Eight healthy 3-day-old foals were given repeated injections of trimethoprim-Sulfadiazine intravenously for 3 days, at a dosage of 15 mg/kg bodyweight (2,5 mg trimethoprim and 12,5 mg Sulfadiazine). Blood samples were collected prior to each administration and for the following 24 hours after last administration. Serum concentrations of trimethoprim and Sulfadiazine were measured and the pharmacokinetics for the substances were studied. The elimination half time (t½) and clearance (ClB) for trimethoprim and Sulfadiazine for the foals did not diverge from adult horses. According to these data it is possible to assume that the same dose interval and dose can be used for foals as for adult horses.

Today most dosages used in horse medicine are based on studies in adult horses. Since there are differences between adult and neonatal horses with respect to different pharmacological parameters this can cause problems when administring drugs to neonatal foals. Neonatal sepsis is a common cause of morbidity and mortality in foals and aggressive antibiotic treatment is needed immediately when sepsis is suspected. In Sweden the combination of trimethoprim/sulphadiazine and bensylpenicillin is often used as the initial treatment. When administering drugs, interactions can occur between the drugs and endogenous substances.

The retrospective part of this study evaluates the susceptibility patterns of bacteria isolated from blood in critically ill foals in Sweden. The most commonly obtained bacteria were Actinobacillus spp. (38 %) and Escherichia coli (27 %). Trimethoprim-sulphonamide (TMS) had good efficacy against most gram negative bacteria with the exception of Klebsiella spp. Among 17 Escherichia coli isolates 94 % were susceptible to trimethoprim-sulphonamide whereas all 24 isolates of Actinobacillus spp.