Welcome to Just Answer. I'm sorry to hear about this situation with your dogs. Can your yard be contaminated? Yes. Decontaminating it can be difficult as doing this will kill the grass. In all honesty, the best thing that you can do for your yard is to make sure it dries out, doesn't have pooling water and is getting plenty of direct sun. The cysts need moist areas to survive. Here is a good link to read on this.
How long apart was each dog affected?