I'm no scientist, but I would fall on the side of its possible.  Ingestion is required, so I guess if the concentration in that specific location is really high, and the water went down the hatch, then I suppose it could happen.  

I was on a pretty gungy looking beaver pond and got no more than a quarter mouthful, which was spit out. It was pretty gross, but I worried the worst was yet to come. So far so good.

It happened Sunday. I think I'll be good. I've heard the horror stories so I'm just being a little nervous I guess.

Splakin; "Scares me to think I used to drink right out of the lake in the mid 90's. Spring thaw thou lol"
I was up in Temagami a few weeks ago and all of our water came straight from the lake just as I'd done several times previously.
I don't know if I have an iron constitution or what, but have never caught it up there.
I did catch it on the Moon river though even though I was using a filter, I found out later that my buddy dropped it in the mud then rinsed it off in the river, contaminating the clean water outlet

I was very much under the impression that Giardia took at least a week to kick in. Sounds like something else to me frankly. Maybe he also contracted Giardia- but a 4 hour response sounds unlikely. And if it were that easy to contract wouldn’t you get it 1/10 times you swam? Hard to swim without getting at least a little water in your mouth.

I would also hope people don’t usually wash their retainers in beaver ponds :D.

The CDC, Mayo Clinic, and healthlineBC, all state that giardiasis typically takes 1-3 weeks to become symptomatic, with BC noting you can start to show symptoms in as little as 3 days. (https://www.healthlinkbc.ca/health-topics/hw186436). All 3 also note that symptoms include diarrhea, but I found no reference to vomiting. And it’s also pretty clear that many people don’t show symptoms even if they’re infected. So I have no doubt that this fellow had contracted Giardia at some point, I just really don’t see how 4 hours after rinsing a retainer in a lake, that parasite can be held responsible for his clearly very uncomfortable situation. It’s just not how that infection progresses.

However, it’s a good reminder as to the need to be cognizant of various potential infection pathways open to you out in the woods. Any number of other infections you could pick up in the water, food poisoning, etc, can all make for a very bad trip.

The reason this is important is that people will often be so afraid of drinking water (due to potential Giardia infection), that they can get themselves into a pretty serious state of dehydration. As far as I've ever heard, in a place like Algonquin, the suggestion is always to stay properly hydrated (let's say, in an emergency situation where you can no longer treat the water, but you also can't just get to civilization) rather than to avoid the water. One reason that I've heard that specific advice given is that the symptoms take long enough to kick in that you would often be out of the woods by then. Anecdotally, I had a family member contract it (lab verified), and it took about 2 weeks after ingestion of the water source for symptoms to appear.

Agreed. As I mentioned, in a case where you can’t treat your water (like an emergency where you’ve lost or broken you treatment device, or you’ve gotten properly lost in the woods and weren’t prepared to stay out), it’s better, in places like Algonquin, to drink the cleanest surface water you can find than to go into dehydration. And one reason for that is that Giardia, which is pretty commonly encountered in the woods, typically takes long enough to kick in that you would be back in civilization anyway and could get treatment if necessity. Don’t become dehydrated because you’re afraid of getting sick. But obviously none of that supersedes the need for some sort of water treatment plan. There are plenty of people who go to Algonquin and voluntarily drink untreated lake water, but I’m not among them nor is it ever recommended.