Pharmacology/Uses/Indications
Although there may be some slight differences in activity against certain organisms, amoxicillin generally shares the same spectrum of activity and uses as ampicillin. Because it is better absorbed orally (in non-ruminants), higher serum levels may be attained than with ampicillin.
Penicillins are usually bactericidal against susceptible bacteria and act by inhibiting mucopeptide synthesis in the cell wall resulting in a defective barrier and an osmotically unstable spheroplast. The exact mechanism for this effect has not been definitively determined, but beta-lactam antibiotics have been shown to bind to several enzymes (carboxypeptidases, transpeptidases, endopeptidases) within the bacterial cytoplasmic membrane that are involved with cell wall synthesis. The different affinities that various beta-lactam antibiotics have for these enzymes (also known as penicillin-binding proteins; PBPs) help explain the differences in spectrums of activity the drugs have that are not explained by the influence of beta-lactamases. Like other beta-lactam antibiotics, penicillins are generally considered more effective against actively growing bacteria.
Contraindications/Precautions
Penicillins are contraindicated in patients who have a history of hypersensitivity to them. Because there may be cross-reactivity, use penicillins cautiously in patients who are documented hypersensitive to other beta-lactam antibiotics (e.g., cephalosporins, cefamycins, carbapenems).
Do not administer systemic antibiotics orally in patients with septicemia, shock, or other grave illnesses as absorption of the medication from the GI tract may be significantly delayed or diminished. Parenteral (preferably IV) routes should be used for these cases.
Reproductive/Nursing Safety
Penicillins have been shown to cross the placenta and safe use of them during pregnancy has not been firmly established, but neither have there been any documented teratogenic problems associated with these drugs. However, use only when the potential benefits outweigh the risks. In humans, the FDA categorizes this drug as category B for use during pregnancy (Animal studies have not yet demonstrated risk to the fetus, but there are no adequate studies in pregnant women; or animal studies have shown an adverse effect, but adequate studies in pregnant women have not demonstrated a risk to the fetus in the first trimester of pregnancy, and there is no evidence of risk in later trimesters.) In a separate system evaluating the safety of drugs in canine and feline pregnancy, this drug is categorized as in class: A (Probably safe. Although specific studies may not have proved the safety of all drugs in dogs and cats, there are no reports of adverse effects in laboratory animals or women.)
Adverse Effects/Warnings
Adverse effects with the penicillins are usually not serious and have a relatively low frequency of occurrence.
Hypersensitivity reactions unrelated to dose can occur with these agents and can be manifested as rashes, fever, eosinophilia, neutropenia, agranulocytosis, thrombocytopenia, leukopenia, anemias, lymphadenopathy, or full-blown anaphylaxis. In humans, it is estimated that up to 15% of patients hypersensitive to cephalosporins will also be hypersensitive to penicillins. The incidence of cross-reactivity in veterinary patients is unknown.
When given orally, penicillins may cause GI effects (anorexia, vomiting, diarrhea). Because the penicillins may also alter gut flora, antibiotic-associated diarrhea can occur, as well as selecting out resistant bacteria maintaining residence in the colon of the animal (superinfections).
High doses or very prolonged use has been associated with neurotoxicity (e.g.,ataxia in dogs). Although the penicillins are not considered hepatotoxic, elevated liver enzymes have been reported. Other effects reported in dogs include tachypnea, dyspnea, edema and tachycardia.
Overdosage/Acute Toxicity
Acute oral penicillin overdoses are unlikely to cause significant problems other than GI distress, but other effects are possible (see Adverse Effects). In humans, very high dosages of parenteral penicillins, especially in patients with renal disease, have induced CNS effects.
Drug Interactions
In vitro studies have demonstrated that penicillins can have synergistic or additive activity against certain bacteria when used with aminoglycosides or cephalosporins.
Use of bacteriostatic antibiotics (e.g.,chloramphenicol, erythromycin, tetracyclines) with penicillins is generally not recommended, particularly in acute infections where the organism is proliferating rapidly as penicillins tend to perform better on actively growing bacteria. In low concentrations, certain penicillins (e.g.,ampicillin, oxacillin or nafcillin) may have additive or synergistic effects against certain bacteria when used with rifampin, but there is apparent antagonism when the penicillin is present in high concentrations.
Doses
Dogs
For susceptible infections:
1. For Gram + infections: 10 mg/kg PO, IM, SC twice daily for at least 2 days after symptoms subside. For Gram-infections: 20 mg/kg PO three times daily or IM, SC twice daily; for at least 2 days after symptoms subside
2. For susceptible UTI's: 10-20 mg/kg PO q12h for 5-7 days. For susceptible systemic infections: 22-50 mg/kg PO q8h for 7 days. For susceptible orthopedic infections: 22-30 mg/kg IV, IM, SC, or PO q6-8h for 7-10 days. NOTE: Duration of treatment are general guidelines, generally treat for at least 2 days after all signs of infection are gone.
3. For Lyme disease: 22 mg/kg PO q12h for 21-28 days
Cats
For susceptible infections:
1. For Gram + infections: 10 mg/kg PO, IM, SC twice daily for at least 2 days after symptoms subside. For Gram-infections: 20 mg/kg PO three times daily or IM, SC twice daily; for at least 2 days after symptoms subside
2. For susceptible UTI's and soft tissue infections 50 mg (total dose per cat) or 11-22 mg/kg PO once daily for 5-7 days. For sepsis:: 10-20 mg/kg IV, SC, or PO q12h for as long as necessary. NOTE: Duration of treatment are general guidelines, generally treat for at least 2 days after all signs of infection are gone.
3. C. perfringens, bacterial overgrowth (GI): 22 mg/kg PO once daily for 5 days
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4. C. perfringens enterotoxicosis: 11-22 mg/kg PO bid-tid for 7 days
5. For treating H. pylori infections using triple therapy: amoxicillin 20 mg/kg PO twice daily for 14 days; metronidazole 10-15 mg/kg PO twice daily; clarithromycin 7.5 mg/kg PO twice daily
Ferrets
1. For eliminating Helicobacter gastritis infections: Using triple therapy: Metronidazole 22 mg/kg, amoxicillin 22 mg/kg and bismuth subsalicylate (original Pepto-Bismol) 17.6 mg/kg PO. Give each 3 times daily for 3-4 weeks.
2. For susceptible infections: 10-35 mg/kg PO or SC twice daily
Rabbits/Rodents/Pocket Pets
1. Hedgehogs: 15 mg/kg IM or PO q12h
Cattle
For susceptible infections:
1. 6-10 mg/kg SC or IM q24h (Withdrawal time = 30 days)
2. For respiratory infections: 11 mg/kg IM or SC q12h
3. Calves: Amoxicillin trihydrate: 7 mg/kg PO q8-12h
4. 13.2-15.4 mg/kg IM or SC once daily
Horses
For susceptible infections:
1. For respiratory infections: 20-30 mg/kg PO q6h
2. Amoxicillin trihydrate: 20 mg/kg q12h IM
3. Foals: Amoxicillin Sodium: 15-30 mg/kg IV or IM q6-8h; amoxicillin trihydrate suspension: 25-40 mg/kg PO q8h; amoxicillin/clavulanate 15-25 mg/kg IV q6-8h
Birds
For susceptible infections:
1. For most species: 150-175 mg/kg PO once to twice daily (using 50 mg/ml suspension)
2. 100 mg/kg q8h PO
3. 100 mg/kg q8h, IM, SC, PO
4. Ratites: 15-22 mg/kg PO twice daily; in drinking water: 250 mg/gallon for 3-5 days
Reptiles
For susceptible infections:
1. For all species: 22 mg/kg PO q12-24h; not very useful unless used in combination with aminoglycosides
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