The protozoan parasite Giardia occurs in two forms. The active (motile) form that lives and multiplies in the intestinal tract is called a "trophozoite." It can be recognized under the microscope by its characteristic appearance, which looks somewhat like a monkey face with two eyes and a nose. The trophozoite only lives in the intestine and cannot survive in the environment for any significant length of time. The other form is called a cyst and is the infective form of the parasite. Each cyst contains two completely formed trophozoites inside of it. Cysts can remain viable in the environment for many months and can cause infection if conditions are cool and moist.

Dogs are infected by ingesting cysts in the environment. Most infections arise from contaminated water, such as puddles, streams, lakes, shallow wells and water contaminated by feces.

Giardia causes disease by damaging the small intestine, which leads to maldigestion (inability to break down nutrients properly) and malabsorption (inability to properly absorb digested nutrients). Giardia also increases intestinal motility, thus decreasing the amount of time the intestine can digest and absorb nutrients. Increased intestinal motility may be manifested by flatulence (excessive gas production) and diarrhea.

The most common symptom of Giardia infection is diarrhea but there are many other causes of diarrhea. Some examples include:

Dietary disturbances: sudden changes in diet, overfeeding, dietary indiscretion, like getting into the garbage and eating too many table scraps

Drugs: aspirin and similar non-steroidal anti-inflammatory drugs such as ibuprofen; many antibiotics; anti-cancer drugs; heavy metals (lead, arsenic) and insecticides

Other parasites including worms (hookworms, roundworms, whipworms, tapeworms) and protozoa (coccidia, Entamoeba, Trichomonas, Balantidium)

Viruses: parvovirus, coronavirus

Bacteria: Salmonella, Campylobacter, Clostridium, E.coli

Obstruction of the intestinal tract by foreign bodies

Tumors of the intestinal tract

Mechanical obstruction of the intestinal tract caused by volvulus (twisting of the intestine) or intususception (telescoping of the intestine on itself)

Metabolic disorders such as kidney failure, liver failure and hypoadrenocorticism