An important obstacle in studying Cryptosporidium parvum is the lack of a permanent in vitro cultivation system of the parasite. While short-term cultures using various host cell lines have been widely employed, long-term cultures that would facilitate the immortalization of C. parvum isolates have not yet been developed. The description of the complete development of C. parvum in cell-free culture in 2004 has been received with great interest and also with some astonishment. Unfortunately, attempts to reproduce these results with different isolates of C. parvum and also C. hominis have failed. In this report, we provide an alternative interpretation of the nature of a parasite stage that occurs 24 hr after excystation of oocysts which, morphologically, is similar to stages that have been regarded as being extracellular trophozoites or merozoites by other investigators.

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