You can definitely machine wash most down comforters, but use cold water and throw tennis balls and several dryer sheets in the dryer with the comforter. Machine dry it partially, then let it air dry the rest of the way. Otherwise, yes, damp warm feathers smell not unlike wet warm dog.

I used to run a laundromat and got pro at this ;)

Definitely check if it lets you do machine wash like username26 said. Mine lets me, so I never had to bring it out.

I used to machine wash mine all the time. I now have a duvet cover because it's easier to wash.

Down is machine washable. I don't know about the fabric exterior of your comforter, though. Still, you may need to go to a laundromat for their larger machines.

If you want to do just a spot clean, try baking soda-based "pacifier wipes". You can find them at Target. I've used them on upholstered furniture and clothing (while out and about) to remove all kinds of stains, new and old. So far, they haven't left any kind of "ring" around the edge of the area cleaned once it dries either. You may have to use more than one, and you might want to try pinching up the area to be cleaned so that the stain, after it's re-wet doesn't just soak through to the next layer. Blot with an absorbent cloth often to get the stain out rather than spreading it.

If it's machine-washable, use at least 50% less detergent than you normally would on a load that size. I don't know the why of it, but the tag on my down duvet has that warning, and the one time I put in the full amount of detergent I had to put it through the rinse cycle two extra times to get all the soap suds out.

If washing it in a machine worries you try using Shout travel spot wipes. They work better than the Shout in a bottle. While still wet work water into it to thin it out. Or you might try peroxide and water, depending on the nature of the stain. If there's protein in it the peroxide will bubble it out. Be careful not to overdo it though, it can create yellow stains if you use too much.

Go to a store that sells camping supplies, e.t., Campmore, EMS, Patagonia - or probably to Amazon - and buy Nikwax Down Wash for about $8. Make sure you find a commercial size washer and dryer for a quilt that size, and clean tennis balls. Nikwax is designed to preserve the down. I use it to wash expensive down jackets. Works great.

We wash our king size duvet about 2x a year. The key is to use only a little bit of soap, it just doesn't rinse out well. Then a couple of spins in the dryer (ours takes two or three goes before it is dry) and it is fluffy and awesome!

We do our king size pillows one at a time too, they take a couple runs through the dryer too.

Make sure it is totally dry, though. Nothing worse than damp down. :(

Perfect timing for this question!! Our dog just decided to, ahem, "claim" our new comforter...

I've got a dog who likes to pee on our bed, so I can attest to the fact that a down comforter can take a machine washing (or 10 or 50)!