Pulmonary disposition of tilmicosin in foals and in vitro activity against Rhodococcus equi and other common equine bacterial pathogens.

This study investigated the effects and effectiveness of tilmicosin, a commonly used antibiotic, in young horses. The researchers looked at how the drug behaved in the bloodstream and lungs, and how it acted against certain common types of bacteria that harm horses.

Methodology

• The research administered a single dose of a new formulation of tilmicosin, dissolved in a fatty acid salt, to seven healthy young foals that were 5 to 8 weeks old.
• The dosage was given intramuscularly (injected into a muscle), at a dose rate of 10 mg per kg of body weight.
• The researchers then measured concentrations of the drug in various locations within the foals’ bodies, including their blood serum, lung tissue, Pulmonary Epithelial Lining Fluid (PELF), Bronchoalveolar Lavage (BAL) cells, and blood neutrophils.

Findings

• The levels of tilmicosin were different in various parts of the foals’ bodies. The drug was most concentrated in the blood neutrophils, followed by the BAL cells, then the PELF, and least concentrated in the serum.
• The antibiotic persisted for varying times in different tissues, with the longest retention time in lung tissue, then PELF, followed by BAL cells, neutrophils, and serum.
• In laboratory tests, tilmicosin showed potency against most of the 56 strains of the Rhodococcus equi bacteria tested. It was also effective against most types of streptococci, a group of bacteria that include the cause of ‘strep throat’ in humans, plus Staphylococcus, Actinobacillus, and Pasteurella species. However, this antibiotic was not effective against Enterococcus, Pseudomonas, and Enterobacteriaceae species.

Implications

• This research provides valuable insights into how the drug tilmicosin behaves in the bodies of young foals. This is particularly important for the treatment of bacterial diseases in horses, as it demonstrates how the antibiotic is distributed and how long it persists in different tissues.
• The findings will help veterinarians to plan the best dosing strategies for treating equine diseases, especially respiratory tract diseases, caused by susceptible bacteria.
• However, the finding that this drug is not active against some bacterial species suggests that its use may be limited in some circumstances, or that other antibiotics may also be required.