Key Insights
  • Erythromycin is an extra-label macrolide antibiotic used mainly in foals with Rhodococcus equi pneumonia
  • Erythromycin is traditionally combined with rifampin when treating R. equi pneumonia
  • Veterinarians may also use erythromycin for equine proliferative enteropathy caused by Lawsonia intracellularis
  • Macrolides are generally avoided in adult horses because they can cause severe and potentially fatal colitis
  • Important side effects in foals include diarrhea, reduced sweating, hyperthermia, and respiratory distress
  • Mares must be protected from erythromycin residue while their foals are receiving treatment

Erythromycin is an antibiotic used primarily in young foals with certain bacterial infections. Its best-established use is treating Rhodococcus equi pneumonia, or rattles, traditionally in combination with rifampin. Veterinarians may also prescribe it for equine proliferative enteropathy caused by Lawsonia intracellularis.

Although erythromycin can reach bacteria that survive inside the horse’s cells, its use carries significant risks. Macrolides are generally avoided in adult horses because they can disrupt the intestinal microbiota and cause severe colitis.

All uses of erythromycin in horses are extra-label and require veterinary supervision. Horse owners caring for a treated foal should understand how the medication is administered, which side effects require prompt attention, and how careful antibiotic use helps limit resistance.

This article is intended for educational purposes only. The information presented here is not a substitute for veterinary advice.

Erythromycin for Horses

Erythromycin is a macrolide antibiotic used primarily in foals to treat Rhodococcus equi pneumonia (rattles). It is usually part of combination treatment, prescribed concurrently with rifampin. It may also be used to treat equine proliferative enteropathy (EPE) caused by Lawsonia intracellularis infection. [1]

This medication requires particular caution in horses. Veterinarians usually do not prescribe macrolides to adult horses because they can disrupt the intestinal microbiota and cause severe, potentially fatal colitis. Foals can also develop side effects, including diarrhea, reduced sweating, hyperthermia, and respiratory distress. [1][2]

Erythromycin also affects gastrointestinal movement independently of its antibiotic action. It stimulates motilin receptors, which can increase gastric emptying and hindgut movement. However, its usefulness as a prokinetic medication is limited, and the effect may decrease with prolonged use. [1]

Erythromycin use in horses is less common with increased availability of newer macrolides, including clarithromycin and azithromycin. [1][2]

Erythromycin is a prescription-only medication and may only be legally obtained and administered under the direction of a licensed veterinarian. Antibiotic resistance is a serious concern in veterinary medicine. Giving antibiotics without veterinary oversight is not safe and contributes to resistant bacteria, making future infections harder to treat in the same horse and across the wider horse population. Resistant bacteria can also affect other animals and, in some cases, people, which is why antibiotics should only be used when prescribed by a veterinarian and when they are likely to be effective.

illustration of erythromycin molecule

Available Forms

There are no erythromycin products specifically approved for use in horses. Veterinarians use formulations for human or other species use in an extra-label manner. [2] Extra-label use is permitted under veterinary prescribing regulations.

Erythromycin is available in several formulations. Because stomach acid can break down the drug, some oral formulations are designed to improve its stability and absorption. [2][3]

Available oral and injectable formulations include: [2]

  • Erythromycin base
  • Erythromycin estolate
  • Erythromycin ethylsuccinate
  • Erythromycin lactobionate

Injectable veterinary erythromycin products are approved for use in cattle, sheep, and swine rather than horses. [2]

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Drug Class: Macrolide Antibiotics

Macrolides are named for the large lactone ring in their chemical structure. Erythromycin is a naturally occurring macrolide, while clarithromycin and azithromycin are closely related derivatives. These are the macrolides most commonly used in horses. [1]

Macrolides spread widely through body tissues and accumulate inside cells. This property helps them reach bacteria that survive within the horse’s cells, where some other antibiotics used in horses may not reach effective concentrations. [1][3]

Macrolides are usually bacteriostatic, meaning they stop susceptible bacteria from growing and multiplying rather than killing them directly. At higher concentrations, some macrolides, including erythromycin, may also kill susceptible bacteria. [2]

Mechanism of Action

Erythromycin interferes with bacterial protein production. It binds to ribosomes within bacteria cells, where it disrupts the movement of the growing peptide chain during protein synthesis. [1][3]

As a result, the ribosome cannot continue assembling proteins normally. This limits production of enzymes, structural proteins, and other molecules the bacteria need to maintain normal cellular functions and continue multiplying.

Susceptible bacteria are unable to sustain normal growth while exposed to effective concentrations of erythromycin. [1][3]

Uses in Horses

All equine uses of erythromycin are extra-label. Extra-label, also called off-label, use means that a drug is used in a species, for an indication, or according to a regimen not included on an approved product label.

Treatment with erythromycin in horses must be directed by a veterinarian. [2]

Antibiotic Spectrum

Erythromycin is most active against gram-positive bacteria, with more limited activity against gram-negative organisms. Its spectrum includes various aerobic and anaerobic bacteria as well as several intracellular and atypical pathogens. [2][3]

Susceptible organisms may include: [2][3][4]

  • Staphylococcus spp.
  • Streptococcus spp.
  • Rhodococcus spp.
  • Clostridium spp.
  • Fusobacterium spp.
  • Pasteurella spp.
  • Actinobacillus spp.
  • Mycoplasma spp.
  • Chlamydia spp.
  • Rickettsia spp.

Some non-enteric gram-negative organisms, including Pasteurella and Actinobacillus species, may be susceptible. However, many enteric gram-negative bacteria, including E. coli and Klebsiella, as well as Pseudomonas, are generally resistant to erythromycin. [2][3][4]

Erythromycin activity may also be reduced in acidic environments, including abscesses. Whenever possible, antibiotic selection should be guided by bacterial culture and susceptibility testing to determine whether the infecting organism is likely to respond. [3]

Antibiotic resistance is a growing concern in veterinary medicine. Using antibiotics without veterinary guidance or when they are not needed can make infections harder to treat across the broader population of horses and livestock. Veterinary oversight is essential for all antibiotic treatment.

Rhodococcus equi Pneumonia (Rattles)

The best-established equine use of erythromycin is treating Rhodococcus equi pneumonia, or rattles, in foals. R. equi is an intracellular gram-positive bacterium that most commonly causes chronic bronchopneumonia in young foals. Because it can survive inside macrophages, a type of immune cell, effective treatment requires antibiotics that can reach bacteria inside cells and infected lung tissue. [5]

Erythromycin is commonly used with rifampin for this condition. The combination became a conventional treatment because both drugs penetrate macrophages and have activity against R. equi. Laboratory testing has demonstrated additive or synergistic activity between erythromycin and rifampin. [5]

Combination treatment also matters for antimicrobial stewardship. Rifampin should not be used alone because resistance can develop rapidly. [2]

Newer macrolides have increasingly replaced erythromycin for treating rattles in foals. A veterinarian may select clarithromycin or, less often, azithromycin because these drugs behave differently in the body and are associated with fewer serious side effects. [1][2]

Lawsonia intracellularis Infection (EPE)

Erythromycin may be used in foals with equine proliferative enteropathy caused by Lawsonia intracellularis. EPE is an intestinal disease of young horses that causes thickening of the small intestinal lining. [6]

Signs of EPE in foals include: [6]

Erythromycin may be administered alone or with rifampin in cases of EPE. [2][4]

Potomac Horse Fever

Erythromycin combined with rifampin has been used to treat Neorickettsia risticii infection, the cause of Potomac Horse Fever.

Clinical signs associated with PHF include: [7]

  • Fever
  • Diarrhea
  • Lethargy
  • Reduced appetite

Oxytetracycline and doxycycline are more commonly used for this condition. [1]

Any foal with fever, diarrhea, or other signs of gastrointestinal illness requires prompt veterinary evaluation.

Administration Routes

Erythromycin is generally administered orally to foals. For R. equi pneumonia, it is commonly given with oral rifampin.

Dosage

The prescribed dosage of erythromycin varies and depends on individual horse factors including:

  • Age
  • Body weight
  • Type of infection
  • Concurrent medications
  • Overall health or concurrent medical conditions

Treatment of R. equi pneumonia is often prolonged and may continue for several weeks, depending on the foal’s clinical response and resolution of pulmonary disease. [2][5]

Medication dosages must be determined by a licensed veterinarian based on the individual horse’s condition. Incorrect dosing can cause serious harm. Always follow the prescription label exactly and never adjust a dose without professional direction. It is important to complete the full course of antibiotics as prescribed, even if the horse appears to be improving. Failure to complete a full course of antibiotics can lead to relapse and contributes to antimicrobial resistance.

Storage

Storage requirements vary by formulation. Erythromycin base tablets and capsules are generally kept at room temperature in tightly closed containers, while estolate preparations should also be protected from light. Oral suspensions may be refrigerated to help maintain palatability. [2]

Erythromycin lactobionate powder for injection is also stored at room temperature before reconstitution. After reconstitution, stability varies with the storage temperature and dilution conditions, and the drug may lose activity in acidic solutions. [2]

Safety Warnings & Precautions

Mares can develop severe and sometimes fatal colitis after being exposed to erythromycin while their foals are receiving treatment. Exposure may occur through contaminated feed or water containers or through contact with drug residue associated with the foal’s treatment. [2]

Treated foals should be managed carefully to minimize the transfer of macrolide residue to the mare. Equine antimicrobial-use guidance recommends rinsing the foal’s mouth after administration.

Foals receiving erythromycin should also avoid prolonged exposure to direct sunlight because the drug can impair sweating and increase the risk of hyperthermia.[4]

Antibiotics can disrupt the balance of beneficial bacteria in a horse’s intestinal tract. This imbalance can interfere with digestive function and allow harmful bacteria to multiply, potentially leading to diarrhea. Although one large equine study found that the overall prevalence of this complication is low, some affected horses developed serious complications, including death. [8]

Gut dysbiosis does not always occur with obvious diarrhea. In healthy horses treated experimentally with other commonly used antimicrobials, fecal bacterial communities changed despite normal appetite, fecal consistency, and physical examination findings. Such changes may reduce beneficial gut bacteria and contribute to inflammation and diarrhea. [9]

Prudent antimicrobial use is especially important in horses. Antibiotics should be used when bacterial infection is known or suspected, with drug selection, dose, and treatment duration chosen to maximize efficacy while limiting side effects and antimicrobial resistance. [8]

Monitoring

Foals receiving erythromycin should be monitored for treatment response and side effects. Important observations include:

For R. equi pneumonia, monitoring may also include physical examination, bloodwork, thoracic ultrasonography, or radiography. Treatment duration is set by the prescribing veterinarian. [5]

It’s important for horse owners to follow the prescription dosage and schedule, even if the horse seems to be improving. Do not stop administration of antibiotics unless instructed to do so by a veterinarian.

Erythromycin may interfere with some laboratory methods used to measure ALT and AST, resulting in falsely increased values. Veterinarians consider this potential effect when interpreting bloodwork from a horse receiving erythromycin. [10]

Antibiotic Resistance

Macrolide resistance can occur through changes to the bacterial ribosomal binding site, active removal of the drug from bacterial cells, or enzymatic inactivation. Resistance to one macrolide may also result in cross-resistance to other drugs in the class. Macrolide-resistant R. equi isolates have been reported. [3]

A 10-year evaluation of R. equi isolates found increasing minimum inhibitory concentrations for erythromycin and rifampin, which indicates these drugs are losing effectiveness against this species over time. [5]

Horse owners play an important role in preventing resistance. Never give leftover antibiotics, share medications between horses, change the dose, skip doses, or stop treatment early unless your veterinarian directs you to do so. Completing the prescribed treatment plan and following veterinary instructions helps protect your horse, other horses, and the long-term effectiveness of important antibiotics.

Contraindications

Contraindications refer to circumstances in which a drug should be avoided or used with caution.

The most important restriction is that erythromycin should generally not be administered to adult horses. Adult horses are particularly susceptible to severe gastrointestinal disturbances caused by macrolides, including antimicrobial-associated colitis. This complication can be fatal. [3]

Caution is recommended in horses older than four months unless R. equi infection has been confirmed. [2]

Macrolides should also be avoided in horses with known hypersensitivity to these drugs. Erythromycin estolate warrants additional caution in patients with pre-existing liver dysfunction. [2]

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Side Effects

Foals receiving erythromycin may develop:

  • Diarrhea or enterocolitis
  • Reduced or absent sweating
  • Hyperthermia
  • Rapid breathing
  • Respiratory distress
  • Gastrointestinal upset

Hyperthermia is an important side effect in treated foals, particularly during hot weather. Erythromycin-associated impairment of sweating can interfere with normal temperature regulation. Provide shade, monitor the foal closely, and protect it from excessive environmental heat during treatment. [2][3]

Signs that require prompt veterinary assessment include: [2][3]

  • Diarrhea or a marked change in manure consistency
  • Colic or abdominal discomfort
  • Reduced or absent sweating
  • High body temperature
  • Rapid or difficult breathing
  • Severe lethargy or weakness
  • Worsening infection
  • Any sign of diarrhea or colitis in an adult horse exposed to erythromycin

Acute Toxicity (Overdose)

Erythromycin is less likely to cause overdose than its recognized side effects, although these can be severe in horses. [2]

If overdose or toxicity is suspected, immediately stop administration and contact your veterinarian without delay. Safely move the horse to a quiet area only if practical, ensure water is available unless otherwise directed, and provide the medication container and treatment records to your veterinary team.

Drug Interactions

Erythromycin can inhibit certain liver enzymes, which may slow the breakdown of some medications. Interactions have been described with theophylline, midazolam, and other drugs metabolized by the liver. [1][2]

Erythromycin may also interact with other antibiotics with a similar mechanism of action, including chloramphenicol, clindamycin, and lincomycin. Competition at the binding site may reduce antimicrobial activity, although the clinical importance of some of these interactions is uncertain. [3]

Sucralfate may decrease erythromycin absorption. Veterinary guidance recommends separating the two medications by approximately two hours when both are prescribed. [10]

Rifampin deserves special attention because it is frequently administered with erythromycin in foals with R. equi. Although the drugs are commonly used together, rifampin can reduce absorption of some macrolides. Equine pharmacology guidance recommends separating the administration of rifampin and macrolides as much as is practical. [1]

If a drug interaction is not listed by the manufacturer, it does not mean no interaction exists. Always notify your veterinarian about all medications and supplements your horse has received before starting treatment with a new medication.

Regulatory Status and Legal Considerations

There are no erythromycin products specifically approved for equine use. Its use in horses is extra-label and requires veterinary oversight. [2]

Extra-label use is permitted under veterinary prescribing regulations. In foals, erythromycin may be selected when the infecting organism, clinical condition, and expected benefits support its use.

Because erythromycin is an antibiotic, antimicrobial stewardship remains essential. Treatment should be based on a veterinary diagnosis and, when practical, culture and susceptibility testing.

The drug, dose, treatment duration, and combination with other antimicrobials should be selected to maximize the likelihood of effective treatment while limiting side effects and the development of antimicrobial resistance.

Status in Competition

Erythromycin is not listed as a Banned Substance or Controlled Medication on the FEI Equine Prohibited Substances List. [11] Erythromycin is also absent from the FEI List of Detection Times. [12]

The FEI distinguishes detection time from withdrawal time. The treating veterinarian should determine an appropriate withdrawal period and safety margin for the individual horse and its circumstances. [12]

Always check the current regulations for your sport before starting your horse on a new medication.

Frequently Asked Questions

Here are some frequently asked questions about erythromycin use in horses:

Summary

Erythromycin is a macrolide antibiotic used primarily in foals with bacterial infections, especially Rhodococcus equi pneumonia and Lawsonia intracellularis infection. It requires veterinary supervision because macrolides can cause serious adverse effects in horses, including potentially fatal colitis in adults.

  • Erythromycin is commonly combined with rifampin to treat Rhodococcus equi pneumonia in foals
  • It is most active against gram-positive bacteria and certain intracellular pathogens
  • The drug is generally avoided in adult horses because of the risk of severe diarrhea and colitis
  • Treated foals may develop diarrhea, reduced sweating, hyperthermia, or respiratory distress
  • Foals receiving erythromycin require protection from excessive heat and close monitoring during treatment
  • All equine use is extra-label, and a veterinarian must direct dosing and treatment
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References

  1. Cole. C. et al. Ed. Equine Pharmacology. Wiley-Blackwell. 2015.
  2. Budde. J. A. and McCluskey. D. M. Plumb's Veterinary Drug Handbook, 10th Edition. Wiley-Blackwell. 2023.
  3. Mercer. M. A. Macrolide Use in Animals. Merck Veterinary Manual. 2026.
  4. The Ohio State University College of Veterinary Medicine. OSU VMC Antimicrobial Use Guidelines. The Ohio State University. 2018.
  5. Buckley. T. et al. Resistance studies of erythromycin and rifampin for Rhodococcus equi over a 10-year period. Irish Veterinary Journal. 2007.
  6. Stewart. A. J. Bacterial Diarrhea in Foals. Merck Veterinary Manual. 2026.
  7. Stewart. A. J. Potomac Horse Fever. Merck Veterinary Manual. 2024.
  8. Barr. B. S. et al. Antimicrobial-associated diarrhoea in three equine referral practices. Equine Veterinary Journal. 2013.
  9. Liepman. R. S. et al. Effects of Intravenous Antimicrobial Drugs on the Equine Fecal Microbiome. Animals. 2022.
  10. Brooks. W. Erythromycin (Ery-tab, Ery-Ped, Eryc). Veterinary Partner, VIN. 2024.
  11. Federation Equestre Internationale. 2026 Equine Prohibited Substances List. Federation Equestre Internationale. 2026.
  12. Federation Equestre Internationale. FEI List of Detection Times. Federation Equestre Internationale. 2024.