Cryptosporidium parvum is a coccidian parasite which causes gastrointestinal disease in humans and a
variety of other mammalian species. Several studies have reported different degrees of pathogenicity and virulence
among Cryptosporidium species and isolates of the same species as well as evidence of variation in host susceptibility
to infection. The study aimed to investigate infectivity and virulence of two Cryptosporidium parvum “Iowa isolate”
(CpI) and a “local water isolate” (CpW).Thirty-three Swiss albino mice have been divided into three groups: Negative
control Group (C), the CpI group infected with “Iowa isolate “and the CpW group infected with C. parvum oocysts
isolated from a local water supply. Infectivity and virulence have been measured by evaluating clinical, parasitological
and histological aspects of infection. Significant differences were detected regarding oocysts shedding rate, clinical
outcomes, and the histopathological picture of the intestine, lung, and brain. It was concluded that the local water isolate
is significantly more virulent than the exported one.
Key words: Cryptosporidium parvum, isolates, infectivity, vi