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Mirror KB Equine Article Series

Equine Drug Therapy
Part 1

by Kim and Kari Baker

     Horse owners, in the everyday care of their animals, routinely need to administer drugs in one form or another. The most common drugs we use 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 illness. Drugs such as tranquilizers or stimulants sometimes are needed under veterinary orders.

     Exactly what are drugs, and which substances should be considered drugs? Paul Schaumberg, DVM, of the Kootenai Veterinary Clinic in Libby, Mont., defines a drug as "any chemical or compound administered to provide a therapeutic benefit. However, there is a current trend in the U.S. 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."

     Certain drugs influence the horse's health by altering natural body processes, such as in the case of controlling inflammation or concealing pain. Other drugs function as soldiers, removing and defending the body against infectious organisms and parasites that invade and cause harm. If properly prescribed and administered, drugs constitute a valued armament in the treatment of many illnesses and injuries that affect the horse. 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.

     As a case in point, oral neomycin is an antibiotic that is sometimes used to treat bacterial diarrhea in foals, but it is useless in treating other infections of body tissues due to its poor absorption from the gut. Your veterinarian will know this, but a wise and conscientious owner should try to learn as much as possible about any medications they give to their horse. Why is this important? You'd be surprised when tidbits like these come in handy. For instance, if your horse has a cold and another owner offers you the leftovers of her foal's oral neomycin medication, then you'll know that this treatment would likely prove to be ineffective and, worse still, could potentially cause undesirable side-effects

     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 antiserums 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. 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

     In order for any drug to be effective, it first must be administered in the appropriate 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. Dr. W David Wilson of UC Davis in California says, "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 amocillin, ampicillin and related antibiotics." 

   Injection is another familiar route. The three major parenteral (situated or occurring outside the intestine) non-oral routes in which injections are given to the horse include subcutaneous (SQ, or under the skin), intramuscular (IM, within the muscle), and intravenous (IV, into the 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. 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 reach a very high level almost immediately when given via an intravenous injection, and in most cases will also decrease quickly as the drug is circulated throughout the body as it is subjected to metabolism and elimination. 

     The increase in concentration of an individual drug may be slower when given via an intramuscular injection (depending on whether the drug is administered as a solution or a suspension), but it may also remain in the bloodstream longer than a drug given intravenously." 

     Many drugs that are aqueous solutions are absorbed very quickly after IM injection. In contrast, Procaine Penicillin G is an aqueous suspension which persists as a depot at the injection site, leading to slower absorption and persistence for a longer period," states Wilson.  

   The absorption of a drug will be even slower following subcutaneous, 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 administered will in part depend on whether an acute, short-term duration of the drug is needed in the bloodstream, or a slower, sustained action of the drug is desired. However, not all drugs can be administered in more than one method, in which case the veterinarian and owner will only have a limited number of options for administering the drug.

      More often than not, the body's reaction to drugs can be well predicted based on previous research and clinical experience. However, there can be a biological variation in the drug's effectiveness. Not all horses respond exactly the same even when the drug is given via the same method and dosage. There are individual differences in the rate and completeness that the drug can be absorbed.

Read Equine Drug Therapy - Part 2

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