A Giardia species was first identified in cats by two investigators in 1925; one investigator named it Giardia cati and the other called it Giardia felis.9 It is now known that Giardia duodenalis is a species complex comprising at least eight major assemblages. The feline Giardia (G. cati) is part of the G. duodenalis species complex. Assemblage A of the G. duodenalis species complex has been reported in people, dogs, cats, and other animals. Assemblage B has been found in people and dogs
but not in cats.10 Other genetic assemblages within the G. duodenalis group appear to be confined to a specific animal host. Cats have been infected by assemblages known to infect people (assemblage
A) as well as assemblages that appear to be specific to cats (assemblage F).
Giardia species occur as trophozoites and cysts. The trophozoite is the active, motile form found in the intestinal tract. It is
teardrop-shaped and is about 15 µm long and 8 µm wide. The cyst is the resistant stage mainly responsible for transmission;
it is oval and about 12 µm long and 7 µm wide (Figure 2). The cyst can survive several months outside the host in wet, cold conditions but is susceptible to desiccation in dry and
Figure 2. Giardia species cysts after zinc sulfate centrifugation. The cysts are about 8 x 12 µm.
The life cycle is direct, and susceptible hosts become infected by ingesting cysts. The prepatent period of giardiasis in
cats ranges from five to 16 days (mean of about 10 days). In contrast to in dogs and people, trophozoites in cats are not
found in the duodenum but in the jejunum and ileum.11 Trophozoites multiply by binary fission in the intestinal tract and then encyst by an unknown mechanism. Shedding of Giardia species cysts by cats may fluctuate from undetectable to concentrations of more than 1,000,000 cysts per gram of feces.11 Peaks of cyst shedding occur sporadically rather than cyclically, and the duration between any two given peaks is generally
from two to seven days.11 Trophozoites can also be passed in feces but rarely survive for long outside the host.
While cats can also be infected with the commensal flagellate P. hominis, T. foetus infections appear to be more common.12 Isolates of T. foetus from cats are genetically similar to those from cattle; dogs are also occasionally infected.13
The organism is present in a cat's ileum, cecum, and colon as a trophozoite, and it reproduces by binary fission.12 The organism does not encyst, so trophozoites are the only recognized stage (Figure 3). After experimental inoculation in cats, T. foetus was detected in the feces of all cats by Day 14.14
Figure 3. Tritrichomonas foetus trophozoite. The trophozoite is about 10 x 26 µm long by 3 x 15 µm wide. (Photo courtesy of Glenda Taton-Allen, Colorado