# Gamithromycin: veterinary dose and monograph

> Gamithromycin is a veterinary antibiotic. A long-acting macrolide (azalide) developed specifically for BOVINE RESPIRATORY DISEASE (BRD) — a SINGLE subcutaneous injection provides several days of therapeutic lung concentrations, treating and controlling (metaphylaxis) Mannheimia… Indications include bovine respiratory disease (BRD) — treatment and metaphylaxis/control; ovine respiratory disease (where licensed); swine respiratory disease (where licensed). This monograph lists 4 reference doses for cattle, horses, sheep and pigs, each with route, frequency and source, plus meat and milk withdrawal periods for food animals.

Source: https://vetmasterji.com/drugs/gamithromycin · Last reviewed: 2026-10-01

## 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

*Gamithromycin doses for cattle*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| BRD (treatment / metaphylaxis) | 6 mg/kg | SC | single dose | — | 35d slaughter |

- 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

*Gamithromycin doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Streptococcus zooepidemicus / R. equi | 6 mg/kg | IM | every 7 days if needed | — |

- 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

*Gamithromycin doses for sheep*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| Footrot (Dichelobacter nodosus) — elimination | 6 mg/kg | SC | single dose | — | Withdrawal NOT established — treat as extra-label |

- 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

*Gamithromycin doses for pigs*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| Not recommended (Papich 5e) | Not recommended in pigs (Papich 5e) | — | — | — | Withdrawal NOT established — treat as extra-label |
| Swine respiratory disease (Plumb’s 10e) | 6 mg/kg | IM | single dose, in the neck | — | Withdrawal NOT established — treat as extra-label |

- 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.

*Gamithromycin doses for buffalo*

| Indication | Dose | Route | Frequency | Duration | Withdrawal |
| --- | --- | --- | --- | --- | --- |
| BRD (treatment / metaphylaxis) | 6 mg/kg | SC | single dose | — | 35d slaughter |

- 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

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

> **Caution:** 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

> **Caution:** 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](https://vetmasterji.com/drugs/enrofloxacin)
- [Erythromycin](https://vetmasterji.com/drugs/erythromycin)
- [Florfenicol](https://vetmasterji.com/drugs/florfenicol)
- [Fusidic Acid](https://vetmasterji.com/drugs/fusidic-acid)
- [Gentamicin](https://vetmasterji.com/drugs/gentamicin)
- [Levofloxacin](https://vetmasterji.com/drugs/levofloxacin)
- [Lincomycin](https://vetmasterji.com/drugs/lincomycin)
- [Linezolid](https://vetmasterji.com/drugs/linezolid)

## Frequently asked questions

### What is the dose of Gamithromycin for cattle?

Reference doses for Gamithromycin in cattle: 6 mg/kg SC single dose (BRD (treatment / metaphylaxis)). Confirm against the label and the patient before use.

### What is the dose of Gamithromycin for horses?

Reference doses for Gamithromycin in horses: 6 mg/kg IM every 7 days if needed (Streptococcus zooepidemicus / R. equi). Confirm against the label and the patient before use.

### What is the dose of Gamithromycin for sheep?

Reference doses for Gamithromycin in sheep: 6 mg/kg SC single dose (Footrot (Dichelobacter nodosus) — elimination). Confirm against the label and the patient before use.

### What is the withdrawal period for Gamithromycin?

Cattle, SC (BRD (treatment / metaphylaxis)): 35d slaughter; Sheep, SC (Footrot (Dichelobacter nodosus) — elimination): Withdrawal NOT established — treat as extra-label; Pigs, IM (Swine respiratory disease (Plumb’s 10e)): Withdrawal NOT established — treat as extra-label. Follow the product label and national regulations where they differ.

### When should Gamithromycin not be used?

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).

### What are the side effects of Gamithromycin in animals?

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

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.