Azithromycin
Azithromycin is an azalide antibiotic related to the macrolides, with prolonged persistence and high concentrations in tissues and phagocytic cells. It is used for selected susceptible bacterial infections, including some intracellular pathogens, and as a partner drug in specific antiprotozoal combinations. Its distribution differs substantially from an antibiotic whose activity is inferred mainly from plasma concentration.
Applications include selected bacterial respiratory infections and specific combination treatment of protozoal disease. Feline upper-respiratory use has limited controlled outcome evidence, and the relationship between tissue exposure, susceptibility and clinical response varies by organism.
Group
Azalide antibacterial
Mechanism of Action
Azithromycin binds the 50S bacterial ribosomal subunit and inhibits steps in protein-chain elongation. Susceptible organisms can no longer synthesize proteins needed for growth and replication, producing predominantly bacteriostatic activity, with the net effect influenced by organism and exposure.
It accumulates within cells and is retained in several tissues, supporting use against selected intracellular pathogens and explaining its long persistence. High tissue concentration does not automatically establish adequate activity at every site or against an organism with an unfavorable susceptibility profile.
Macrolide-related immunomodulatory effects can alter inflammatory-cell function, mediator release and some bacterial virulence processes. These effects may contribute to improvement in airway signs independently of direct bacterial growth inhibition. Activity with atovaquone against selected protozoa reflects a separate combination application.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Susceptible bacterial infection — Initial | 10 mg/kg | PO | Every 24 h | Up to 7 days; Initial phase lasts 5–7 days; follow with the alternate-day phase. |
| Susceptible bacterial infection — Maintenance | 10 mg/kg | PO | Every 48 h | Follows the initial daily course. |
| Susceptible bacterial infection – 5 mg/kg every 24 hours | 5 mg/kg | PO | Every 24 h | |
| Susceptible bacterial infection – 5 mg/kg every 48 hours | 5 mg/kg | PO | Every 48 h | |
| Pyoderma – sequential study course — Initial | 10 mg/kg | PO | Every 24 h | Up to 1 days; Day 1 only; continue with the lower amount on days 2–5. Not a preferred pyoderma antibiotic. |
| Pyoderma – sequential study course — Maintenance | 5 mg/kg | PO | Every 24 h | Up to 4 days; Days 2–5 after the initial dose; not a preferred pyoderma antibiotic. |
| Pyoderma – intermittent study course | 5 mg/kg | PO | Up to 21 days; Given on 2 days per week for 3 weeks; exact spacing is not supplied. Not a preferred pyoderma antibiotic. | |
| Susceptible infection – short course | 5–10 mg/kg | PO | Every 24 h | Up to 5 days; 3–5 days. |
| Susceptible infection – intermittent five-dose course — Initial | 5 mg/kg | PO | Every 24 h | Up to 2 days; First 2 daily doses; then repeat every 3–5 days for 5 doses total. |
| Susceptible infection – intermittent five-dose course — Maintenance | 5 mg/kg | PO | Every 3–5 days after 2 initial daily doses, for 5 doses total. Administration-count limit is unsupported by the calculator. | |
| Dermatologic infection – daily then spaced course — Initial | 5–10 mg/kg | PO | Every 24 h | Up to 7 days; 5–7 days; if continued treatment is necessary, the described alternative repeats every 5 days. |
| Dermatologic infection – daily then spaced course — Maintenance | 5–10 mg/kg | PO | Every 120 h | Only after an initial 5–7-day daily course and if further treatment is needed. |
| Pyoderma – 5-to-10-day course | 10 mg/kg | PO | Every 24 h | Up to 10 days; 5–10 days; not a preferred first-choice pyoderma antibacterial. |
| Babesia gibsoni combination | 10 mg/kg | PO | Every 24 h | Up to 10 days; With atovaquone 13.3 mg/kg PO every 8 hours with a fatty meal for the same 10 days. |
| Chronic lymphoplasmacytic rhinitis adjunct | 5 mg/kg | PO | Every 24 h | A long-term immunomodulatory regimen combines piroxicam 0.3 mg/kg PO daily; assess NSAID contraindications and gastrointestinal risk. |
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Susceptible bacterial infection — Initial | 5–10 mg/kg | PO | Every 24 h | Up to 7 days |
| Susceptible bacterial infection — Maintenance | 5–10 mg/kg | PO | Every 48 h | After the initial 7-day phase. |
| Respiratory infection – twice-weekly alternative — Initial | 10–15 mg/kg | PO | Every 24 h | Up to 3 days; Then the same amount twice weekly; this regimen did not eliminate Chlamydophila felis despite improved signs. |
| Respiratory infection – twice-weekly alternative — Maintenance | 10–15 mg/kg | PO | Twice weekly after 3 daily doses; exact spacing is unspecified. Did not eliminate Chlamydophila felis in the described study. | |
| Upper respiratory infection — Initial | 15 mg/kg | PO | Every 24 h | Up to 3 days |
| Upper respiratory infection — Maintenance | 15 mg/kg | PO | Every 72 h | After the initial 3-day course. |
| Susceptible infection – short course | 5–10 mg/kg | PO | Every 24 h | Up to 5 days; 3–5 days. |
| Susceptible infection – intermittent five-dose course — Initial | 5 mg/kg | PO | Every 24 h | Up to 2 days; First 2 daily doses; then repeat every 3–5 days for 5 doses total. |
| Susceptible infection – intermittent five-dose course — Maintenance | 5 mg/kg | PO | Every 3–5 days after 2 initial daily doses, for 5 doses total. Administration-count limit is unsupported by the calculator. | |
| Dermatologic infection – daily then spaced course — Initial | 7–15 mg/kg | PO | Every 12 h | Up to 7 days; 5–7 days; this is a separate higher-frequency alternative. |
| Dermatologic infection – daily then spaced course — Maintenance | 7–15 mg/kg | PO | Every 120 h | After 5–7 initial days if continued treatment is necessary. |
| Upper respiratory infection – lower-dose staged course — Initial | 5–10 mg/kg | PO | Every 24 h | Up to 5 days |
| Upper respiratory infection – lower-dose staged course — Maintenance | 5–10 mg/kg | PO | Every 72 h | Up to 56 days; Only if initially responsive; a total 6–8-week treatment approach is described. Does not establish eradication of Chlamydophila felis. |
| Bartonellosis | 10 mg/kg | PO | Every 24 h | Up to 21 days; Include flea control and reassess alternative or concurrent disease if response is poor. |
Horses
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Rhodococcus equi infection — Initial | 10 mg/kg | PO | Every 24 h | Up to 5 days; Foals; change to every-other-day treatment after a response is observed. |
| Rhodococcus equi infection — Maintenance | 10 mg/kg | PO | Every 48 h | Foals; after response to the initial course. |
| Foal Rhodococcus high-risk prophylaxis | 10 mg/kg | PO | Every 48 h | Up to 14 days; Described during the first 2 weeks after birth on high-risk endemic farms; not a universal prophylaxis recommendation. |
| Foal bacterial infection – general listing | 10 mg/kg | PO | Every 48 h | Foals; indication is not otherwise specified in the dosage listing. |
| Adult equine infection – general listing | 10 mg/kg | PO | Every 48 h | Adult horses; monitor for diarrhea. |
Cattle
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Injectable bacterial infection – conflicting listing | 10 mg/kg | IM | The dose section lists IM administration, but the precautions prohibit IM use of the injectable preparation. Do not calculate or administer until product and route are clarified. | |
| Cryptosporidiosis | 33 mg/kg | PO | Every 24 h | Up to 7 days; Calves. |
Rabbit
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Staphylococcal osteomyelitis | 50 mg/kg | PO | Every 24 h | Combined with rifampin 40 mg/kg PO every 12 hours; monitor gastrointestinal tolerance. |
| Jaw abscess – postoperative treatment | 15–30 mg/kg | PO | Every 24 h | Up to 28 days; 2–4 weeks after surgery; discontinue and reassess if anorexia or diarrhea occurs. |
Guinea pigs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Pneumonia | 15–30 mg/kg | PO | Every 24 h | Discontinue and reassess if soft stools develop. |
Cockatiel
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Chlamydiosis | 40 mg/kg | PO | Every 48 h | Up to 21 days; Studied using human oral suspension delivered into the crop by a feeding tube; do not extrapolate to all bird species. |
Indications
- Selected susceptible gram-positive, Mycoplasma and intracellular bacterial infections.
- Rhodococcus equi infection in foals, generally within a suitable combination protocol.
- Reported treatment of Lawsonia-associated proliferative enteropathy in horses.
- Selected feline upper-respiratory infections; controlled outcome evidence is limited.
- Reported treatment of cryptosporidiosis in calves.
- Combination with atovaquone for canine Babesia gibsoni infection and feline Cytauxzoon felis infection.
Contraindications
- Use cautiously with a vomiting history.
- Adult horses can develop diarrhea; long-term safety is uncertain.
- Do not administer injectable solution as an IV bolus or IM.
- Not appropriate for serious gram-negative infections.
- Macrolide hypersensitivity. Use cautiously with hepatic dysfunction.
Interactions
- Potential hepatic enzyme interactions are less prominent than with erythromycin or clarithromycin.
- Separate aluminum/magnesium antacids by 2 hours.
- Potential cyclosporine or digoxin increases warrant monitoring; use cautiously with cisapride and avoid pimozide.
Adverse effects
- Vomiting, diarrhea and altered fecal consistency.
- Human cardiac-arrhythmia warnings exist; the cited animal reports did not demonstrate this effect.
Toxicity
Acute oral overdose mainly causes vomiting, diarrhea and cramping. Provide supportive care and assess hydration and hepatic function when clinically indicated.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Azithromycin 250 mg tablet | Tablet | 250 mg / tablet | PO |
| Azithromycin 500 mg tablet | Tablet | 500 mg / tablet | PO |
| Azithromycin 600 mg tablet | Tablet | 600 mg / tablet | PO |
| Azithromycin 100 mg per 5 mL suspension | Oral Liquid | 100 mg / 5 mL | PO |
| Azithromycin 200 mg per 5 mL suspension | Oral Liquid | 200 mg / 5 mL | PO |
| Azithromycin 1 g oral packet One complete packet; final liquid concentration depends on the mixing volume. | Powder | 1000 mg / sachet | PO |
| Zito 500 azithromycin tablet | Tablet | 500 mg / tablet | PO |
Monitoring and Administration
Clinical evidence is insufficient for many empirical animal uses. Azithromycin did not eradicate feline Chlamydophila felis despite symptomatic improvement and was ineffective for Mycoplasma haemofelis in the described studies.
For IV preparation, add 4.8 mL sterile water to a 500 mg vial, then dilute to 1 or 2 mg/mL using 500 or 250 mL diluent respectively. Infuse 1 mg/mL over 3 hours or 2 mg/mL over 1 hour; never give as a bolus or IM.
Keep preparations in their original formulation and apply the product-specific stock-versus-infusion stability instructions.
Foal Rhodococcus treatment commonly combines a macrolide with rifampin; a comparison favored clarithromycin/rifampin over azithromycin/rifampin. A 1 g oral packet may be mixed with water for a foal, but no universal resulting concentration is given.
The susceptible MIC threshold is ≤2 mcg/mL and is based on a human clinical breakpoint; it is not a species-specific veterinary validation.
For the described multidose oral suspension, a 10-day period at 5–30°C is given; other products can have different limits. Shake before use and follow the exact product storage instructions. Administer single-dose packets immediately after mixing. Reconstituted 100 mg/mL IV stock has a separate 24-hour period below 30°C; final diluted infusions have separate 24-hour room-temperature or 7-day refrigerated chemical-stability data, subject to aseptic-use requirements.