A microbiological assay to estimate the antimicrobial activity of parenteral tildipirosin against foodborne pathogens and commensals in the colon of beef cattle and pigs

Effect of metaphylactic administration of tildipirosin on the incidence of pneumonia and otitis and on the upper respiratory tract and fecal microbiome of preweaning Holstein calves

The objectives of this study were to evaluate the effect of the metaphylactic use of a semi-synthetic long-acting macrolide (tildipirosin) on the prevention of pneumonia and otitis in preweaning Holstein calves, as well as its effects on the microbiome of their upper respiratory tract (URT) and feces. Newborn healthy Holstein heifers, collectively housed, were randomly allocated to 1 of 2 treatment groups: treatment (TRT; n = 932) or control (CTR; n = 927). Calves in the TRT group received a single subcutaneous injection of 4 mg/kg tildipirosin (Zuprevo, Merck Animal Health) at 7 ± 7 d of life. Calves in the CTR group received no drug injection. All enrolled calves were evaluated from 1 to 63 ± 3 d of life (weaning age) and monitored daily for any adverse health events during this period. Daily physical examination was performed to diagnose pneumonia and otitis, and body weight was measured weekly in all animals. From a randomly selected subset of 217 calves, blood samples for biochemical variables analysis and swabs were collected weekly from the URT and rectum for analysis of the nasal and fecal microbiome, respectively, via next-generation sequencing of the 16S rRNA gene. Total bacterial load was evaluated using quantitative PCR. In addition, another subset of 26 calves was randomly selected and fecal swabs were collected in a more intensive sampling to investigate the short-term effect of tildipirosin administration on the fecal microbiome. We performed general mixed linear models and logistic regression to analyze continuous and binary outcomes, respectively. Tildipirosin metaphylaxis reduced the incidence of otitis (CTR = 47.03%; TRT = 37.55%) and tended to reduce the incidence of pneumonia (CTR = 20.71%; TRT = 17.38%) and the overall mortality risk (CTR = 6.69%; TRT = 4.94%). We observed no significant differences between groups for mortality due to pneumonia (CTR = 0.86%; TRT = 0.97%) or mortality due to otitis (CTR = 2.05%; TRT = 1.39%). Calves in the TRT group had a higher average daily gain than calves in the CTR group. Furthermore, metaphylaxis had no significant effects on the total bacterial load, genus, or phylum analysis of the fecal microbiome from the 2 subset groups. However, for the URT microbiota, we observed a significant decrease in total bacterial load for the TRT group compared to the CTR group 1 week after metaphylactic injection. Tildipirosin metaphylaxis decreased the mean relative abundance of the genera Mannheimia, Moraxella, and Pasteurella but significantly increased the mean relative abundance of Mycoplasma. Although tildipirosin had no positive effect on Mycoplasma, it reduced the mean relative abundance of important pathogenic bacteria in the URT and had positive effects for the control of otitis. The metaphylactic use of tildipirosin can be a suitable strategy for the control of otitis on farms with a high prevalence of this disease.

Pharmacokinetics and Pharmacodynamics of Tildipirosin Against Pasteurella multocida in a Murine Lung Infection Model

Tildipirosin, a 16-membered-ring macrolide antimicrobial, has recently been approved for the treatment of swine respiratory disease and bovine respiratory disease. This macrolide is extensively distributed to the site of respiratory infection followed by slow elimination. Clinical efficacy has been demonstrated in cattle and swine clinical field trials. However, the pharmacokinetic/pharmacodynamic (PK/PD) index that best correlates with the efficacy of tildipirosin remains undefined. The objective of this study was to develop a PK/PD model following subcutaneous injection of tildipirosin against Pasteurella multocida in a murine lung infection model. The PK studies of unbound (f) tildipirosin in plasma were determined following subcutaneous injection of single doses of 1, 2, 4, 6, and 8 mg/kg of body weight in neutropenic lung-infected mice. The PD studies were conducted over 24 h based on twenty intermittent dosing regimens, of which total daily dose ranged from 1 to 32 mg/kg and dosage intervals included 6, 8, 12, and 24 h. The minimum inhibitory concentration (MIC) of tildipirosin against P. multocida was determined in serum. The inhibitory effect I_max model was employed for PK/PD modeling. The area under the unbound concentration-time profile over 24 h to MIC (fAUC_{0-24 h}/MIC) was the PK/PD index that best described the antibacterial activity in the murine infection model. The fAUC_{0-24 h}/MIC targets required to achieve the bacteriostatic action, a 1-log₁₀ kill and 2-log₁₀ kill of bacterial counts were 19.93, 31.89, and 53.27 h, respectively. These results can facilitate efforts to define more rational designs of dosage regimens of tildipirosin using classical PK/PD concepts for the treatment of respiratory diseases in pigs and cattle.

Veterinary Medicine Needs New Green Antimicrobial Drugs

Given that: (1) the worldwide consumption of antimicrobial drugs (AMDs) used in food-producing animals will increase over the coming decades; (2) the prudent use of AMDs will not suffice to stem the rise in human antimicrobial resistance (AMR) of animal origin; (3) alternatives to AMD use are not available or not implementable, there is an urgent need to develop novel AMDs for food-producing animals. This is not for animal health reasons, but to break the link between human and animal resistomes. In this review we establish the feasibility of developing for veterinary medicine new AMDs, termed "green antibiotics," having minimal ecological impact on the animal commensal and environmental microbiomes. We first explain why animal and human commensal microbiota comprise a "turnstile" exchange, between the human and animal resistomes. We then outline the ideal physico-chemical, pharmacokinetic, and pharmacodynamic properties of a veterinary green antibiotic and conclude that they can be developed through a rational screening of currently used AMD classes. The ideal drug will be hydrophilic, of relatively low potency, slow clearance and small volume of distribution. It should be eliminated principally by the kidney as inactive metabolite(s). For oral administration, bioavailability can be enhanced by developing lipophilic pro-drugs. For parenteral administration, slow-release formulations of existing eco-friendly AMDs with a short elimination half-life can be developed. These new eco-friendly veterinary AMDs can be developed from currently used drug classes to provide alternative agents to those currently used in veterinary medicine and mitigate animal contributions to the human AMR problem.