Cryptosporidium is a widespread parasite transmitted through contaminated food and water; it could lead to severe gastrointestinal disease in immunocompromised hosts. To date, no specific therapy proved to be effective against Cryptosporidium parvum (C. parvum). The present work studied the efficacy of Chenopodium ambrosioides (CA) oil extract on C. parvum infected dexamethasone (DEX) immunosuppressed mice. Infected im unosuppressed mice (n=70) were divided into 14 groups of five mice each according to different concentrations of received treatments either CA oil extract at doses of 75, 100, 125, 150 & 175mg/kg/day alone or in combination with nanoparticles or nitazoxanide
at doses of 50 & 100mg/kg/day alone or in combination with nanoparticles. Five infected non-treated mice were included as an infected control group; five non- infected & non-treated mice were enrolled as a normal control one. Anti-Cryptosporidium efficacies of different treatment regimens were evaluated in the 16th mice groups microscopically by detection of C. parvum oocysts in fecal pellets and histopathological examination of their small intestines. The results showed that different concentrations of CA oil extract induced a significant reduction in the mean number of C. parvum oocysts. The rate of oocysts reduction was parallel to the increasing concentrations of the drug. The inhibitory effects of CA oil extract on the parasite were enforced by adding nanoparticles. Histopathological examination of mice intestine revealed that CA did not show toxic effect on intestinal mucosa even with high doses.