a therapeutic benefit. However, there is a current trend in the United States to exclude plant and animal parts that are given in a whole organic form. The Food and Drug Administration classifies only the purified and quantified compounds extracted from plant and animal parts--or laboratory synthesized copies or enhancements--as drugs. The whole form, which often contains many active and inactive biochemical substances, is called a nutraceutical."
Some drugs influence health by altering natural body processes, such as by controlling inflammation, or concealing pain. Other drugs remove infectious organisms and parasites before they can cause harm. If properly prescribed and administered, drugs are a valued weapon against many equine illnesses and injuries. It is the owner's mission to make sure that any drug he or she administers is given at the right dosage, administered by the correct route, and is suitable for the horse's condition as determined by a veterinarian.
Safe Drug Management
- Consult your veterinarian if you're unsure of a health concern and its appropriate treatment.
- Drugs should be stored far out of reach of children, away from exposure to direct sunlight, and within the temperature range suggested by the manufacturer.
- Always note the expiration date on drug labels and don't use any product that has expired or is discolored. Even if the expiration date is okay, if it looks odd in any way don't use it!
- Always administer drugs according to the manufacturer's or veterinarian's directions, whether topically, orally, or by injection. Some drugs can only be given intramuscularly, such as procaine penicillin and most vaccines. Others, like the injectable phenylbutazone (Bute), are very irritating to the muscles and should only be given through an intravenous injection.
- Know how to administer drugs properly. Get help from your veterinarian if you are unsure of the proper sites and methods used when administering an injectable medication.
- Use proper hygiene when giving an injectable medication--avoid contaminating the bottle. Cleansing of the injection site is also important to avoid contaminating underlying tissues.
- Never mix products unless specifically recommended by the manufacturer.
- Do not save partially used vaccine vials for later use.
- When using a multiple-dose vial, never enter the container with an unsterile needle.
- Dispose of used containers, vials, needles, and syringes properly. Do not leave them where they will become a hazard to man or animal. (Beware of needles dropping in bedding!)
- Along with the product's caution statement for appropriate handling, read the directions for the drug's proper use and method of administration, then follow them exactly.
- Learn about the possible side effects of each drug you intend to use and what emergency steps you must take if your horse experiences a reaction to the drug.
Kim and Kari Baker
Case in point--oral neomycin is an antibiotic that is sometimes used for treatment of bacterial diarrhea in foals, but is useless in treating other infections of body tissues due to its poor absorption from the gut. Your veterinarian will know this, but a conscientious owner should learn as much as possible about any medications given to the horse. (This is one more reason for owners not to use "leftover" medications on their horses; the drug might not work on the problem, or could cause more problems.)
Another important thing to know might be how a drug is made. A drug can be acquired in a variety of ways, but is it important to know exactly how a drug is derived? "FDA-approved drugs can be derived from plants, then purified, while others are by-products of bacteria or fungi. Others are synthetically formed in laboratories, and others are genetically engineered proteins," says Schaumberg. "In such, safety, efficacy, absorption, and excretion are well determined. How a drug is derived would matter more if you are using non-FDA-approved 'drugs' like herbals and other naturopathic (using natural agents) preparations."
Iron preparations, such as those used in treating anemia, are derived from mineral-based agents, as are electrolytes and certain astringents, disinfectants, and laxatives.
Hormones such as insulin and various reproductive agents, along with antisera such as tetanus antitoxin and vitamins A and D, come from animal sources.
Plants of all sorts have provided mankind with medicinal therapy since the time of primitive man. And as such, a wide variety of chemicals that are naturally produced by plants are also used in equine medicine. One such chemical is atropine, best known as a treatment for organophosphate poisoning and often used as a topical treatment for eye injuries. Sarapin, which has been used as a treatment of neuromuscular pain, is a plant extract distilled from Sarracena purpurea, better known as the pitcher plant. Veterinarians sometimes use plant flavonoids such as hesperidin and citrus bioflavonoids to control equine nose bleeds. Antibiotics have generally been defined as that which is produced by living microorganisms (bacteria), but many antibiotics can also be derived synthetically in the lab.
Administration and Absorption
For any drug to be effective, it first must be administered in the right form, dosage, and route of administration. Besides topical treatments, the oral route is one of the most common methods of administering a drug, as it is reasonably easy and unlike injection sites, it does not require a sterile (or some semblance of sterile) preparation of the site. However, due to slow absorption from the gastrointestinal tract, the drug might require an hour or longer to take effect. Unfortunately, many drugs that are well absorbed from the gastrointestinal tract of people or dogs are not absorbed sufficiently well when given orally to horses to be of therapeutic benefit. Examples include ampicillin and related antibiotics.
Injections are another familiar route. The three major routes in which injections are given to the horse include subcutaneous (SQ, or under the skin), intramuscular (IM, into a muscle), and intravenous (IV, into a vein). Depending on the chosen route of administration, the practitioner will encounter varying results based on the availability of the drug to the target tissues.
"From the administration site, a drug is able to enter the bloodstream and circulate throughout the body due to its ability to pass through the walls of small blood and lymph vessels. The rate a drug will take effect is determined by the site of administration, the drug used, and its ability to be absorbed," explains Schaumberg.
"Once administered, the fate of a drug can be subdivided into four steps," he adds. "These in order of action are absorption, dispersion (distribution), breakdown (metabolism), and elimination. Once a drug is absorbed into the bloodstream, it can either be distributed evenly throughout the body, concentrated within specific tissues, restricted from entry into certain regions of the body, or use a mixture of the three."
The concentration of a drug in the bloodstream will spike almost immediately when given via IV injection, and in most cases will also decrease quickly as the drug is circulated throughout the body, metabolized, and eliminated (see "Drug Concentrations in Blood with Various Routes of Administration" on page 34). The increase in concentration of a drug will generally be much slower when given via IM injection (depending on whether the drug is administered as a solution or a suspension), but it will also remain in the bloodstream longer. (Many aqueous solutions are absorbed very quickly after IM injection. In contrast, some medications are aqueous suspensions that persist as depots at the injection site, leading to slower absorption and persistence for a longer period.)
Medication absorption will be even slower following SQ, intradermal (into the skin), or oral methods of administration, yet the drug will be available in the bloodstream for a longer time than with the previous two injection methods. Therefore, the route by which the drug is given will in part depend on whether an acute, short-term duration or a slower, sustained action of the drug is needed in the bloodstream. However, not all drugs can be administered in more than one method, in which case the veterinarian and owner will have limited options for administering the drug.
Usually the body's reaction to drugs can be well predicted based on previous research and clinical experience. However, there can be biological variation in the drug's effectiveness. Not all horses respond exactly the same even when a drug is given via the same method and dosage. There are individual differences in the rate and completeness that the drug can be absorbed.
"The age of the horse is often a factor," says Schaumberg. "Drug dosage in adults and foals is ascertained by weight, but since foals can't metabolize or eliminate drugs as effectively, the drug dosage is usually slightly reduced. A decrease in drug dosage might also be called for in the geriatric horse due to the slowing of body functions.
Another factor that can affect the horse's responsiveness to a drug is the condition of the digestive system. When administering an oral drug, the extent of absorption will differ depending on conditions in the gut.
"The pH (acidity) of the stomach acid varies within a normal range in a horse," says Schaumberg. "Diet, hormonal influences on stomach mucosa, stress, and many other factors affect stomach pH. A one-point change in pH can dramatically influence the stomach's absorption of a drug." An example of this might be if the horse has diarrhea. The effectiveness of the drug will generally be decreased due to the rapid rate of passage through the gut. On the flip side, constipation might enhance the absorption of the drug due to slowed movement through the intestinal tract.
"Drugs are also excreted through the kidneys, as well as through sweat and exhalation, and can be excreted whole or broken down as metabolites," he adds. "For instance, urinary excretion is also affected by pH. Acidic urine increases the excretion of basic drugs, while a more alkaline urine readily excretes drugs with a low pH." (Many drugs are metabolized by the liver and eliminated in bile or recirculated through the kidneys for elimination via urine.)
Other factors that must be considered when you're selecting a drug and the proper dosage for that particular horse include determining the horse's temperament, general health, body lean to fat ratio, and whether the drug is safe for a pregnant mare (if that's the intended use).
"You should know the range of dosage in accordance with the horse's weight and age and adjust up and down according to the factors that you're aware of," notes Schaumberg. "This is why doing a blood work-up to establish cholesterol, triglyceride, or albumin (proteins soluble in water) levels allows the veterinarian to make a more informed decision." Evaluation of liver and kidney function are also valuable.
While many drugs can be safely used together, it can be dangerous to the horse to give more than one drug. Thus, when giving any drug, it's very important to understand how it will affect the horse in the presence of an additional drug. "There are literally hundreds of drug interactions," Schaumberg says. "An antibiotic like tetracycline is bacteriostatic (slows growth) and will interfere with an antibiotic like penicillin that is bactericidal (kills only growing bacteria). Drugs, like Lasix (a diuretic now named Salix), will cause any drug that is excreted by the kidneys to be excreted faster."
Keep diligent records of drug usage handy so they can be referenced before giving any medication. If there is a question of drug compatibility, consult your veterinarian before giving a second drug (including supplements and dewormers). For example, any promazine tranquilizer can't be used safely if a horse has recently been dewormed with a piperazine, phenothiazine, or organophosphate product, as these will significantly increase the risk of toxicity.
With only a few exceptions, drug dosage is presented as a weight unit of the drug for each weight unit of the horse. Keep in mind formulations of like drugs are not always equal. They can vary in the concentration of active and inactive ingredients depending on the manufacturer of the drug. In particular, concentrations vary in injectables, such as the vitamin B complexes and various antibacterial agents.
When medicating a horse, always understand the function of the chemicals you plan to use, how each drug works, its intended use, and what it will and won't do. Medications should never be used for a purpose other than what they are designated to do unless prescribed by a veterinarian. At no time should a drug be used in an indiscriminate manner, on a horse other than the one originally on the prescription, or on other animals or humans.
Adams, R.H.; Chalkley, L.W.; Buchanan, T.M. Veterinary Treatments & Medications for Horsemen. Equine Research Publications, 1977.
Kellon, E.M. Equine Drugs and Vaccines. Breakthrough Publications, 1995.
About the Author
Kim and Kari Baker are equine photographers as well as writers. The twins live on a ranch in northwest Montana and have been in the equine industry for more than 35 years, raising, showing, and training Appaloosas.
POLL: Horse Height