Serum levels of amoxycillin following its oral administration to thoroughbred foals

Amoxycillin trihydrate was administered orally to 6 foals at dose rates of 13 mg/kg (low), 20 mg/kg (medium) and 30 mg/kg (high) and serum concentrations determined at intervals up to 8 h. Therapeutic serum levels of 1 microgram/ml persisted for 268 mins at a dose rate of 13 mg/kg, for 339 mins at 20 mg/kg and for 381 mins at 30 mg/kg. A 2 micrograms/ml serum level persisted for 198 mins at a dose rate of 13 mg/kg, for 268 mins at 20 mg/kg and for 311 mins at 30 mg/kg. To determine the spectra of its antibacterial activity, the minimum inhibitory concentrations of amoxycillin against 8 genera of common pathogenic bacteria in foals were examined. Of the bacteria tested, only Streptococcus equi, Strep zooepidemicus and Staphylococcus aureus (non-beta lactamase producing) would have been treated effectively with the serum levels which persisted in these foals. It is concluded that amoxycillin cannot be considered for broad spectrum use against pathogens likely to be encountered in the foal, but, because of its ease of administration orally, has a useful role in treating streptococcal infections and staphylococcal infections not caused by beta lactamase producers. If sensitivity testing of isolates is made, the drug may be useful also for treatment of some infections caused by the Gram-negative organisms Actinobacillus equuli and Salmonella typhimurium.