This strategy will bring the use of these drugs under veterinary supervision so that they are used only when necessary for assuring animal health.

The VFD final rule outlines the process for authorising use of drugs, and provides veterinarians in all states with a framework for authorising the use of medically important antimicrobials in feed, when needed for specific animal health purposes.

This means that the use of antimicrobials purely for production purposes, for example to enhance growth or improve feed efficiency, will be halted, bringing the US into line with Europe on the issue.

Antimicrobials will now only be used for prevention, control or treatment of a specifically identified disease, which should help to reduce the development of antibiotic-resistant strains of bacteria.

According to the US' Centers for Disease Control and Prevention (CDC), at least 2 million people in the US become infected annually with bacteria that are resistant to antibiotics. At least 23,000 people die each year as a result of these infections.

“The actions the FDA has taken to date represent important steps toward a fundamental change in how antimicrobials can be legally used in food-producing animals,” said Michael R. Taylor, FDA deputy commissioner for foods.

“The VFD final rule takes another important step by facilitating veterinary oversight in a way that allows for the flexibility needed to accommodate the diversity of circumstances that veterinarians encounter, while ensuring such oversight is conducted in accordance with nationally consistent principles.”

In December 2013, the agency published a guidance document, which calls on animal drug manufacturers of approved medically important antimicrobials that are put into water or feed of food-producing animals to voluntarily stop labelling them as drugs that can be used to promote animal growth.

They were also asked to change the labelling of their products for the remaining uses to require veterinary oversight of these drugs when they are used for therapeutic purposes. All of the affected makers of these drugs have committed in writing to participate in the strategy.

"Medically important" antimicrobials covers those that are of importance in human medicine. Some antimicrobials are used extensively in animal health but not human health, and do not seem to be affected by the ruling.

However, other drugs covered by the ruling include those currently FDA-approved for production purposes, those available over-the-counter and those used in feed or drinking water of food-producing animals.

The FDA's strategy does not require producers and drug companies to have veterinarian oversight straight away, in order to reduce the disruption to the industry caused by these changes. The deadline is December 2016, and until then the agency will use a phased enforcement approach, together with education of the people involved in the antimicrobial use chain.