Antibiotics (antibacterials) are drugs that kill bacteria or stop bacterial growth. Each antibiotic has a profile of characteristics that make it more or less useful for particular bacteria in a particular medical situation.

The word SYSTEMIC in this case means body-wide treatment. This is in contrast to TOPICAL (applied to skin) OPHTHALMIC (applied to eye), for example. Systemic antibiotics are given by 2 ROUTES: oral or injectable. Injectable includes intravenous (IV=in vein), intramuscular (IM= In Muscle) and (rarely in horses) subcutaneous (under skin) injection.

After giving an antibiotic by one of these routes, we expect the drug to be absorbed into the blood over a given period of time, and then to end up in different body tissues, at different levels, at different times, and then to stay in those tissues there for expected periods of time.

Commonly used systemic antibiotics in equine veterinary practice include sulfadiazine/ trimethoprim, sulfamethoxazole/ trimethoprim, procaine penicillin, gentamicin, enrofloxacin and ceftiofur. Less commonly used antibiotics include metronidazole, amikacin and chloramphenicol, among others. There are only a few antibiotics LABELED by the FDA specifically for use in horses. The rest are used in an EXTRA-LABEL fashion.

There are many other antibiotics that might be used in very specific circumstances, for resistant bacteria, but usually only based on culture and sensitivity results.

YOUR VET'S ROLE

Equine vets know the most common bacterial infections of various organ systems and can make educated guesses as to the bacteria causing them. When we talk about an antibiotic's SPECTRUM, we refer to the range of bacteria that is killed or controlled by that antibiotic. Generally, we may choose to use drugs with complementary spectra, meaning a combination of antibiotics that kills a wide variety of both gram negative and gram positive bacteria. An example of this sort of combination is sulfadiazine with trimethoprim as a single medicine, or penicillin and gentamicin given separately.
In a perfect world, our choice of an antibiotic would be determined by culture and sensitivity of the organism, but this is relatively rare. Much more, we make an educated guess about the bacteria, and then base our choice of antibiotic on that. A vet's preference is to use one of the few antibiotics labeled for equine use, but this is not always feasible.

YOUR ROLE

Don't take antibiotic use in the horse lightly! As a responsible horse person, you should understand the general benefits but also the limitations and dangers of systemic antibiotic use in horses. You should only use antibiotics in horses as directed by a licensed veterinarian. You should recognize how fragile the equine intestinal microbiome is compared to that of other species, and know that disturbance of this through use of antibiotics can mean the death of the horse.

You should also understand and respect the danger of development of bacterial resistance to antibiotics. Recognize the few antibiotics labeled for use in the horse. Other use of antibiotics is extra-label. Understand what this means. IF there is an FDA-approved antibiotic for a given use, that should be selected over an extra-label antibiotic.