It binds to penicillin-binding proteins involved in bacterial cell wall synthesis, thereby decreasing cell wall strength and rigidity, affecting cell division, growth and septum formation. It acts in a time-dependent fashion.

Uses of Ampicillin

Ampicillin is active against many Gram-positive and Gram-negative aerobic organisms and obligate anaerobes but is ineffective against bacteria that produce beta-lactamases (e.g., Escherichia coli, Staphylococcus aureus). It is commonly used for:

Respiratory tract infections
Urinary tract infections
Gastrointestinal infections
Skin and soft tissue infections
Surgical prophylaxis
Sepsis caused by susceptible organisms

Dose of Ampicillin in Animals

Dose of Ampicillin in Dogs:

Routine infections: 10–20 mg/kg IV, IM, SC, or PO every 6–8 hours.
CNS or serious bacterial infections: Up to 40 mg/kg IV every 6 hours.
Leptospirosis: 20 mg/kg IV every 6 hours if oral doxycycline is not tolerated.
Sepsis or systemic infections: 20–40 mg/kg IV every 6–8 hours, often combined with aminoglycosides or fluoroquinolones for broad-spectrum coverage.

Dose of Ampicillin in Cats:

10–20 mg/kg IV, IM, SC, or PO every 6–8 hours.
CNS or serious bacterial infections: Up to 30 mg/kg IV every 6 hours may be used in severe cases.

Dose of Ampicillin in Horses:

Respiratory infections: 20 mg/kg IV or IM every 6–8 hours.
Streptococcal lower airway infections: 15 mg/kg IV every 12 hours.
Surgical prophylaxis: 5 g IV as a single dose ≈15 minutes prior to the first incision.

Dose of Ampicillin in Cattle:

Respiratory infections: 4.4–11 mg/kg IM once daily (label dosage); 22 mg/kg SC every 12 hours (extra-label use).
Surgical prophylaxis: Ampicillin sodium 5 g IV as a single dose before surgery.

Dose of Ampicillin in Ferrets:

5–10 mg/kg IM, SC, or IV twice daily.

Dose of Ampicillin in Rodents and Small Mammals:

Gerbils, Mice, Rats: 20–100 mg/kg PO, SC, or IM every 8–12 hours.
Hedgehogs: 10 mg/kg IM or PO once daily.
Rabbits, Guinea pigs, Chinchillas, Hamsters: Not recommended due to the risk of fatal enterocolitis.

Dose of Ampicillin in Reptiles:

Respiratory infections: 50 mg/kg SC or IM every 12 hours.

Dose of Ampicillin in Birds:

Specific dose recommendations are not well-documented but extrapolation from mammals suggests cautious use under veterinary guidance.

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Side Effects of Ampicillin in Animals

Common: Nausea, diarrhea, and skin rashes.
Severe: Hypersensitivity reactions, including anaphylaxis, neutropenia, agranulocytosis, and eosinophilia.
Rare: Ataxia (neurotoxicity) at very high doses, hepatotoxicity, elevated liver enzymes, tachypnea, and tachycardia.

Contraindications of Ampicillin in Animals

Hypersensitivity to penicillins or cephalosporins.
Avoid oral administration in critically ill patients due to unreliable gastrointestinal absorption.
Do not use the trihydrate formulation (Polyflex®) intravenously due to the risk of anaphylaxis and sudden death.

Some Notes:

Therapeutic Monitoring: Maintaining plasma levels above the MIC is crucial for efficacy; prolonged intervals or missed doses can lead to therapeutic failure.
Drug Interactions:
- May antagonize bacteriostatic antibiotics (e.g., tetracyclines, erythromycin).
- Inactivated by aminoglycosides in vitro; avoid mixing in the same syringe.
Resistance: Gram-negative organisms like Pseudomonas aeruginosa and Klebsiella are typically resistant.
Excretion: Ampicillin is excreted primarily through urine and bile, achieving high concentrations in both.
Storage:
- Sodium salt is stable for 8 hours if refrigerated after reconstitution but should be used within 2 hours at room temperature.
- Oral formulations have reduced bioavailability when administered with food.