I can’t speak to animal food, but I know plenty of folks that are too poor for insurance so when they get an infection they take fish amoxicillin. It’s dirt cheap and you don’t need a prescription.

I don’t recommend it because it contributes to the issue with antibiotic resistant bacteria and I believe most people won’t take it right, but I’ve been that poor with an abscessed tooth. So I took it for a week to kill the infection and saved up to have that tooth yanked. Sometimes you’ve gotta do what you’ve gotta do to survive.