Ampicillin Trihydrate IM Administration in Horses
The recommended dosage for ampicillin trihydrate administered intramuscularly in horses is 10-15 mg/kg body weight every 12 hours for most infections, with the option to increase to every 6-8 hours for severe infections.
Dosage and Administration Schedule
Standard Dosing
- Dose range: 10-15 mg/kg body weight
- Frequency: Every 12 hours (twice daily)
- Duration: Minimum of 48-72 hours beyond clinical resolution
Severe Infections
- Dose: 15 mg/kg body weight
- Frequency: Every 6-8 hours (3-4 times daily)
- Duration: Minimum of 10 days for streptococcal infections
Pharmacokinetics in Horses
Ampicillin trihydrate administered via IM route in horses has distinct pharmacokinetic properties that influence dosing recommendations:
- After IM administration, ampicillin is absorbed rapidly with peak plasma concentrations occurring between 18-21 minutes post-injection 1
- The elimination half-life is relatively short, necessitating multiple daily administrations to maintain therapeutic levels
- Bioavailability via IM route is significantly higher than oral administration, which is poorly absorbed in horses 2
Clinical Applications
Appropriate Uses
- Respiratory tract infections caused by susceptible streptococci
- Soft tissue infections
- Wound infections
- Susceptible bacterial infections where parenteral therapy is indicated
Bacterial Spectrum
Ampicillin is most effective against:
- Streptococcus equi
- Streptococcus zooepidemicus
- Non-beta-lactamase producing Staphylococcus aureus
- Some strains of Actinobacillus equuli
Administration Technique
- Reconstitute ampicillin trihydrate powder with the appropriate amount of sterile water for injection to achieve a concentration of 250 mg/mL 3
- Use freshly prepared solutions within one hour of preparation
- Administer deep into a large muscle mass
- Rotate injection sites to prevent local irritation
Important Considerations
Therapeutic Monitoring
- Continue treatment for at least 48-72 hours after clinical signs resolve
- For streptococcal infections, a minimum of 10 days of treatment is recommended to prevent complications 3
Potential Limitations
- Limited spectrum against gram-negative pathogens
- Not effective against beta-lactamase producing organisms
- Multiple daily injections may be required for severe infections
Alternative Approaches
- For severe systemic infections, consider sodium ampicillin at 15 mg/kg four times daily which provides better coverage for systemic infections 4
- For prolonged therapy, consider switching to appropriate oral antibiotics if available and if the infection is responding
Cautions
- Hypersensitivity reactions can occur
- Local tissue irritation at injection sites is possible
- Maintain adequate hydration during therapy
- Monitor for signs of superinfection during prolonged therapy
The efficacy of ampicillin trihydrate administered IM depends on maintaining adequate plasma concentrations above the minimum inhibitory concentration (MIC) for the target pathogen. Research indicates that a dosage of 15 mg/kg administered four times daily would successfully treat most ampicillin-susceptible bacterial infections in well-vascularized tissues 1.