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Summary In Sweden, mares sometimes develop acute, often fatal, colitis when their foals are treated orally with erythromycin and rifampicin for Rhodococcus (R.) equi infection. Clostridium (C.) difficile, or its cytotoxin, was demonstrated in faecal samples from 5 of 11 (45%) mares with diarrhoea. By contrast C. difficile was not found in the faecal flora of 12 healthy mares with foals treated for R. equi infection or in 56 healthy mares with healthy untreated foals. No other enteric pathogen was isolated from any diarrhoeic mare. Of 7 investigated treated foals, 4 had a high (1651.0, 1468.3, 273.0 and 88.8 μg/g) faecal concentration of erythromycin. The dams of those 4 foals developed acute colitis, whereas the dams of 3 foals with a lower (26.3, 4.6 and 3.7 μg/g) faecal erythromycin concentration remained healthy, indicating that there might have been an accidental intake of erythromycin by mares. The foals treated with antibiotics were regarded as asymptomatic carriers and potential reservoirs, as C. difficile was found in 7 of 16 foals investigated, while 56 untreated foals proved negative. The isolated C. difficile strains proved resistant to both erythromycin (MIC >256 mg/l) and rifampicin (MIC >32 mg/l), a fact that may have favoured the growth of C. difficile in the foal intestine. All mares found positive for C. difficile were, or had recently been, hospitalised together with their foals, indicating that C. difficile may be a nosocomial infection, in horses. The results emphasise that routine testing for C. difficile and its cytotoxin is recommended when acute colitis occurs in mares when their foals are treated with erythromycin and rifampicin. Preventive measures in order to avoid accidental ingestion of erythromycin by mares from the treatment of their foals are suggested.

Reasons for performing study When managing strangles in horses, it is crucial to detect chronic infection with Streptococcus equi (SE), i.e. silent carriers. Objectives Evaluate diagnostics for SE carriers over time in a farm. Study design Longitudinal observational study. Methods Sixty-three Icelandic horses isolated on an island were studied 4 to 26 months after remission of acute strangles, including repeated clinical examination and collection of blood, nasopharyngeal lavage (NPL), and guttural pouch lavage (GPL) samples. Twenty-two horses were treated with penicillin locally and systemically. Serology for S. equi was examined by iELISA [1]. Nasopharyngeal lavage and GPL samples were investigated for S. equi and S. zooepidemicus by real-time PCR [2]. Results Thirty-three per cent were SE carriers after 15 months, despite repeated penicillin treatment. In 16/18 carriers, GPL samples were PCR-positive, but not NPL samples, whereas the opposite was true in 2 horses. Several carriers with persistent aerocystitis were not detected by 3 consecutive NPL samples. Five of 18 carriers were seronegative (27.8%) at 15 months. Following conservative treatment, 7 of these 18 carriers were still carriers at 20 months, 8 were negative, and 3 were lost for sampling. Only GPL samples were positive at this point, and only 1/7 carriers were seropositive. Conclusions To detect chronic carriers of strangles, RT-PCR analysis from both GPL and NPL samples may be necessary. Serological screening at individual level often misses individual carriers in long-term cases. Ethical animal research: The testing was approved by the Swedish Ethical Committee on Animal Experiments and horse owners gave their informed consent for inclusion of animals in the study. Source of funding: The Swedish-Norwegian Foundation for Equine Research. Competing interests: None declared.

Summary In Sweden there are several reports of mares developing acute colitis while their foals were being treated orally for Rhodococcus equi pneumonia with the combination of erythromycin and rifampicin. In this study 6 adult horses were given low oral dosages of these antibiotics, singly or in combination. Within 3 days post administration of erythromycin, in one case in combination with rifampicin, 2 horses developed severe colitis (one fatal). Clostridium difficile was isolated from one of the horses, whereas no specific pathogens were isolated from the other. Both horses had typical changes in blood parameters seen in acute colitis. Clostridium difficile was also isolated from the faeces of a third horse given an even lower dosage of erythromycin in combination with rifampicin. This horse developed very mild clinical symptoms and recovered spontaneously. In the fourth horse given erythromycin only, very high numbers of Clostridium perfringens were isolated. The horses given rifampicin only did not develop any clinical symptoms and there were no major changes in their faecal flora. In conclusion, it has been demonstrated that low dosages of erythromycin ethylsuccinate can induce severe colitis in horses associated with major changes of the intestinal microflora. Clostridium difficile has been demonstrated as a potential aetiological agent in antibiotic-induced acute colitis.

Summary Clostridium (C.) difficile, or its cytoxin, was demonstrated in faecal samples from 10 of 25 (40%) mature horses investigated with acute colitis treated primarily with antibiotics for disorders other than diarrhoea. C. difficile was not found in faecal samples from 140 horses without signs of enteric disorders, from 21 nondiarrhoeic horses treated with antibiotics, nor from 22 horses with colitis untreated with antibiotics. Except for C. difficile neither Salmonella nor any other investigated intestinal pathogen was isolated in any of the diarrhoeic horses. The findings strongly support some earlier reports that C. difficile is associated with acute colitis in mature horses treated with antibiotics. Of the 10 horses, 4 proved positive for C. difficile both in culture and in the cytotoxin test, 4 in culture only and 2 only in the cytotoxin test. Eight of 10 horses positive for C. difficile were or had recently been hospitalised, indicating that C. difficile may be a nosocomial infection in horses. All horses positive for C. difficile were treated with β-lactam antibiotics. The authors are grateful to A. Hellander-Edman, B. Green and J. Skidell for fruitful cooperation and for providing specimens and to G. Holmstrom, G. Sigstam and H. Ljung for excellent technical assistance. This research was supported by ATG (the Swedish Horserace Totalizator Board) and the AGRIA Foundation for Research.