Horse Side Vet Guide ®

Equine Health Resource

Antibiotics, Systemic Antibacterials & Antimicrobials



Antibiotics (antibacterials) are drugs that kill bacteria or stop bacterial growth. Today, some antibiotics are produced directly from microbes, whereas some are synthesized in laboratories. Each has a profile of characteristics that make it more or less useful for a particular medical situation.

Systemic antibiotics are given by 3 main routes: orally, intravenously and intramuscular injection. After giving an antibiotic by one of these routes, we expect the drug to be absorbed over a given period of time, and to end up in different body tissues at different levels for an expected period of time.

Commonly used systemic antibiotics in equine veterinary practice include sulfamethoxazole/ trimethoprim, procaine penicillin, gentamycin, and ceftiofur. Less commonly used antibiotics include metronidazole, enrofloxacin, amikacin and chloramphenicol.

There are dozens of other antibiotics that might be used in very specific circumstances, for resistant bacteria or based on sensitivity results for known cultured bacterial pathogens in a specific case.

Vets know the most common bacterial infections of various organ systems and can make educated guesses as to the bacteria causing them. Generally, we may choose to use drugs with complementary spectra, meaning a combination that kills a wide variety of both gram negative and gram positive bacteria. An example of this sort of combination is penicillin and gentamicin. In other cases, our choice of antibiotic is determined by culture and sensitivity.

Related DiagnosesThis Treatment Might Be Used for these Diagnoses

Wound or Laceration into Joint, Tendon Sheath or Bursa
Clostridial Muscle & Fascia Infection (Myonecrosis)
Potomac Horse Fever, Neorickettsiosis
Septic or Infected Flexor Tendon Sheath (in Adult)
Infection of Extensor Tendon Sheath
Foal or Newborn, Septic or Infected Joint or Tendon Sheath
Bastard Strangles
Foal, Umbilical Infection or Abscess
Rectal Tear
Infected Joint, Septic Arthritis (in Adult)
Sporadic Lymphangitis
Internal Ear Infections
Cholangiohepatitis, Cholelithiasis
Ulcerative Lymphangitis
Purpura Hemorrhagica, PH
Lyme Disease, Borreliosis
Meningo-Encephalitis, Meningitis
Dermatophilosis, Rain Rot
Brucellosis, Poll Evil, Fistulous Withers
Pigeon Fever
Endometritis, Chronic
Foal or Newborn, Patent Urachus
Equine Proliferative Enteropathy, EPE
Sinus Infection, Sinusitis
Mouth or Tongue Infection, Actinomyces, Actinobacillus
Cystitis, Bladder Inflammation or Infection
Retained Placenta
Guttural Pouch Empyema & Chondroids
Smoke Inhalation, Pneumonitis
Abdominal or Internal Abscess, Generally
Burn, Thermal or Fire
Pneumonia, Pleuropneumonia & Pleuritis, Generally
Liver Abscess
Foal or Newborn, Septicemia
Clostridial, Clostridium Colitis (in Adult)
Sabulous Cystitis, Urolithiasis
Neurotoxic Snakebite, Coral Snake, Cobra
Bladder & Urethral Stones
Proud Flesh
Choke, Esophageal Feed or Foreign Body Obstruction
Kidney & Ureteral Stones
Abscessed or Infected Molar, Periapical Abscess
Infarcted Intestine or Colon
Foal or Newborn, Failure of Passive Transfer
Endotoxemia, Endotoxic Shock
Duodenitis-Proximal Jejunitis, DPJ
Poisonous Snake Bite, Rattlesnake or Pit Viper
Metritis, After Foaling
Foal or Newborn, Necrotizing Enterocolitis, Bloody Diarrhea
Salmonella Colitis (in Growing Foal or Adult)
Blister Beetle Toxicity
Shoe Boil, Olecranon Bursitis
Temporohyoid Osteoarthropathy
Quittor, Infected Collateral Cartilage
Equine Canker
External Ear Infections
Cauda Equina Neuritis
Right Dorsal Colitis
Viral Upper Respiratory Tract Infections, Generally
Vesicular Stomatitis, VS
Small Strongyle Infestation
Fracture of Mandible or Lower Jaw
Foal or Newborn, Fractured or Broken Ribs
Fracture of Splint Bone
Dust Cough, Irritated Airway from Dust
Dorsal Pharyngeal Lymphoid Hyperplasia, DPLH
Fractured or Broken Ribs (in Adult)

Consider Potential Side Effects & Complications

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 function of the gastrointestinal tract. In some cases, this disruption can cause overgrowth of undesirable organisms and life threatening colitis. Horses can have allergic reactions to any drug, including antibiotics.

Consider Reasons Not To Use This Treatment

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 the antibiotic. In that case, the drug loses its effectiveness.

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 look for improved lung sounds, blood gases, and respiratory rate.

Questions To Ask My 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?
Author: Doug Thal DVM Dipl. ABVP


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