Clinical pharmacology of antimicrobial drugs for the treatment of septic neonatal calves

Neonatal adjustments

Pharmacodynamic responses by neonates are the same as those for adults, but physiologic differences and pathophysiologic changes can affect pharmacokinetic values sufficiently to require adjustments in dosage regimens used for calves. Adjunctive care may be necessary for some patients to correct or maintain perfusion and temperature of tissues so that absorption and distribution may be adequate. Intravenous administration may be the only route appropriate for some critically ill patients. Anatomic sites and formulation of products administered by extravascular parenteral routes markedly can affect absorption of those products and subsequent clinical response to treatment.

Clinical pharmacology of cefixime in unweaned calves

Cefixime is a unique third-generation oral cephalosporin. Its in vitro activity and pharmacokinetic properties have been studied to assess its potential for use in the therapy of newborn calf infections due to gram-negative bacteria. The minimum inhibitory concentrations of cefixime for 90% (MIC90) of field isolates of Escherichia coli, Salmonella and Pasteurella were 0.10-0.40 micrograms/mL. The serum disposition kinetics of cefixime following intravenous and oral administration was evaluated. The elimination half-life of cefixime after intravenous and oral administration was 3.5-4.0 h, the steady-state volume of distribution was 0.34 L/kg and approximately 90% of the drug was bound to serum proteins. Oral absorption was comparatively slow and bioavailability values for single 5 mg/kg doses were 20.2% after the administration of 200 mg of cefixime in capsules, 28.3% after dosing an aqueous solution of cefixime and 35.7% after fasted calves received the solution of cefixime. Mean serum drug concentrations 12 h after the cefixime solution was administered orally (5 mg/kg) were 1.05 micrograms/mL for the milk-fed calves and 1.76 micrograms/mL for the fasted calves. Computations showed that mean free drug concentrations equal to the MIC50 of the drug for gram-negative pathogens associated with newborn calf infections can be maintained in tissues by multiple treatments at 5 mg/kg every 12 h or 10 mg/kg every 24 h.

Weakness in the newborn calf

The peak time period for the average beef producer to experience the majority of calf losses has consistently been from the time of birth through the first seven days of life. Weakness is a principal clinical sign of diseases or conditions responsible for mortality including birth trauma, prematurity or dysmaturity, congenital malformations, metabolic defects, intrauterine infection, anoxia or hypoxia, hypothermia, starvation, extremes in birth weight, and post-natal infection. This article discusses anoxia/hypoxia and septicemia in greater detail because of their involvement as a common cause of weakness in the newborn calf.