For all of you wondering...I had a loooong talk with my vet today about what happened with my mom's dog. He said he's 99% sure it was cardiomyopathy (sp?), which is essentially a heart condition. It can cause heart attacks and blood clots, either of which could have killed her. What's scary is that there really aren't any symptoms (you can't detect it with a stethoscope like a heart murmur), so my mom would not have been able to tell.
Anyway, he said it's actually fairly common in any large breed around middle age. The same exact thing happened to one of his dogs (a Mastiff) when it was 7. So...I guess the moral of the story is that you never know.
I did want to say thanks to all of you who checked things out for me, and to thank you all for your kind words.
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"We live in an age where unnecessary things are our only necessities." --Oscar Wilde
well, at least you have an idea of what caused it, and confirmation for your mom that there was nothing she could do - I'm sure she's probably been feeling pretty torn up about it - its a really sad loss.
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"Fashion can be bought. Style one must possess." ~ Edna Woolman Chase