Not sure if you can get giardia from changing dog foods . . .giardia is a parasite. I found some info by looking online:
A dog becomes infected by eating the
cyst form of the parasite. In the small intestine, the cyst opens and releases an active form called a trophozoite. These have flagella, hair-like structures that whip back and forth allowing them to move around. They attach to the intestinal wall and reproduce by dividing in two. After an unknown number of divisions, at some stage, in an unknown location, this form develops a wall around itself (encysts) and is passed in the feces. The
Giardia in the feces can contaminate the environment and water and infect other animals and people.
and for cleaning your home
Solutions of Lysol, bleach, and quaternary ammonium compounds are effective against
Giardia.: