The impact of antibiotics on the fecal flora of the horse - A review

Antimicrobiotic-associated diarrhoea (AAD) is a well known phenomenon in horse medicine, but information about the incidence and severity of these cases is scarce. There is even less information about the nature of antibiotic-induced gastrointestinal disturbances, which are assu- med to be responsible for the clinical picture. This article enlists the substances which are said to have a high risk of inducing diarrhoea in the horse and reviews the existing case reports. Because of this risk lincomycin and clindamycin are contraindicated for the use in horses. Macrolides, tylosin, metronidazole, oxytetracyclin, potentiated sulfonamides and penicillins are said to possibly induce severe diarrhoea, especially in combination with stress factors. Bacteriological examinations of the gastrointestinal flora are rare in cases of AAD and often concentrate on the search of pathogens like clostridia and/or salmonella. Therefore, most of the article reviews the results of studies, which were performed to analyse the fecal flora of initially sound horses or ponies under the influence of antibiotics, namely lincomycin and clindamycin, oxytetracycline, potentiated sulfonamides, ampicillin, penicillin, a penicillin-streptomycin-combination and gentamicin. In cases of AAD, induced by lincomycin, clindamycin or oxytetracyclin the most consistent findings in several studies by different authors are rises in Clostridium counts, which are frequently identified as Clostridium perfringens. There are hints, that the proliferation of Clostridia is preceded by rises in coliform counts and accompanied by rises in streptococci counts in clinically manifest cases. In Salmonella carriers oxytetracyclin is able to induce diarrhoea and to prolong the shedding of these bacteria. However, up to now, there are no results which allow conclusions on the mechanisms of the proliferation of pathogens like clostridia or salmonella. Fecal flora studies of faeces with normal consistency from antibiotic treated horses revelled in most cases decreases in bacterial numbers or no relevant changes in bacterial counts. However, studies are very limited and further evaluations are needed tc ensure and probably differentiate these results.