Abstract
Since various pathogens or factors have been implicated in the development of diarrheic disease, laboratory testing is necessary for accurate diagnosis. The current study was designed to obtain an idea about the pathological effect of Salmonella, E.coli and Cryptosporidium on GIT in rat model. A total of 48 early mature, male albino rats (Swiss Webster) were used to study the course of the infection with E.coli O18, S. Enteritidis and Cryptosporidium during a 30-day period, with special emphasis on clinical signs, bacterial dissemination, oocyst shedding and finally histopathologic changes in target organs. The results revealed that E. coli 018 had pathogenic potential in rats. Shedding of E. coli organisms in faeces detected from 2nd day post infection (PI) by conventional culture methods. While, using quantitative real time PCR (qPCR) it was detected for longer periods. In Salmonella infected group, shedding o f S. Enteritidis was detected only in 2nd day PI by conventional methods while by using qPCR was detected for longer periods. Cryptosporidium oocyst shedding in immunocompetent rats began on the day 4 PI and reached the peak on day 11 PI. The intensities of oocyst shedding were significantly greater in the immunosuppressed rats than in the non-immunosuppressed ones. The results of this study confirmed the use of rat model in the experimental infections to evaluate the negative impacts of diarrheic agents on human health is of great value and it is required to understand the pathogenesis of these infectious agents as these models help us to make simulation for human host. They also help to evaluate the diagnostic procedures that would help in development of control programs.

Key words: Salmonella, E. coli, Cryptosporidium, Experimental, Studies, Rats,