Gamithromycin: veterinary dose and monograph

At a glance

Drug class

Antibiotic

Also known as

Zactran

Species with doses

Cattle, Horses, Sheep and Pigs

General dose range

6 mg/kg SC single dose — check the species tables for the indication

Routes

SC, IM

Last reviewed

1 October 2026

Indications

• Bovine respiratory disease (BRD) — treatment and metaphylaxis/control
• Ovine respiratory disease (where licensed)
• Swine respiratory disease (where licensed)
• Single-dose respiratory therapy in feedlot/group settings

Gamithromycin dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Cattle

• BRD (treatment / metaphylaxis): Papich 5e (Zactran label): 6 mg/kg SC in the neck once (2 mL per 110 lb). US label slaughter withdrawal 35 days; not for female dairy cattle 20 months or older or veal calves.

Horses

• Streptococcus zooepidemicus / R. equi: Papich 5e: 6 mg/kg IM every 7 days if needed. Plumb’s 10e p.572 (extra-label): mild-moderate Rhodococcus equi bronchopneumonia in foals — 6 mg/kg IM in the semimembranosus/semitendinosus once a week for at least 40 days (efficacy similar to azithromycin + rifampin but more adverse effects). Injection-site reactions and colic-like signs possible.

Sheep

• Footrot (Dichelobacter nodosus) — elimination: Plumb’s 10e p.572 (extra-label): 6 mg/kg SC cranial to the shoulder or in the axilla. Papich 5e gives no sheep dose. No sheep withdrawal is printed — obtain a FARAD estimate.

Pigs

• Not recommended (Papich 5e): Papich 5e: gamithromycin is not recommended in pigs. REFERENCE CONFLICT: Plumb’s 10e p.572 gives an extra-label swine dose (next rows). Where a local swine label exists, follow that label and its withdrawal.
• Swine respiratory disease (Plumb’s 10e): Plumb’s 10e p.572 (extra-label): bacterial swine respiratory disease — 6 mg/kg IM once in the neck; Bordetella bronchiseptica-associated respiratory disease in piglets — 6 mg/kg IM once. No swine withdrawal is printed — obtain a FARAD estimate. Papich 5e does not recommend use in pigs.

Buffalo

No buffalo-specific doses are on file, so buffalo are dosed as cattle.

• BRD (treatment / metaphylaxis): Papich 5e (Zactran label): 6 mg/kg SC in the neck once (2 mL per 110 lb). US label slaughter withdrawal 35 days; not for female dairy cattle 20 months or older or veal calves.

Contraindications

Adult horses / hindgut fermenters (fatal colitis — macrolide class). Known macrolide hypersensitivity. Lactating dairy cattle producing milk for human consumption (not approved — residues). Observe meat withdrawal. Injection-site reaction (give SC at the labelled site/volume).

Species-specific warnings

Horses: Fatal colitis in adult horses (macrolide class) — strictly avoid. A cattle product.

Cattle: Not approved for lactating dairy (residues); observe the ~35-day meat withdrawal (verify nationally).

Adverse effects

• Injection-site swelling/pain (transient)
• Fatal colitis in adult horses/hindgut fermenters (class effect)
• Rare GI effects

Drug interactions

• Other macrolides/lincosamides: cross-resistance / no added benefit
• Antagonism with bactericidal cell-wall agents (theoretical)
• No major small-molecule pharmacokinetic interactions in cattle

Pregnancy and lactation

Considered safe in pregnant cattle/sheep at label dose.

Monitoring

• Respiratory clinical response over 2–4 days
• Injection-site reaction
• Meat withdrawal compliance
• Antimicrobial stewardship (macrolides are important in human medicine)

Toxicity and overdose

Wide margin in target species — overdose may cause injection-site reactions and GI signs. The catastrophic toxicity is species-related: fatal antibiotic-associated colitis if given to adult horses or other hindgut fermenters.

Management: Supportive; no specific antidote. Manage injection-site reactions and GI signs. Treat colitis aggressively if a macrolide is accidentally given to an equid.

Clinical pearls

• A one-shot, long-acting macrolide for BRD — single SC injection covers the key respiratory pathogens for days, ideal for feedlot treatment/metaphylaxis
• Concentrates in lung and leukocytes (high tissue:plasma ratio) — that is why one dose suffices
• Never let it reach adult horses/rabbits (fatal colitis); not for lactating dairy
• Use prudently for metaphylaxis — macrolide stewardship matters

Mechanism of action

A semi-synthetic macrolide (azalide) that binds the 50S ribosomal subunit (23S rRNA), inhibiting bacterial protein synthesis — bacteriostatic to bactericidal against respiratory pathogens. It is extensively taken up and concentrated in lung tissue and phagocytes and has a long half-life, so a single dose maintains therapeutic pulmonary levels for days — the basis of its single-injection BRD efficacy.

Formulations and products

• Injection 150 mg/mL (Zactran)
• Zactran (Boehringer Ingelheim (Merial)) — 150 mg/mL, SC injection

Storage

Injection: 15–30 °C, protect from light; do not freeze.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other antibiotics

• Enrofloxacin
• Erythromycin
• Florfenicol
• Fusidic Acid
• Gentamicin
• Levofloxacin
• Lincomycin
• Linezolid

Frequently asked questions