Chapter 2 – Amphenicols


This family has both chloramphenicol which use is prohibited in food animal species because it causes idiosyncratic aplastic anemia. Florfenicol is a newer molecule that does not have this side effect but can cause bone marrow suppression if used for a longer period of time. Florfenicol is used in swine to control respiratory disorders as second-line antibiotics.

Useful molecules to know in swine medicine:

  • Chloramphenicol → Prohibited to use in food animal species!!
  • Florfenicol

Mechanism of Action

Amphenicols bind to the 50S subunit of the ribosome and stop the addition of new amino-acids, inhibiting protein synthesis.

Check your learning: Amphenicols and Mechanism of Action



Amphenicols are broad spectrum and are efficient against aerobic and anaerobic Gram + and Gram – bacteria. They can also be used to treat Mycoplasma spp. infections.


Florfenicol has a good per os bioavailability in swine.


Amphenicols are lipid soluble and are neutral molecules which means that they have a good volume distribution. The antimicrobial tissue concentration is similar to plasma concentration. Additionally, they can access parts of the body that can be difficult to reach such as cerebro-spinal or synovial fluids.


50% of florfenicol is excreted unchanged in the urine. Phase I and Phase II metabolism occur in the liver, resulting in the creation of more water-soluble molecules that are excreted in the urine.

Adverse effects

Reversible bone marrow suppression may happen if florfenicol is administered for a prolonged period of time. Stopping the treatment will usually reverse the effect. Perianal inflammation that can evolve into a rectal prolapse has been observed in swine. This effect also subsides when the treatment is stopped.

There are fewer resistances against florfenicol than there are against chloramphenicol but some Gram – bacteria have been found to carry plasmids with resistant genes.

The binding site of amphenicols on the ribosomal 50S subunit is located close to the binding site for other antimicrobials such as macrolides and lincosamides. Therefore, molecules from these antimicrobial families should not be given at the same time.

The elevated cost of florfenicol, associated with some of the side effects such as rectal prolapse, has limited its use to the second-line treatment of swine infections despite its broad spectrum and good distribution.


Swine Antibiotherapy Handbook Copyright © by Perle Zhitnitskiy. All Rights Reserved.

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