Sick as a Dog

This is Chester, an almost three month old Great Dane, at the Great Dane Service Project in Ipswich Massachusetts:

He actually has been sick for a day or so.  But I really don't know the origin of the phrase "sick as a dog."

I have been sick as a dog for the past 30 or so hours.  Bad stomach bug.  Violent vomiting. 

I am on the other side of it now, I hope, but still feeling bad.  Very sore muscles from the heaving.

Apparently I am in good company. Garrison Keillor has had it, and wrote this on his Facebook page yesterday:

Never puked in Dubuque,
Or yorked in Grand Forks,
Or hugged the throne in San Antone,
Never hurled in MOST of the world,
But I got sick something silly in Picadilly.