Erythromycin

Erythromycin is a macrolide antibacterial with activity mainly against susceptible gram-positive organisms and selected respiratory or intracellular pathogens. It has been used for Rhodococcus equi infection in foals and various livestock infections. At lower exposures, its separate motilin-receptor activity has been used to promote gastrointestinal motility in selected patients.

Group

Macrolide antibacterial and motilin agonist

Mechanism of Action

Erythromycin binds the bacterial 50S ribosomal subunit and interferes with translocation of the growing peptide chain, suppressing protein synthesis and usually producing bacteriostatic activity. Tissue and intracellular distribution support activity against some intracellular pathogens. Independently of antibacterial action, stimulation of motilin receptors can increase coordinated upper gastrointestinal contractions and promote gastric or abomasal emptying.

Dose Rates

Dog

Use / stage	Dose	Route	Schedule	Instructions
Bacterial infection	10–20 mg/kg	PO	Every 8–12 h
Prokinetic adjunct	0.5–1 mg/kg	PO, IV	Every 8–12 h	IV use requires a suitable soluble salt.

Cat

Use / stage	Dose	Route	Schedule	Instructions
Bacterial infection	10–20 mg/kg	PO	Every 8–12 h
Prokinetic adjunct	0.5–1 mg/kg	PO, IV	Every 8–12 h	IV use requires a suitable soluble salt.

Horse

Use / stage	Dose	Route	Schedule	Instructions
Rhodococcus equi infection	37.5 mg/kg	PO	Every 12 h	Foals; phosphate or estolate formulation, often in a defined rifampin combination.
Rhodococcus equi infection	25 mg/kg	PO	Every 8 h	Alternative foal regimen; phosphate or estolate formulation.
Bacterial infection	5 mg/kg	IV	Every 4–6 h	Lactobionate injection only.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Abscesses or pododermatitis	2.2–8.8 mg/kg	IM	Every 24 h
Pneumonia	2.2–8.8 mg/kg	IM	Every 24 h
Pneumonia	15 mg/kg	IM	Every 12 h
Rumen motility	8.8 mg/kg	IM		Experimental calf regimen.
Rumen motility	10 mg/kg	IM		Cows after surgery for left displaced abomasum.

Ferret

Use / stage	Dose	Route	Schedule	Instructions
Susceptible bacterial infection	10 mg/kg	PO	Every 6 h

Bird

Use / stage	Dose	Route	Schedule	Instructions
Susceptible bacterial infection	60 mg/kg	PO	Every 12 h	Oral suspension regimen.

Ratite

Use / stage	Dose	Route	Schedule	Instructions
Susceptible bacterial infection	5–10 mg/kg	PO	Every 8 h

Sheep

Use / stage	Dose	Route	Schedule	Instructions
Respiratory bacterial infection	2.2 mg/kg	IM	Every 24 h	Older animals.
Neonatal dysentery prevention	Reference only	IM		The source prints 123 mg/kg once soon after birth, an unusually high amount relative to its other regimens. The dose requires clarification before calculation.

Indications

Treatment of susceptible bacterial respiratory, skin and soft-tissue infections in veterinary patients with an appropriate species-specific formulation.
Combination treatment of Rhodococcus equi infection in foals, traditionally with rifampin.
Prokinetic treatment of selected gastric or abomasal hypomotility disorders, including some cattle after correction of displaced abomasum.
Specialist adjunctive treatment of selected equine gastrointestinal hypomotility, with limited clinical efficacy evidence.

Contraindications

Do not give orally to rabbits or rodents.
Never inject an IM formulation IV.

Interactions

Inhibits drug-metabolizing enzymes and can raise concentrations of coadministered substrates.

Adverse effects

Vomiting in small animals
Severe diarrhea in horses and exposed nursing mares
Hyperthermia in foals

Preparations

Preparation	Form	Strength	Route
Erythromycin base 250 mg	Tablet	250 mg / tablet	PO
Erythromycin base 500 mg	Tablet	500 mg / tablet	PO
Erythromycin ethylsuccinate suspension 40 mg/mL Salt/ester-labelled strength; confirm base-equivalent label before conversion.	Oral Liquid	40 mg / mL	PO
Erythromycin phosphate powder (base equivalent) 260 mg phosphate per gram explicitly equivalent to 231 mg erythromycin base.	Powder	231 mg / g	PO

Monitoring and Administration

Monitor GI tolerance, temperature and infection response. Only an appropriate lactobionate/gluceptate formulation is suitable IV.

Base tablets are poorly absorbed in horses; the formulation matters. Prokinetic efficacy is better documented experimentally than in clinical patients.

Equine GI-motility dose of 1 mg/kg is described without a fully specified route/schedule; retained outside calculator dosing.

Intramammary protocols require distinct dry-cow and lactating-cow products: dry-cow treatment is once at drying off; the lactating regimen repeats after each of three milkings. The syringe strength is not specified, and intramammary administration is unsupported by this schema.