[header] How to diagnose and treat giardia in cats [title] Watch for changes in bowel movements. [step] If you want to diagnose giardia, pay attention to your cat's litter box. The main sign of giardia is unusual bowel movements. 
Cats will have foul-smelling, watery diarrhea that may contain trace amounts of blood. [substeps] Some cats may also lose weight with giardia due to the diarrhea.