Recent studies show that Giardia isolates are heterogeneous but fall into at least three groups as determined by a number of complementary techniques. Giardia undergoes surface antigenic variation, both in vitro, and in humans and other animal model infections. Many of the characteristics of antigenic variation and the proteins involved, called variant-specific surface proteins (VSPs), are unique. The sequences of five VSPs reveal a family of cysteine-rich proteins. Here Theodore Nash reviews the relationship between antigenic variation and Giardia heterogeneity.

Giardia lamblia is one of the most common parasites of humans. Ingested cysts excyst in the small intestine and multiply as motile, flagellated, binucleated trophozoites. These encyst in the lower intestine and colon and are then excreted in the feces, sometimes in enormous quantities (approaching 10 7 per gram). Molecular comparison of cultured isolates 1 and clones has led to an understanding of the genetic relationship among isolates and the finding that Giardia trophozoites undergo surface antigenic variation.