Giardia lamblia: in vitro cytopathic effect of human isolates.

The variable clinical course of human giardiasis may be due in part to differences in the virulence of various strains of Giardia lamblia. To address this issue, the in vitro cytopathic effect of isolates obtained from human symptomatic or asymptomatic infections was assessed by ultrastructural and electrophysiological methods. Axenic trophozoites of 10 strains of G. lamblia isolated from children with infections in Mexico City were cultured for 12 to 24 hr on live MDCK epithelial cells. No decrease in transepithelial resistance of MDCK monolayers mounted in Ussing chambers was detected with any of the isolates analyzed. On the contrary, trophozoites or media in which the isolates grew produced up to a twofold increase in transepithelial resistance. Transmission and scanning electron microscopy revealed that all isolates of G. lamblia, irrespective of their origin, gave rise to focal regions of microvilli depletion. These modifications were induced by the close attachment of the ventrolateral flange of the parasite adhesive disk to the apical surface of MDCK cells. The circular imprints evolved progressively to larger areas devoid of microvilli. In conclusion, under in vitro conditions, isolates of G. lamblia trophozoites derived from symptomatic or asymptomatic human infections damage epithelial cultured cells mainly by depleting their microvilli. None of the isolates showed evidence of an invasive effect.