Well, I've joined the club...

[Aquattro]

Andrew, from everything I've read about this, I'm pretty sure your clams would already be gone if they had this problem. It generally hits quickly. I'd also hold off on any future clam purchases in the next little while.
I'm personally going to re-assess my quarintine procedures. If I had quarintined the new clam, I feel the others would still be here.