I lost my little Yorkie/Maltese Rusty like this on 9/19/11. It came on so fast and by the time I got him to the vet, it was too late, he passed away just 3 hours later.
It wasn't even 24 hours of him being sick. Then, within less than a week, two of my other dogs got it. My vet said that she didn't know what caused this with Rusty and that we will probably never know.
My other two dogs recovered quickly on antibiotics and steroids.
Sending positive thoughts and prayers for a speedy recovery for your other dog.
