The intestinal protozoan Giardia is a leading cause of waterborne illness in humans. Like Cryptosporidium, Giardia has a simple life cycle, alternating between an active form that colonizes the intestines of the host and causes diarrhea, and resistant cysts that are passed from the body in the feces. The cysts can survive for long periods in the environment and cause infection when ingested by another host. Unlike Cryptosporidium, there are effective drugs for treating Giardia infection.

More than 100 outbreaks of waterborne giardiasis have occurred in the United States since 1965. Most of these outbreaks have been attributed to contamination of surface water with human sewage. Giardia is also commonly spread by direct person-to-person, fecal-oral transfer of cysts, and giardiasis outbreaks have been associated with poor hygiene or inadequate sanitary conditions in day-care centers, nurseries and public institutions. Foodborne transmission from infected workers or family members can also occur.

Although Giardia infection is common in a wide variety of wildlife and livestock species, there is considerable controversy over the issue of whether cysts shed by animals can infect humans. Investigators have been unable to determine conclusively the host origin of Giardia cysts detected in the majority of waterborne outbreaks.

The evidence for zoonotic transmission of giardiasis from animals to humans is circumstantial. Giardia isolates from humans have been experimentally transmitted to other animals, including beavers and muskrats. In the wild, the reported prevalences of Giardia infection in these aquatic mammals are 7 to 16% for beavers and over 95% for muskrats, but the Giardia species isolated from these animals appear to be different from those that infect humans.

Sheep, goats, pigs, horses and cattle have also been implicated as potential sources for transmission of giardiasis to humans. We have found up to 27% of California range beef cattle to be shedding Giardia in their feces, with the majority of infections occurring in calves. We also detected shedding of Giardia in 4 to 6% of California horses. Despite the isolation of Giardia from these hosts, there exists considerable controversy among researchers as to whether Giardia from livestock is infectious to humans under natural conditions.

Taxonomic confusion is a major impediment to resolving the question of zoonotic transmission. Some investigators have named Giardia species based on their host origin, while others have based species designations on morphological features of the protozoans. Currently, researchers are using various molecular analyses to distinguish different species. It remains unclear, however, how many species of Giardia infect humans or what species are involved in waterborne outbreaks.

Just as Cryptosporidium oocysts are ubiquitous in surface water throughout North America, Giardia cysts are also found in a majority of surface water samples. As with Cryptosporidium oocysts, however, available tests cannot determine whether the detected cysts are infectious to humans. From a practical standpoint, given the scientific uncertainties, any Giardia cysts found in water must be considered potentially infectious to humans. Giardia cysts can be removed by filtration and are not as resistant to chlorine-based disinfectants as Cryptosporidium oocysts.