It is allowed. I would consider that preventing disease in a particular herd for which it's known there's a disease pattern, as well as treating a disease, is important. Preventing disease, and the snowball effect of many more animals becoming much more sick, and having to use more antimicrobials, and maybe second line and third line treatments, is a reasonable practice. Yes, prophylactic drugs are approved for use in food animals in both prevention and treatment. They are permitted and considered reasonable.
On June 13th, 2017. See this statement in context.