Murine Giardiasis: Localization of Trophozoites and Small Bowel Histopathology during the Course of Infection *

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Abstract
A method was developed for quantitative localization of Giardia muris trophozoites in the small intestine of the mouse. Using this method, trophozoite distribution as a function of infecting dose of G. muris cysts, duration of infection, and mucosal histopathology was examined. Mice inoculated with 10 and 104 G. muris cysts were compared. The low dose group required longer to reach maximal levels of cyst excretion and trophozoite counts and had less marked histopathology. The groups were not different, however, in maximum rate of cyst excretion, maximum trophozoite count, and distribution of trophozoites and histopathology. The histopathological lesions were predominantly jejunal and transient, with beginning resolution despite continued presence of near maximal numbers of trophozoites on day 14. Early (day 4 and before) and late (day 90 and after) in the infection, cyst excretion was not a reliable indicator of the level of infection, since there were significant numbers of small bowel trophozoites in most animals without detectable cysts in the stool.