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The present study investigated the efficacy and mode of action of Ponazuril(BayVi9143), a symmetrical triazine trione, in artificial infection with lsospora suisin piglets.Infection with 1 X 104 sporulated oocysts produced diarrhoea andoocyst excretion inuntreated controls. A single oral treatment with 20 mglkgbody mass Ponazuril (BayVi 9143) two days after infection was able to control theinfection completely. Weightgain up to age 28 days was significantly higher intreated piglets than in sick controls.In the investigation of the mode of action of Ponazuril (Bay M 9143), the same dosage was administered at various times post infection (2, 3 and 4 days p.i.) in order to elucidate the action of the product on the various stages of parasite development and thus to derive recommendations on the best time of treatment. The parameters investigated were diarrhoea, oocyst excretion and histopathological changes in the gut on dissection (5 and 7 days p.i.).The infected control animals developed patent isosporosis characterised by massive histopathological changes: villous atrophy, fibrinoid necrosis at he tips of the villi, and inflammatory reactions caused by granulocytic infiltration of the villi. The parasites isolated from the sections of the jejunum and ileum that were examined comprised predominantly sexual stages of development and a small number of asexual stages. Neither clinical nor parasitological symptoms were identified in animals given early treatment with Ponazuril (Bay Vi 9143) (2 days p.i.). Histopathology revealed only a few parasitic stages. The gut villi were significantly longer than those in the control animals. Treatment on days 3 and 4 p.i. reduced the clinical and parasitological findings but was unable to suppress them completely because of the advanced degree of intestinal damage caused by the various stages of Isospora suis.Early treatment with Ponazuril (Bay Vi 9143) prevented the development of asexual and sexual parasitic stages of lsospora suis and thus the development of thedisease. Treatment 2 days p.i. thus controls not only the clinical (diarrhoea) and parasitological (oocyst excretion) manifestations but also the histopathological changes in the target organ, the small intestine.Treatment at a later point of time interrupts the development of the parasite but cannot suppress lesions in the gut epithelium which have already developed.For this reason, treatment of piglets against isosporosis in stocks with known problem in the first few days after birth is recommended under field conditions.