For these surgery topics, the most common systemic antibiotics in horses are
Trimethoprim sulfonamide or sulfadiazine
Given orally, TMS has a relatively broad spectrum of action and penetrates into tissues well due to lipophilicity.
Crosses blood brain barrier. Elimination via renal excretion. Time dependent activity.
Side effects are rare but include hypersensitivity reactions, folate deficiency, and colitis.
Doses range from 15-30 mg/kg po every 12-24 hours.
Resistance patterns can help determine which formulation is best for a given population.
The combination of oral administration and broad spectrum activity makes it a favorite for at home care of horses. Owners are typically able to administer it. Generally it is crushed or dissolved in water and then mixed with flavoring and squirted into the horses mouth or added to the grain (some horses will not eat it in the grain).
“Baytril” can be administered iv or orally. It works through concentration dependent killing, meaning it works best when used at high levels less often. Enrofloxacin works best against gram negative organisms and some gram positive organisms (Staphylococcus). It is not very effective against anaerobes.
Highly lipophilic with high concentrations found in the liver, spleen and kidney; moderate concentrations are found in most tissues; mild concentrations in CSF and eyes.
Elimination is primarily in the urine.
Enrofloxacin can cause joint disease in young animals as well as diarrhea and neurological disease in all groups. Recent research suggests it is safe for use in pregnant mares after the first trimester.
Generally, enrofloxacin should be reserved for cases with known resistance to other drugs or for cases in which other drugs have been shown ineffective.
- 5mg/kg IV q24h or 5-7.5mg/kg PO q24h
Procaine penicillin G
PPG is given intramuscularly and can cause anaphylactic reactions and death if given iv. It is most effective against gram positive agents and anaerobes.
Distributes widely in the plasma but penetrates poorly through biological membranes. Does not penetrate abscesses or sites of tissue necrosis well.
Elimination is via renal tubular secretion.
Time dependent activity.
The label dose is ineffective. Therapeutic and effective preventive doses start at 22,000 IU/kg.
It is cheap but does require im administration which can be tricky for some owners and with some horses.
Gentamicin is generally given iv. It is effective against gram negative agents.
It is not effective in acidic (pus filled) or anaerobic environments and does not penetrate into tissues. It must be carried by the vascular system.
Elimination occurs via glomerular filtration
Concentration dependent activity.
Side effects are primarily renal damage. Be careful using gentamicin in dehydrated animals, particularly in combination with other renal toxic drugs (NSAIDs). Older references may suggest 2.2 mg/kg TID but it is safer to administer at 6.6 mg/kg SID as this creates the most bacterial kill with the least side effects.
Gentamicin is rarely given by owners; it is typically administered by veterinarians or in hospital settings.
Amikacin is primarily used as an agent for iv regional perfusion (local antibiotic therapy) and in foals. It is otherwise too expensive. As with gentamicin, it targets gram negative organisms and side effects are primarily renal damage.
- Adults: 10-20 mg/kg IV q24h
- Foals: 20-25mg/kg IV q24h
“Excede” is the most common formulation used today. It has good broad spectrum coverage into tissues but does not enter the CSF in adequate levels.
Hydrolyzed by the liver, eliminated by the kidneys.
Time dependent activity.
A single intramuscular dose lasts 4 days. The most common side effect is diarrhea, which can be severe. “Naxcel” is often used in hospitals but must be given daily for treatment.
Due to the prolonged effect, Excede is a favorite drug for castration surgeries – it covers prophylactic and treatment needs.
Dose ranges for ceftiofur:
- Adults 2.2-4.4 mg/kg im 24h (not more than 10 days of treatment)
- Foals 10mg/kg im, iv or sq, q6-12h
- CRI 2.2 mg/kg iv bolus, then 12 ug/kg/min
Dose ranges for Excede: 6.6 mg/kg im at 0 and 96h
Other antimicrobials are used for specific diseases or disorders.
See Perioperative medications for timing information and duration of therapy.
Overview of the use of antimicrobial drugs for the treatment of bacterial infections in horses, EVE 2021. Discusses choice, appropriate use and misuse, as well as bacteria+ drug combos
Principles of Antimicrobial Therapy: What Should We Be Using? Vet Clin Equine 22 (2006) 279–296- good review of surgical use of antibiotics
Adverse Effect of Antibiotics Administration on Horse Health: An Overview – JEVS 2021 – nice overview of drug actions, effects, and side effects
OSU VMC Antimicrobial use guidelines – great place to start when choosing a drug
Veterinary Advances: equine drugs (untested)
Vin.com drug formulary (membership is free for students)