Azithromycin
**Azithromycin** is a broad-spectrum, semi-synthetic macrolide antibiotic belonging to the azalide subclass. It is widely used in veterinary medicine due to its highly favorable pharmacokinetic profile, which allows for less frequent dosing compared to older macrolides like erythromycin. **Key Clinical Features:** * **Exceptional Tissue Penetration:** Azithromycin is actively taken up by phagocytes (macrophages and polymorphonuclear leukocytes) and fibroblasts, which then migrate to sites of infection. This results in tissue concentrations that can be up to 100 times higher than serum concentrations. * **Long Half-Life:** The drug exhibits a very long tissue half-life (up to 90 hours in dogs), allowing for pulse-dosing regimens (e.g., every 3 to 5 days) after an initial loading phase. * **Spectrum of Activity:** Effective against a variety of Gram-positive organisms (e.g., *Streptococcus*, *Staphylococcus*), some Gram-negative organisms (*Haemophilus*, *Bordetella*), and atypical pathogens including *Mycoplasma pneumoniae*, *Borrelia burgdorferi*, *Toxoplasma* spp., and *Bartonella* spp. * **Specific Uses:** Frequently utilized for canine *Bordetella* pneumonia, feline upper respiratory infections, feline Bartonellosis, *Babesia gibsoni* in dogs (in combination with atovaquone), and *Rhodococcus equi* infections in foals. > **Clinical Pearl:** Despite its broad utility, azithromycin has been shown to be ineffective for clearing *Chlamydophila felis* or *Mycoplasma haemofelis* in cats.
Mechanism: Azithromycin exerts its antibacterial effects by binding to the **50S ribosomal subunit** of susceptible microorganisms. * **Mechanism:** By binding to the 50S subunit, it blocks **transpeptidation** and **translocation** processes โ inhibits RNA-dependent protein synthesis. * **Action Type:** It is generally considered **bacteriostatic**, but can be bactericidal at high concentrations or against highly susceptible organisms. * **Targeted Delivery:** The drug is actively transported to the site of infection by host phagocytic cells. During active phagocytosis, azithromycin is released locally, providing high concentrations directly at the site of bacterial invasion.
Dosing by species
- Susceptible infections ยท 5-10 mg/kg PO once daily for 3-5 days ยท PO ยท q24h ยท 3-5 days
- Susceptible infections (pulse dosing) ยท 5 mg/kg PO once daily for 2 days, then every 3-5 days for a total of 5 doses ยท PO ยท q24h then q3-5d ยท 5 doses total
- 'Derm' infections ยท 5-10 mg/kg PO once daily for 5-7 days. For animals that are difficult to pill, a dose given every 5 days (after the initial 5-7 day course of therapy) may be effective if continued treatment is necessary. ยท PO ยท q24h then q5d ยท 5-7 days initially
- Canine pyoderma ยท 10 mg/kg PO once daily for 5-10 days ยท PO ยท q24h ยท 5-10 days
- Babesia gibsoni (Asian genotype) infections ยท Atovaquone 13.3 mg/kg PO q8h with a fatty meal and Azithromycin 10 mg/kg PO once daily. Give both drugs for 10 days. ยท PO ยท q24h ยท 10 days ยท Reserve immunosuppressive therapy for cases that are not rapidly responding (3-5 days) to anti-protozoal therapy.
- Idiopathic lymphoplasmacytic (chronic) rhinitis ยท Doxycycline 3-5 mg/kg PO q12h, or azithromycin 5 mg/kg PO q24h in combination with piroxicam 0.3 mg/kg PO q24h. ยท PO ยท q24h ยท Long term ยท Used for immunomodulatory effects.
- Susceptible infections ยท 5-10 mg/kg ยท PO ยท q24h ยท May increase dosing interval to q48h after 3-5 days of treatment ยท Doses are empirical and subject to change
- Chlamydiosis (experimentally infected cockatiels) ยท Azithromycin 40 mg/kg PO once every other day (q48h) for 21 days ยท PO ยท q48h ยท 21 days ยท Birds were dosed via metallic feeding tube into crop using the commercially available human oral suspension.
Routes of administration
Contraindications
- Hypersensitivity to azithromycin or any other macrolide antibiotics
Adverse effects
- Vomiting (especially at high doses)
- Diarrhea
- GI cramping
- Potential hepatotoxicity
- Local tissue reactions (IV site)
- Muscle damage (IM injection)
- Diarrhea in foals
Drug interactions
- Antacids (magnesium- and aluminum-containing) ยท May reduce the rate of absorption of azithromycin; separate dosages by 2 hours
- Cisapride ยท Other macrolides are contraindicated with cisapride due to potential for fatal arrhythmias; use with extreme caution or avoid
- Cyclosporine ยท Azithromycin may potentially increase cyclosporine blood levels; monitor carefully
- Digoxin ยท Other macrolides can increase digoxin levels; monitor carefully ยท major
- Pimozide ยท Contraindicated; acute deaths have occurred in humans
- Methylprednisolone ยท May increase serum levels of methylprednisolone ยท moderate
- Theophylline ยท May increase serum levels of theophylline ยท major
- Terfenadine ยท May increase serum levels of terfenadine ยท major
Monitoring
- Clinical efficacy (resolution of infection signs)
- Adverse effects (especially GI distress or signs of hepatic impairment)
Overdose
Acute oral overdoses are unlikely to cause significant morbidity other than gastrointestinal distress, including **vomiting, diarrhea, and GI cramping**. Treatment should be supportive and symptomatic.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.