Assessment of therapeutic and prophylactic effectiveness of levofloxacin in experimental colibacillosis of chicks, caused by Escherichia coli with various susceptibility to the drug

Antimicrobials remain one of the main means to address and control colibacillosis of poultry, but their effectiveness has been noticeably decreasing lately. The cause for that is widely spread strains of Escherichia coli, resistant to most of the used drugs. Thus treatment may prove ineffective quite often and infections continue to spread. The goal of the present research has been the assessment of therapeutic and prophylactic effectiveness of levofloxacin for treatment of experimental colibacillosis in chicks, caused by Escherichia coli with various susceptibility to the drug. For our research we employed 800 Hisex Brown chicks, divided in 2 groups (n=400). Each group was again divided in 16 subgroups (n=25 in each subgroup), among them 8 experimental groups, receiving levofloxacin at a concentration 200 mg/L of drinking water at 24 h prior to the challenge, the treatment lasting 5 days, and 8 control groups, receiving no treatment. The chicks were challenged via intraperitoneal injection with either of the 2 E. coli cultures in suspension: susceptible to levofloxacin and resistant to levofloxacin at doses in the interval of 1.25×10^6 to 1.5×10^8 CFUs/0.5 mL. We noted that levofloxacin had ensured very high survivability of chicks in groups, previously challenged with a strain, susceptible to levofloxacin (survivability 88% to 100%). But when the chicks had been challenged with a strain, resistant to levofloxacin, the therapeutic effectiveness of the aforementioned drug was considerably lower for the whole interval of the used infectious doses (survivability 20% to 80%). The data we have obtained in our research show that effectiveness of levofloxacin for treatment of colibacillosis in chicks is dependent on resistance of aetiological agent. We also stress the necessity for preliminary antimicrobial susceptibility tests of the isolated strains prior to administration of a drug, such tests must be performed in order to try and avoid ineffective regimens and to prevent selection of resistant bacteria.