| Summary: | Abstract Background Toltrazuril is frequently administered for the metaphylactic control of piglet cystoisosporosis. In a previous study, the efficacy of parenteral toltrazuril (45 mg/piglet, Group Forceris®) applied on the 2nd day of life (dol), and of oral toltrazuril (20 mg/kg of body weight, Group Baycox®) applied on the 4th dol was evaluated in an experimental model with Cystoisospora suis infection on the 3rd dol (late infection, LI). In a follow-up study, efficacy and safety were evaluated against infections with C. suis on the 1st dol (early infection, EI). Parameters included oocyst excretion and faecal consistency, body weight development, bacteriological examinations and animal health. Results All control piglets (n = 12) shed oocysts and had diarrhoea, while parasite excretion was completely suppressed in both treatment groups (n = 13 each) and diarrhoea was reduced to a single animal (Forceris® group), resulting in significant differences for these parameters between the treated groups and the controls without significant differences among the treatment groups. No treatment-related adverse events were noted. Body weight gain was reduced in the control group during the acute phase of infection, resulting in significantly lower body weight on the 15th dol. Sows and piglets shed high numbers of Escherichia coli. Clostridium perfringens type A was only detected in low amounts in pooled litter samples. In comparison to the LI study oocyst shedding was more intense in the control animals in EI, while diarrhea was more frequent in LI. In both infection models a high efficacy of toltrazuril in the control of parasitological and clinical outcomes of experimental C. suis infection could be demonstrated. Since in the LI study high numbers of Cl. perfringens type A were detected, it is hypothesized that colonization with these opportunistic pathogens has synergistic effects with C. suis and may explain variable clinical outcomes in untreated animals as well as the sporadic occurrence of diarrhea in toltrazuril-treated piglets. Conclusions Parenteral and oral toltrazuril administered on the 2nd or 4th dol is safe and effective against experimental infections with C. suis on the 1st to 3rd dol. The clinical outcome of experimental infections seems influenced by bacterial coinfections.
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