In bulls, the use of antibiotics is essential in various reproductive clinical conditions [[1], [2], [3]]. One of the most common reproductive diseases in young and old bulls is the vesicular adenitis syndrome [3,4], and its treatment often involves the use of local or systemic antibiotics [3,5]. In bulls, a variety of microorganisms have been isolated from cases of vesicular adenitis syndrome including Actinobacillus actinoides, Aeromonas hydrophila, Brucella abortus, Chlamydophila psittaci, Corynebacterium renale, Corynebacterium pseudotuberculosis, Escherichia coli, Histophilus somni, Mycobacterium tuberculosis, Mycobacterium paratuberculosis, Mycoplasmas, Proteus mirabilis, Streptococcus, Staphylococcus, Ureaplasmas, Trueperella pyogenes, and Tritrichomonas foetus [3]. In bulls the most frequent microorganism isolated is Trueperella pyogenes [3,5]. However, most of the studies the selection of antibiotics for this last condition and other genital infections (e.g. epididymitis) was based on personal experience, anecdotal evidence, extrapolation from other species, and few were based on microbiological culture with identification and sensitivity test to the antibiotics [3,5]. To ensure the appropriate choice of antibiotics, it is crucial to rely on microbiological culture, isolation, identification, and sensitivity tests [3,5,6]. Administering the chosen antibiotic at the precise dose, route, and frequency for an appropriate duration (antibiotic stewardship) is essential [7]. Careful and responsible use of antibiotics is critical in minimizing the risk of microbial resistance. Recent research has reported new information about two families of antibiotics found in bull's seminal plasma: macrolides and tetracycline [8,9]. This new knowledge is essential not only for designing effective treatment regimens but also for avoiding the unnecessary use of antibiotics and preventing the development of antibiotic-resistant microorganisms. To minimize medication errors and potential harm, following the “five rights” is proposed—ensuring the right patient, the right drug, the right dose, the right route, and administering the treatment for the appropriate duration [10]. Adhering to these principles can significantly improve treatment outcomes and patient well-being.

Florfenicol exhibits a broad spectrum of antibacterial activity that includes all microorganisms sensitive to chloramphenicol that comprised gram-negative bacilli, gram-positive cocci, several anaerobes, such as Bacteroides fragilis, Rickettsia and Chlamydia spp., among other atypical microorganisms such as Ureaplasma and Mycoplasma [[11], [12], [13], [14]]. It belongs to the family of antibiotics known as amphenicols, in which mode of action is by inhibiting microbial protein synthesis through binding to the 50S subunit of the 70S ribosome, thereby impairing peptidyl transferase activity and preventing peptide elongation. The typical effect is bacteriostatic, but high concentrations can exhibit bactericidal properties against certain microorganisms. Thiamphenicol and FLO, while structurally related to chloramphenicol, have modifications that enhance their efficacy, reduce toxicity, and, in the case of FLO, decrease bacterial resistance by containing fluorine molecules [11,12]. Florfenicol is considered a time-dependent antibiotic, although some information suggests that it may also exhibit concentration-dependent or codependent behavior [[11], [12], [13], [14]]. It possesses characteristics such as high bioavailability, lipophilicity, and adequate tissue penetration, enabling it to achieve high levels within cells and cross certain anatomical barriers, making it effective against intracellular pathogens [11,[13], [14], [15]]. Importantly, FLO is not susceptible to the actions of acetyltransferase, an enzyme used by bacteria to develop resistance to chloramphenicol and thiamphenicol [16,17]. Florfenicol amine is the major metabolite of degradation of FLO by acid-catalyzed hydrolysis. Florfenicol amine has not antibiotic activity but is an important standard for monitoring animal and environmental residues of FLO [18].

The uses of FLO in veterinary medicine include the prevention and treatment of bovine respiratory disease (BRD) associated with Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni. It is also employed for the treatment of bovine interdigital phlegmon associated with Fusobacterium necrophorum and Bacteroides melaninogenicus [11,12]. In addition, FLO has demonstrated efficacy in treating calves with either naturally occurring or experimentally induced keratoconjunctivitis [18,19], and its presence has been detected in synovial fluid after regional intravenous perfusion [20] and parenteral administration [21].

To the best of our knowledge, the pharmacokinetics of FLO have not been investigated in either bulls or semen. Given its known features mentioned above, this antibiotic presents an intriguing opportunity for examination of its presence in semen. In addition, FLO was the most effective antibiotic against Trueperella pyogenes, the most common pathogen of vesicular adenitis, with more than 95% of in vitro susceptibility of 144 isolates [22]. Moreover, having a long-acting antibiotic available would reduce the frequency of administration and animal handling, thereby minimizing animal stress and improving compliance. Investigating the pharmacokinetic parameters of FLO at two doses, 20 mg/kg, or 40 mg/kg body weight, not only adds new knowledge but also has practical significance. Bulls are large animals, requiring high-volume doses. It is recommended to administer no more than 10 mL per injection site with at least a 10 cm space between administration sites [23]. Therefore, dispensing FLO in mature bulls would necessitate multiple application sites.

The objectives of this study were to compare the serum and seminal plasma pharmacokinetic profile of FLO and its major active metabolite FLA after administration of FLO either by IM or SC routes in beef bulls.