A few thoughts on the sick dog and dogs drinking from water sources:
1) Dogs have major amounts of bacteria in their mouths. Don't be offended; it's how they are. It's largely because of poor canine dental hygiene, but, for that matter, most people do too. The difference is that dogs tend to slobber when drinking, and their saliva gets into water sources. This is bad, bluntly, even if the water source is moving. Dogs - like people - should drink water from containers. Put your dog's water in his bowl when you stop for water.
2) Giardia and other water pathogens will affect almost any mammal. This is part of the 'hard choice' in bringing a dog: Why take precautions for your own health that you won't take for your dog's? If you want to protect him/her, purify their water too. Dogs do NOT have some kind of superior kind of GI tract to humans. Impure water will make them sick, too.
3) Dogs can't talk. When they are sick, their sickness can only be identified from external symptoms. If your dog is sick, why try to make them "tough it out" when it can be a dangerous condition? Either you care for the dog - and will take it to a vet when it's ill (especially if the nature of the illness is unknown) or your shouldn't have the dog in the first place. Another hard choice, but one that comes with having a dog as a life companion.
The Weasel