It might be our national animal, but the beloved beaver doesn’t always love us back.

Whether he’s busy taking down your apple trees or building dams that flood your favourite farmland (I know all about this after a recent trip to Quesnel involving a tour over swampy, once viable fields with a shotgun toting friend), the oversized rodent can be a pest.

But he’s truly at his worst when you drink from the pond he’s been pooping in.

Giardia (or beaver fever as it’s commonly called) is a parasite passed along through feces — from beavers and other animals, including humans.

Though it can be transmitted hand to mouth, people are most commonly infected after drinking contaminated water — and since beavers live in water — you get the idea. If you ingest giardia, you develop giardiasis. And it’s nasty.

My brother ought to know; same with a good friend of mine recently diagnosed.

We’ll start our beaver tale with my brother. An avid outdoorsman, Michael was more often lost in the backwoods surrounding our home than found in front of the TV.

As a teen, he once sipped from what seemed to be a clear-running mountain stream. A couple of weeks later he was plagued with fatigue, diarrhea, and nausea. As his weight plummeted, our mom’s fear rose until she insisted he see our family physician.

Dr. Chan put Michael’s hiking history and presenting symptoms together to conclude he had beaver fever. A stool sample confirmed the diagnosis.

Not uncommon, the American Centers for Disease Control and Prevention calls giardia the most frequently diagnosed intestinal parasitic disease in the United States. Ditto for Canada.

Symptoms generally present themselves about seven to 14 days after exposure. Tiny but tough, giardia cysts are covered in an outer shell — a sort of miniature armour — that allows them to survive outside their host for long periods of time, especially in cold water. This also protects them from chlorine disinfection.

It won’t take an army of them to take you down. Ingestion of just

10 of the microscopic cysts can make you sick.

Once through the front door of our bodies, our stomach acid helps the cysts along their lifecycle, turning them to trophozoites, which go about the business of multiplying in our intestine, causing all kinds of unpleasant symptoms like gas, smelly diarrhea, nausea, vomiting, weight loss and fatigue. The parasites then produce more cysts, which are passed in our stool, ready to rally if they get ingested by another animal.

Cases of beaver fever rise in the summer as campers and hikers sip from contaminated streams, rivers, and lakes, but you can contact giardia riding your bike in Cuba; ask my friend.

I knew something was wrong when we met for lunch, a week after her return to Canada. Kindred spirits in many ways, including our voracious appetites, while I inhaled my lunch, she took just a few tentative spoonfuls of soup before pushing her bowl away, her face distorted in dislike.

Despite intense hunger, every time she ate, she felt ill.

She and her husband started their trip relaxing at a resort with no issues. When they embarked on their bike tour, things got gut-ugly. Within a few days, she wasn’t the only biker looking for the loo. Fellow riders including Brits and Kiwis were suffering gastro symptoms too.

It got worse when she got home. “I could barely get off the couch,” she said.

While the rest of us were gaining Christmas weight, she lost seven pounds. Severe diarrhea, vomiting, and near constant nausea left her weak and thin and headed to the doctor with a stool sample.

It’s likely she picked up giardia from fresh fruits and vegetables washed in contaminated water.

According to healthlinkbc.ca, about half of people infected with giardia will clear the infection without symptoms, but if you’re in the unlucky lot, you’ll suffer for weeks, maybe even months, as symptoms peak and subside, then rise again.

About 40 per cent of infected people develop acquired lactose intolerance (no wonder my friend declined her cream of mushroom soup).

Her physician suggested trauma to the gut, together with the parasite, deletes the lactose enzyme, which can take some time to build back up.

Now on a course of the antibiotic metronidazole, my friend says she’s planning the next trip to Europe, where it seems safer — at least from parasites.

To avoid these nasty little critters, boil water for at least one minute (two if you’re 6,500 feet above sea level) to disinfect it. Keep your camping latrine at least 30 metres from your water source. Avoid unpasteurized milk and juice and wash your hands after using the potty, changing diapers, handling pets and before eating.

If you’re travelling, bring hand-sani and stick to cooked food and bottled water.

And hey, don’t blame the beavers. Recent research suggests these ecosystem engineers can play a vital role in restoring salmon habitats.

And we Canadians love our salmon.

Shannon Linden writes a blog, magazine articles, and grocery lists. Her health and humour column runs Saturdays. Visit her at shannonlinden.ca.