A way to improve or resolve a condition or diagnosis. This might include resolving the underlying cause or treating the signs of disease (symptomatic treatment).

Cost: Under $100

These cost ranges are approximate and may vary from region to region.
Additional charges may also apply.

YOUR VET MAY Recommend

Antibiotics, Systemic Antibacterials & Antimicrobials

Cost: Under $100

These cost ranges are approximate and may vary from region to region.
Additional charges may also apply.


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.


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.


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.

my vet's role


Related Observations

This treatment might be used for a horse exhibiting these signs.

Very Common
Less Common
more observations

Related Diagnoses

This treatment might be used to treat these conditions or ailments.

Very Common
Less Common
more diagnoses

Know Related Treatments

Your vet considers other treatments that might be used along with or instead of this treatment.

Very Common
Less Common
more treatments


Antibiotics must always be used cautiously. Horses rely on a massive population of bacteria that colonize the intestine. This population is critical for fermentation and digestion of roughage.
Antibiotics kill a percentage of that population, and can disrupt the balance of bacteria and thus the function of the gastrointestinal tract. In some cases, this disruption can cause overgrowth of the tract by undesirable organisms and life threatening colitis.

Horses can have allergic reactions to any drug, including antibiotics.

Beyond this, specific antibiotics have specific side effects that will be discussed by your vet.


Antibiotics must be used cautiously in horses that are susceptible to colitis for any reason.
Vets carefully choose antibiotics, because the more antibiotic is put into the environment, the more chance for bacteria to develop resistance to that antibiotic. If resistance develops in a bacterial population over time, the antibiotic loses its effectiveness.
Vets should choose an FDA- approved antibiotic when this makes sense for treatment.

your role

Is it working? Timeframe for effect.
This depends on the specific drug. Each drug class works in an entirely different way.

Generally, we expect to see evidence that infection is being controlled within hours to days, depending upon the specific nature of the infection.

Signs of success might include a reduced fever, less swelling, improved attitude and improvement in the specific parameters of the organ system we are treating. In the case of pneumonia, for example, we might look for improved lung sounds, improved ultrasound findings, improved blood gases, and reduced respiratory and heart rates. In the case of an infected limb (cellulitis or sporadic lymphangitis), we might hope to see reduced lameness and swelling.
Questions To Ask Your Vet:
  • What is your rationale for my horse being kept on antibiotics?
  • Is there any risk to my horse being on antibiotics?
  • What is the risk of colitis with the particular antibiotic combination you have chosen?
  • Is the antibiotic you have chosen FDA approved for use in horses, for this use?
  • Is this extra-label use of the drug?

further reading & resources


Brand Name Products

Equine health related brand name products and services.

  • Aurora Pharmaceutical
    Equisul-SDT® (Sulfadiazine/Trimethoprim)

Author: Doug Thal DVM Dipl. ABVP