In vitro susceptibility to antimicrobial drugs of 62 Salmonella strains isolated from horses in The Netherlands.

The in vitro activity of 17 antimicrobial drugs against strains of Salmonella typhimurium (n = 52), Salmonella thompson (n = 2), Salmonella heidelberg (n = 3), Salmonella hadar (n = 2), Salmonella enteritidis (n = 1), Salmonella infantis (n = 1) and Salmonella derby (n = 1) was tested using the agar dilution method. The strains were isolated from horses admitted to the Large Animal Clinics of Utrecht University. The majority of strains were susceptible to gentamicin, amikacin, kanamycin, enrofloxacin, ciprofloxacin, flumequine, colistine, furazolidone and ceftiofur. However, all strains of Salmonella typhimurium phage type 200 (n = 14), were multiresistant i.e. were resistant to ampicillin amoxycillin, amoxycillin in combination with clavulanic acid, chloramphenicol, nitrofurantoin, trimethoprim, aditoprim and baquiloprim. Two of these strains were also resistant to gentamicin. Based on the susceptibility data found in the present study in combination with pharmacokinetic data available in the literature a rationale for antimicrobial therapy in equine salmonellosis is given. As first choice, gentamicin at a dosage of 3 mg/kg combined with ampicillin at a dosage of 20 mg/kg given with a 8-12 hour dosing interval by intravenous route is advised. As an alternative, the intravenous administration of trimethoprim/sulfonamide combinations given twice daily at a combined dose of 30 mg/kg is suggested.