In vitro Development of Cystoisospora suis in Swine Kidney Cell Line, SK-6.

Cystoisospora suis (syn. Isospora suis) is an apicomplexan parasite which causes an important piglet coccidiosis resulting in commercial losses worldwide. Successful coccidiosis control using chemoprevention and chemotherapy as the recommended approach, nonetheless, conduces the extensive use of anticoccidial drugs that may lead to the emergence of drug-resistant parasites. Consequently, it is necessary to continuously engage in drug discovery for alternative anticoccidials. To achieve the drug discovery aspect, an appropriate in vitro model for the endogenous development of the coccidium has to be established. Moreover, the in vitro model can reduce the cost and experimental animal use necessitated by in vivo systems. The aim of this study was to investigate the development of C. suis throughout its endogenous life cycle using swine kidney cell line, SK-6 for the in vitro system. The results showed that SK-6 cells seemed to be an excellent support system for the complete endogenous life cycle of C. suis. For ability to infect, sporozoites were able to penetrate into the host cells, first observed by 2 hours post inoculation. The intracellular penetrated sporozoites underwent asexual multiplications producing presumed type I meronts and merozoites. The meronts each formed 2-8 zoites by 3 days post infection (dpi) which were prominently seen on 3-5 dpi. Developing oocysts were initially observed on 5 dpi and went on sequential sexual development resulting in intracellular mature oocysts from 7-14 dpi. These became prominent from 9-12 dpi. Recycling of merozoites to form more meronts and merozoites also appeared by 10 dpi, indicated by merozoites burst from initially infected cells observed re-entering host cells. In conclusion, the presented in vitro system was able to support complete endogenous development of C. suis that was comparable to its in vivo life cycle. Furthermore, this provides an appropriate model for the study of a vast array of aspects relating to the coccidium, especially in applications with regard to the sexual development of the parasite.