Corticosteroid treatment increases parasite numbers in murine giardiasis.

Corticosteroid therapy is known to be hazardous in patients with occult infection but the mechanism by which the host parasite relationship is altered by steroids is not known. We have used an intestinal protozoal parasite, Giardia muris, to examine the effects of corticosteroids on the number of parasites in the intestine in the course of a primary infection. A single injection of cortisone acetate, subcutaneously, one day before oral inoculation of CBA mice with 1000 cysts of Giardia muris, resulted in significantly higher trophozoite counts in animals studied at one, two, three, four, and eight weeks post-infection, when they were compared with saline injected controls. Recrudescence of occult infection was also achieved by cortisone acetate treatment of mice which had been infected with Giardia muris eight months previously. Clinical studies are required to establish if recrudescence of occult protozoal infection is an important cause of morbidity when immunosuppressive therapy is given to patients in areas where giardiasis is endemic.