Sometimes this dog has a bad rash, sometimes "he's fine." Also he looks like a brown cousin of Dennis the Menace's dog, Ruff, but I wouldn't worry about that... too much. I will be awake all night worrying about it for you.
For those not nerdy enough to know, the strip is a swipe of the hysterical ending of the 1945 Bugs Bunny cartoon "Hare Tonic", in which Bugs convinces Elmer he's come down with "Rabbititis", for no more reason than tormenting the poor bastard. "Hare Tonic" holds a special place in my heart because it disproves the older Chuck Jones' misguided theory that Bugs only reacts to those who have wronged him, and is never an aggressor: in "Hare Tonic", Bugs tortures Elmer for no offense at all. The young Jones and Jim Davis know that pure impish meanness is funnier than just desserts.
The title panel shows similar nasty Garfield behavior: he's either too lazy to move the pillows off the couch, or too arrogant to obstruct a view of his name, even for physical comfort.
Weird Three Stooges reference in final panel adds little besides making me retroactively give the dog a Curly Howard voice.