The factors that determine the severity of giardiasis are poorly understood. Host factors are important, but parasite virulence may also play a role. The aim of this study was to compare the apparent virulence of three genotypically different Giardia isolates (P01, VNB3, and WB).

Infection rates, parasite loads, structural damage, disaccharidase activity, and water and electrolyte absorption were observed at 10 days after inoculation in a neonatal rat model of infection.

DNA fingerprinting showed differences between isolates studied. The infective rate varied between 67% and 100%. There were no differences in intestinal parasite load. Infection with strains P01 and WB, but not with VNB3, was associated with a reduction in villus height. There was precocious expression of sucrase at 10 days after inoculation in all infected groups. Water absorption of a plasma electrolyte solution was decreased in VNB3-infected animals when compared with P01- and WB-infected animals and controls. Water absorption and lactose hydrolysis were impaired during perfusion with a lactose-containing solution in all infected groups.

Three genotypically different Giardia isolates that infect neonatal rats with the same trophozoite load differ in their ability to cause functional mucosal damage. Infection with Giardia lamblia induced precocious expression of sucrase activity and impaired mucosal absorption