Abstract

The course of infection with Giardia lamblia in gerbils was studied after oral inoculation with either cysts isolated from humans or cultured trophozoites. Primary infections occurred in 87% of exposed gerbils; cyst release peaked during the second and third weeks post-infection; cysts and trophozoites were observed for up to 49 and 87 days post-infection, respectively. Gerbils were refractory to challenge infection with infective human source cysts at 5 months post-infection. Overall, 50% of animals given subcutaneous injections of hydrocortisone acetate, during or following the late stages of their primary infection, showed recrudescence of infection as evidenced by passage of cysts; percentages showing recrudescence were: 80% at 50 days post-infection, 55% at 70 days post-infection, 27% at 7 months post-infection, and 50% (2 of 4) in gerbils challenged unsuccessfully at 5 months post-infection and subsequently injected with hydrocortisone acetate at 7 months post-infection. Both infected animals and uninfected controls injected with hydrocortisone acetate were immunocompromised as evidenced by significantly reduced plaque forming cell responses to sheep red blood cells. We conclude that infections with G. lamblia in gerbils can last longer than standard examinations for cysts would indicate, and that long-term immunity to this organism can be nonsterile.