Sick as a dog!

Can someone tell me where this absurd statement came from? People here usually use it in reference to a pretty serious gastrointestinal upset or problem. It really makes no sense to me, though.

Um, more descriptive than sick as a gerbil?

http://kidshealth.org/kid/talk/come_...ck_as_dog.html

I honestly think Cookie had IBS or something similar on top of all his allergies. He would be very ill every time I had to be away from him over night, so ill he wouldn't even be able to keep water down and my parents would rush him to the vets.
I hated leaving him, but sometimes there was nothing I could do about it. It would only be once a blue moon really, not even annually.
The saying seems appropriate in our house!

I once discussed this very saying with a friend and she was of the opinion that the "sick" in the phrase is more a direct comparison with the actual act of vomitting or as it is termed "being sick". Although not a very pleasant subject, when dogs vomit it tends to be quite spectacular in as much as they will bring back the entire stomach contents and it was this that she felt had brought about the saying. Obviously, as with so many other phrases and sayings, things get bent and twisted to suit local language, dialect, expression etc. So it seems quite a logical step to go from the vomitting act to a general sickness or ill feeling, whether this is in dogs themselves or by a slight shift to applying the same to humans.
The friend concerned is headmistress of a large school and keeper of springer spaniels, so seems to me reasonably qualified to hold forth on this subject, although she did insist that she had no concrete grounding for this and it was just her opinion.
Not sure if this helps as it may or may not be right!