ok.. this is getting interesting... have to review my parasitology to update completely... basically it is the runs and lack of appetite that you really have to watch out for. and the poop has a distinctive Fishy smell ( but you have to be familiar with what normal poop smell like in the first place.) the only prblem with giardiasis is that almost as soon as the dogs are infected they become the source of spread of the disease....so proper disinfection has to be done
A good rule of thumb that we like to recommend is your dog can go for 3 days without food AS LONG AS there is no vomiting or diarrhea, now as for the diarrhea problem.. it's a little more complicated.. the term dehydration is not only with regards the water or h2o in the body but as with humans it has a lot to do with the electrolyte balance in the body.. thats why gatorade is recommended for re hydration. there are also newer products that are designed with rehydration in mind.. such as royal canin's rehydration support which come in sachets that you mix with 500ml of water and is a balanced solution for your dogs re hydration....the sachets are not very expensive and should be worth looking into for us who go on extended hikes and/or who are in very hot areas.
Also some dogs get so engrossed in the hike or play or whatever that they forget to drink sometimes... it is important to let your dogs drink from time to time and not wait for them to be really thirsty.
As for the rattlesnake vaccine i found a good article which pretty much sums everything up...
http://www.hvjournal.com/articles.php?id=1794&art_title=New_Rattlesnake_Vaccine_for_Dogs"
but as with any vaccine it is designed to help the body fight off the disease in the case of the rattle snake vaccine it is primarily focused on preventing an anaphylactic reaction (read: Allergic reaction) to the snake venom.. it will not stop the effect of the venom on the body thus if your dog is bitten it is STILL a medical emergency and proper measures must be taken.
Whew.... that was a long one.. keep the questions coming or if you need clarifications let me know...
Guille