In vitro testing of antimicrobial agents for proliferative enteropathy (ileitis)

Steven McOrist, DVM, PhD; and Connie J. Gebhart, PhD

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Proliferative enteropathy (ileitis) is a common disease of grower and finisher pigs. Recent studies by the authors have illustrated reliable methods of in vitro culture of the etiologic agent, an obligate intracellular bacterium, known as ileal symbiont (IS) intracellularis. Characteristic lesions of proliferative enteropathy are reproduced when IS intracellularis is used as oral inocula in challenge experiments in pigs. No clear pattern of the antibiotic sensitivity of IS intracellularis has emerged during clinical usage of antibiotics in field treatment situations over the past 20 years, and few controlled treatment trials have been reported. We used the in vitro cell culture system necessary for the growth of IS intracellularis to test the in vitro effect of 18 antimicrobial agents. The minimum inhibitory concentration (MIC) of each agent was determined for up to three strains of IS intracellularis.The minimum bactericidal concentrations (MBC) of selected agents were determined for one strain. Penicillin, erythromycin, difloxacin, virginiamycin, and chlortetracycline were the most active compounds tested, each with an MIC of < 1 micro-g per mL. Tiamulin and tilmicosin were the next most active compounds, with MlCs of < 4 micro-g per L. These results indicate that compounds capable of entering the host cell cytoplasm and blocking protein synthesis, such as macrolides, tetracyclines, fluoroquinolones, and virginiamycin are active against IS intracellularis in vitro. Some of these drugs have previously been recommended clinically for the treatment of ileitis. The MlCs of aminoglycosides, aminocyclitols, ceftiofur, bacitracin, and avoparcin were generally greater than their normal recommended dose rate, usually >32 micro-g per mL. The MBC results were broadly similar to the MIC results for the aminoglycosides and other groups of agents tested. However, the aminoglycosides may have some effect in controlling secondary infections to a primary IS intracellularis-induced lesion of the intestine. There is clear need for in vivo treatment trials in experimental and field situations to support a list of drugs that is likely to be effective in the treatment and control of ileitis.

Keywords: proliferative enteropathy, antimicrobial, ileitis

RIS citationCite as: McOrist S, Gebhart CJ. In vitro testing of antimicrobial agents for proliferative enteropathy (ileitis). J Swine Health Prod 1995;3(4):146-149.

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