This summer I fulfilled my longstanding ambition to hike the Rae Lakes loop in King’s Canyon National Park. The forty-mile trek was beautiful, and I had the opportunity to spend two nights at the stunning lakes which are the crown jewels of the loop. Most of the hike follows rivers or streams, so water is abundant.

On the hike I saw something that surprised me: Most of my fellow hikers drank water from the streams and lakes without filtering or treating it. That seemed dangerous to me; stock animals are allowed on the trails, and they don’t follow the National Park Service’s rule of defecating at least 100 feet away from water. Drinking untreated water where stock animals are common is a good way to contract E. coli. More disgustingly, many of my filthy fellow hikers liked to swim in the lakes after a long day on the trail. Given their lack of regard for hygiene, the lake water probably had plenty of human-sourced E. coli as well.

And then there was the concern about Giardia. Giardia are single-celled organisms known as protozoa. Giardia is supposedly a camper’s worst nightmare. It can be present even in waters, such as those in King’s Canyon, that appear completely pristine. Infection results in spastic diarrhea that can render a person unable to do much other than sit on a toilet (if one is available). Fortunately, there is some good news for hikers. Giardia’s incubation period is usually around 40 days in people. Most people are off the trail by the time they get sick, and they usually blame their diarrhea on the previous night’s dinner rather than the crystal-clear unfiltered stream water from which they drank a month before.

But enough about hikers. It turns out that Giardia can infect dogs as well. And Giardia has been a major problem for dogs and their owners since a new test was developed several years ago.

You see, Giardia had previously been really tough to diagnose in dogs. The organisms can be identified in microscopic evaluations of specially prepared stool samples, but they are fragile. Infected dogs don’t shed the organisms continuously, so false negatives were common. A diagnosis of giardiasis was rare in any dog.

But then came the Giardia antigen test. It was a chemical assay rather than a microscopic test. The new stool test checked for proteins released by Giardia organisms. Once vets started using the new test, dogs started testing positive for Giardia all the time. Rates of infection ranged from 10 percent in average household dogs, to 30 percent to 50 percent in puppies, to 100 percent in some shelters and breeding scenarios.

This led to quite a bit of panic. Dogs could get Giardia and so could people. The millions of dogs now testing positive for Giardia needed to be treated to protect their owners. They needed to be treated whether they were suffering from diarrhea or not.

Except for one thing. People weren’t catching Giardia from dogs. And most dogs that tested positive for Giardia weren’t exhibiting symptoms.

In fact, in the entire history of Giardia in North America not a single case of human giardiasis has been documented to have been contracted from a dog. (Source: Today’s Veterinary Practice, September/October, 2013, page 46.)

There are two things to consider. First, Giardia comes in many different varieties. The varieties are called assemblages. Humans appear susceptible to assemblages A and B and rarely E and F. Dogs are usually infected with C or D. In other words, it appears unlikely that canine Giardia can sicken people. (Note that F is the most common assemblage in cats, so spread from cats to humans appears to be a greater threat than spread from dogs.)

Also, one must consider that many dogs infected with Giardia do not become ill. In fact, many experts now believe that Giardia is a natural part of many canines’ intestinal flora.

Can Giardia cause dogs to become sick? The answer appears to be yes — in some circumstances. Giardia appears to be an opportunistic pathogen. Healthy mature dogs can tolerate its presence in their guts without any problem. Puppies, immune-compromised dogs, and dogs with other intestinal problems can experience diarrhea or exacerbation of pre-existing symptoms as a result of the bug.

What does this mean for dog owners? If your dog has diarrhea and tests positive for Giardia then the Giardia should be treated. If your dog is healthy and tests positive for Giardia on routine screening tests then it possibly should not.

And the antigen test should not be used serially. A dog with diarrhea and a positive Giardia antigen test should be treated for Giardia, but the antigen test should not be used to monitor that treatment because it will probably stay positive forever. Instead, microscopic evaluation of the stool should be used to assess for presence of pathological quantities of the parasite.

And what about spreading to humans? As I mentioned, it is not likely. However, basic hygiene should always be practiced. Dog feces should be cleared from the environment rapidly. People should wash their hands after picking up dog poop, and they should keep their dog’s hind ends clean.

These common sense steps don’t merely protect against the theoretical risk of Giardia transmission from dogs to owners. They protect against the 100% real and ubiquitous threat of E. coli as well.

Read more by Dr. Eric Barchas:

• Why Do Some Dogs Keep “Showing Their Lipsticks”?
• Let’s Talk About Dogs and Euthanasia: When Is It Time? Should You Be Present?
• What to Do Before You Get to the Vet in 12 Emergency Dog Situations
• 12 Dog Emergencies That Need Immediate Veterinary Attention
• Just How Dangerous Is It to Falsely Call a Pet a Service Dog?

Got a question for Dr. Barchas? Ask our vet in the comments below and you might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)