Using florfenicol in calf feeding may contribute to E. coli resistance to antibiotics.

Florfenicol is a derivative of thiamphenicol with the same mechanism of action as chloramphenicol (an antibiotic that inhibits protein synthesis). However, florfenicol demonstrates greater activity than chloramphenicol or thiamphenicol and may have a more bactericidal effect on some pathogens (e.g., agents causing Bovine Respiratory Disease, BRD).

BRD, also known as bovine respiratory disease or shipping fever pneumonia, is caused by several factors, with the main bacterial agents being Mannheimia haemolytica, and less frequently, Pasteurella multocida, Histophilus somni, or Mycoplasma bovis. Viral pathogens such as bovine herpesvirus 1, parainfluenza-3 virus, and bovine respiratory syncytial virus can also be involved.

BRD results from complex interactions between environmental factors, host factors, and pathogens. Environmental factors such as weaning, transportation, mixing, overcrowding, adverse weather conditions, dust, and poor ventilation act as stressors that negatively impact the immune and non-immune defense mechanisms of the host animals.

Additionally, certain environmental factors such as overcrowding and poor ventilation may facilitate the transmission of infectious agents among animals. Many infectious agents are associated with the development of BRD. The primary pathogen, such as a virus, can alter the body's defense mechanisms, allowing bacteria to colonize the lower respiratory tract.

BRD is most commonly associated with the transportation and gathering of large groups of recently weaned calves in feedlot settings. The incidence in such calves typically peaks within the first 7-10 days after arrival at the feedlot. The morbidity levels can reach 35-50%, with a mortality rate of 5-10%.

The use of broad-spectrum antibiotics, including florfenicol, provides therapeutic protection against BRD.

Florfenicol has a broad antibacterial spectrum, covering all organisms sensitive to chloramphenicol, gram-negative bacteria, gram-positive cocci, and other atypical bacteria such as mycoplasma.

Florfenicol is a lipophilic drug, which ensures a sufficiently high intracellular penetration concentration for the treatment of pathogens and overcoming certain anatomical barriers (penetration through the blood-brain barrier in cattle is 46%). Pharmacokinetics of florfenicol include a half-life of 2-3 hours after intravenous administration in cattle, 18 hours after intramuscular injection, and 27 hours after subcutaneous injection of 40 mg/kg.

However, new research by Brazilian scientists has shown an interesting paradox: calves that were not exposed to florfenicol exhibited resistance of E. coli to this drug.

The main goal of the study was to investigate whether feeding calves with milk residues containing antibiotics not approved for sale after treatment contributed to bacteria resistance to antibiotics in calves.

The study, published in the Journal of Dairy Science, involved 63 dairy calves of different breeds - Holstein and Gir - divided into three treatment groups: calves consuming bulk tank milk (without antibiotics), calves drinking pasteurized waste milk, and calves drinking unpasteurized waste milk.

Researchers found that pasteurization reduced bacterial load in waste milk but did not affect antibiotic residues. They also noted previous studies that indicated the influence of antibiotic residues on changes in rumen microbiota and rumen fermentation profile in calves, depending on the amount of waste milk fed during feeding.

Scientists analyzed tissues and fecal samples of calves to assess the resistance of 3 strains of E. coli to 7 common farm antibiotics. These antibiotics included ampicillin, amoxicillin, ceftiofur, florfenicol, enrofloxacin, streptomycin, and tetracycline.

According to the evaluations, the authors noted the following. Calves consuming pasteurized and unpasteurized waste milk showed a higher frequency of fecal E. coli resistance to 7 antibiotics on days 30 and 60 of life compared to calves consuming bulk tank milk.

Calves fed unpasteurized waste milk exhibited resistance to 5 out of 7 antibiotics studied.

Calves fed pasteurized waste milk exhibited resistance to 4 out of 7 antibiotics studied.

Interestingly, all three groups of calves showed E. coli resistance to florfenicol, even though this drug was not used for animal treatment on the farm.

Scientists explain that resistance to florfenicol can be explained by "resistance dissemination," where the gene providing resistance to a specific drug develops along with other resistance genes, and then resistant bacteria spread among animals.

In general, scientists emphasize the need for careful use of antibiotics on the farm. "The presence of antimicrobial residues in milk waste raises serious concerns within the scientific community. The use of milk residues poses a risk, creating selective pressure and contributing to the emergence and spread of resistant bacteria in the intestinal microbiota of calves." Further research will focus on the transition of calves from liquid to solid feed after weaning. The goal is to develop new feeding strategies and use of milk residues in calf rearing systems.