Giardia is a family of protozoan parasites that infest the digestive tract of a variety of species, from mice to amphibians – even you and your pets! Each Giardia species tends to be specific to the animal it infects, with Giardia duodenalis (syn. G. intestinalis, G. lamblia) being attributed to humans and our domesticated animals.

Like many intestinal pathogens, Giardia is passed from “fecal-oral transmission,” where something contaminated with infected feces finds its way into you or your pet’s mouth. This is most often water, but can include grass, rocks, sticks, toys, and even your hands. Giardia is very contagious, with as few as 10-100 organisms being able to infect a new host. To put that in perspective, an infected host can shed as many as 10 billion organisms with each bowel movement!

Once in the digestive tract, Giardia feeds and replicates in the small intestine, damaging the intestinal lining and interfering with digestion. Many infected hosts may show no symptoms at all, but symptoms can range from mild flatulence to severe diarrhea. Depending on the severity of damage to the small intestine, some cases may continue to have foul-smelling, diarrhea for several months while the intestinal lining recovers.

Your veterinarian may detect Giardia with a “fecal test,” which looks for Giardia with a microscope. They may also recommend an “antigen” test, which detects specific proteins on the organism surface. Giardia is a very treatable disease, but may require one or more treatments to clear completely.

Giardia can persist in the environment for weeks, so the best thing you can do to reduce risk to you or your pet is to follow good hygiene practices – wash those hands and toys! – and drink clean water.

Veterinarians pay close attention to the potential for diseases to be spread from animals to humans – a process called zoonosis. With DNA testing, scientists further differentiate Giardia duodenalis into smaller, more specific sub-species groups. Since 1998, studies of Giardia in cats, dogs and humans found that different sub-species of Giardia were specific to the species they infected – some infected humans, some infected cats and others infected dogs. It wasn’t until 2005 that studies in Mexico, Brazil and India began to show evidence of dogs carrying strains previously attributed to humans – in essence, a “reverse zoonosis!”

While there has not yet been any evidence of dog or cat strains of Giardia in humans, the potential for dogs to carry the human strains of Giardia has veterinarians on alert. Until we know more, the philosophy “better safe than sorry” seems the wisest approach, especially when one of the highest risk human groups for contracting Giardia is children from the ages of 1-9 years old.

If you’re out hiking, think twice before drinking from that mountain stream – it could contain many infectious pathogens, including Giardia. The best way to protect yourself is to bring bottled water or use a specialized filter.