Giardia Colonizes and Encysts in High-Density Foci in the Murine Small Intestine

is a highly prevalent yet understudied protistan parasite causing significant diarrheal disease worldwide. Hosts ingest cysts from contaminated sources. In the gastrointestinal tract, cysts excyst to become motile trophozoites, colonizing and attaching to the gut epithelium. Trophozoites later differentiate into infectious cysts that are excreted and contaminate the environment. Due to the limited accessibility of the gut, the temporospatial dynamics of giardiasis in the host are largely inferred from laboratory culture and thus may not mirror physiology in the host. Here, we have developed bioluminescent imaging (BLI) to directly interrogate and quantify the temporospatial dynamics of infection, thereby providing an improved murine model to evaluate anti- drugs. Using BLI, we determined that parasites primarily colonize the proximal small intestine nonuniformly in high-density foci. By imaging encystation-specific bioreporters, we show that encystation initiates shortly after inoculation and continues throughout the duration of infection. Encystation also initiates in high-density foci in the proximal small intestine, and high density contributes to the initiation of encystation in laboratory culture. We suggest that these high-density foci of colonizing and encysting likely result in localized disruption to the epithelium. This more accurate visualization of giardiasis redefines the dynamics of the life cycle, paving the way for future mechanistic studies of density-dependent parasitic processes in the host. is a single-celled parasite causing significant diarrheal disease in several hundred million people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of infections in the host has remained limited and largely inferred from laboratory culture. To better understand physiology and colonization in the host, we developed imaging methods to quantify expressing bioluminescent physiological reporters in two relevant animal models. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. We also show that high parasite density contributes to encystation initiation.