Giardia species cysts excyst in the duodenum after exposure to gastric acid and pancreatic enzymes and release two trophozoites that
then separate, mature, and attach to the brush border of the villous epithelium throughout the intestinal tract in cats. The
pathogenesis of diarrhea may be related to secretory-excretory products of the parasite. As with Cryptosporidium species, clinical signs may be related to immunodeficiency, coinfections, other gastrointestinal diseases, host susceptibility,
or infection with more-pathogenic strains.
Several elements, such as adhesion, secretion of proteases, and release of lytic factors, are involved in the mechanism of
tissue damage of T. foetus.31
In a study of naturally infected cats, mild-to-moderate lymphocytic-plasmacytic and neutrophilic colitis and crypt epithelial
cell hypertrophy were common.32 Coinfection with other organisms occurs and influences the pathogenesis of disease. For example, cats with preexisting Cryptosporidium species infection had more severe signs of disease when experimentally inoculated with T. foetus than did cats inoculated with T. foetus alone.12 However, coinfections with Cryptosporidium species and T. foetus are rare in naturally exposed cats. Other predisposing factors as described for Cryptosporidium and Giardia species may also play a role in the pathogenesis of this disease.
Most cats with Cryptosporidium species infection have subclinical disease.15,33,34 Some infected cats develop small bowel diarrhea, weight loss, and anorexia. Clinical signs associated with infection can
occur in immunocompetent or immunodeficient cats.28,35,36 Coinfection with other protozoans including Giardia species and T. foetus may aggravate clinical signs of cryptosporidiosis in cats.13,37,38
Although most of the cats shedding Giardia species do not show clinical signs of disease, some develop acute or chronic diarrhea and weight loss. The diarrhea is usually
mucoid, pale, and soft and has a strong odor; steatorrhea may be present.9,11,39,40 Most infected cats are afebrile, do not vomit, and have normal total protein and hemogram values.9
Kittens with T. foetus-associated illness usually present for evaluation of chronic large bowel diarrhea. The diarrhea is often semiformed and malodorous
and contains blood or mucus. The anal area frequently becomes edematous, and feces can fall from the anus. Clinical signs
of disease are often intermittent and usually resolve with antimicrobial therapy, only to recur after therapy is discontinued.