Understanding Giardia in Dogs Symptoms Causes and Treatment Giardia is a protozoan parasite that inhabits the intestines of infected dogs, leading to a disease known as giardiasis. Dogs typically contract the parasite by ingesting Giardia cysts found in contaminated water, soil, or feces. The disease can affect dogs of all ages, but puppies, older dogs, and dogs with weakened immune systems are more susceptible. The parasite thrives in moist, cool environments, making areas with stagnant water, puddles, and unsanitary conditions hotspots for Giardia contamination.