Giardia colonizes and encysts in high density foci in the murine small intestine

Abstract Giardia is a highly prevalent, yet understudied protistan parasite causing diarrheal disease worldwide. Hosts ingest Giardia 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 is largely inferred from laboratory culture and thus may not mirror Giardia physiology in the host. Here we have developed bioluminescent imaging (BLI) to directly interrogate and quantify the in vivo temporospatial dynamics of giardiasis, thereby providing an improved murine model to evaluate anti -Giardia drugs. Using BLI, we determined that parasites primarily colonize the proximal small intestine non-uniformly in high-density foci. By imaging encystation-specific bioreporters, we show that encystation initiates shortly after inoculation and continues throughout the entire duration of infection. Encystation also initiates in high-density foci in the proximal small intestine, and high-density laboratory cultures of parasites are also stimulated to encyst. This work overturns the assumption that parasites encyst later during infection as they are dislodged and travel through the colon. We suggest that these high-density regions of parasite colonization likely result in localized pathology to the epithelium, and encystation occurs when trophozoites reach a threshold density due to local nutrient depletion. This more accurate visualization of giardiasis redefines the dynamics of in vivo Giardia life cycle, paving the way for future mechanistic studies of density-dependent parasitic processes in the host. Significance Giardia is a single-celled parasite causing both acute and chronic diarrheal disease in over one billion people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of Giardia infections in the host has remained limited, and largely inferred from laboratory culture. To better understand giardiasis in the host, we developed imaging methods to quantify Giardia expressing bioluminescent physiological reporters in live mice. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. Furthermore, this work provides evidence of a parasite density-based threshold for the differentiation of Giardia into cysts in the host. These findings overturn existing paradigms of giardiasis infection dynamics in the host.