Someone asked me a question that made me think of the science of giardia yesterday, or the day before, so I read up on all the government pdfs and pages.
I wish I'd seen this thread, so I could post all the urls for the pdf stuff I read, cdc etc, but the overall thing that all the real sources agreed on were simple:
backpacking is a key infection vector, or activity. All sources mentioned it specifically, without exception.
The reports and articles all stated that 1 cyst is all that is required. Something like 1 to 10 cysts per liter can be expected in running water (assuming infected matter upstream or in your area), that's a number all the sources seem agreed on, and would account for the dilution factor. I kept looking for that 10 cyst ingestion requirement number I've seen referenced in backpacking sources like bpl, but not one of the gov webpages or papers mentioned that higher count of 10 being required, in fact, the only place I saw it mentioned was places like bpl or other backpacking oriented information pages/sites. BPL is not a health science resource, so I don't care what people believe or don't believe, but if that's a myth, it should be terminated and stopped before people get hurt.
One page was kind of interesting, and was discussing why beavers were associated with it, and it noted that association was almost always caused by beavers being close to campgrounds, and getting infected by campers, then their fe ces contaminates the water, and so on.
The notion I come across that one can 'see' if water is good or not good struck me as particularly fantasy based, if there is some fecal matter in the water upstream from where you are, it's not good, and you certainly cannot see that fact, or smell it, maybe you can intuit it if you have a particularly excellent connection with nature and it's flows and patterns, but for average use, you certainly cannot do any such thing.
Again, I wish I'd seen this thread, I'd post every article I read, but they really seemed to agree in general. The two vectors were water and spread fe ces, which actually explains the observation that people make about handling other backpacker's food and getting giardia after that, that fits exactly, poor sanitary habits, then touching the food, then you touching it certainly could spread it, and that seems to be a very common vector.
The thing I was looking for, as scientific fact, not speculation, was specifically, how long giardia cysts can live out of the water, when it's dry. What confuses that question is a common fector, fe ces on doorknobs, trailmix etc, in trace amounts, which I guess lets giardia move to your body. I didn't find anywhere that told me that, but I did find some interesting stuff about temperature and cyst survival, in water, it's a few months when cooler, but drops steadily when warmer, if i remember right, 54C water they lived only 10 minutes, for example, boiling water I believe killed them almost immediately.
I just footnote this by observing that the cdc does not have a wonderful track record, and often lags behind the times, but they lag from what I have seen in underestimating, not overestimating, risk factors. Every single government document listed backpacking specifically as a source of infections, without exception.
This fits nicely with what Edward Abbey noted many years ago in Desert Solitaire, when he observed that prior to 1965 (or was it 1955? been a long time since I read it) or so, every free flowing water source in the USA was drinkable, talking about streams and so on, not of course rivers that flow through farming and urban areas. It took only some 10 years for that situation to change, due to live stock grazing on leased national park lands and backpackers practicing poor sanitation. Since we can read long threads covering people's experiences with that poor sanitation, ie, toilet paper on trails, non buried or inadequately buried waste, I'd certainly consider that to be a primary vector at this point, along with animals also spreading it.
So my quest for the science of it showed almost no variation in the data or information, and all of it said the same thing. Where exactly is the alleged 'science' going on that is disproving categorically the cdc and other government findings, making for an actual scientific disagreement? It's just one guy, right, who publishes some stuff online, correct? Typical internet stuff, right? Or is there real science going on somewhere?
As correctly noted in the sources, you can't test a stream for good or bad status, it's good when there's no contaminated fe ces upstream, and bad when there is. The beaver thing above mentioned that issue directly, and it's kind of obvious anyway, in cooler water giardia cysts only lived some 2 months max. Cysts floating in the water have no smell or flavor. Heavily contaminated water of course would be suspect as dirty, but water being clean shows you almost nothing.
This is why I carry a real filter, mechanical. What one paper explicitly stated was that bleach, chlorine, does NOT kill giardia, that's why people use aquamira, it's a tweaked bleach, so it would be good for that myth to be terminated once and for all, bleach does not kill giardia, unless the science is wrong, their outsides are too tough. Bleach could be a good option post filtering however, to kill smaller things, that's probably a good idea in very bad water.
Speaking for myself, I don't care what people do, but since I do somewhat try to follow the science of such matters, I'm not into pointless risks (for example, if I am carrying a filter already, why would I not use it? Sawyer squeeze is light and leaves no taste behind) when it comes to contracting long term weird parasite stuff, it's really not fun, trust me.
You can find all the cdc and other gov stuff easily, just google giardia and pick the authority sites, read the pdfs, etc. Maybe there's some other science out there, could well be, but that would be in scientific journals and medical journals, which usually are not free to read.
By the way, the bpl word filter really needs to be updated, it's kind of silly, this is an adult oriented site, grownups can handle the word fe ces.