Sick as a Dog

What, exactly, does this phrase mean? How was it first derived? The bigger boggle...since my sweet baby Honey is ill, does that mean she's as sick as a human?

And why are animal medications so much more expensive than people medications? Are the ingredients/formulations more complex? Are generic forms available?

My children often and frequently accuse me of treating my chihuahuas better than I treat them (which is sometimes true I suppose). So why should I not be allowed to apply for social security for Honey? I feed her, clothe her (when she allows me to put the cute little sweaters on her), I pay for a sitter when I leave her for extended periods of time, she has never been to "school" (only home-schooled, lol), but still, on paper she receives the same attention and monetary support as the rest of my family & children. I think I should be allowed to claim her on my taxes at least...insurance coverage would be nice too. Just a random thought...