Hello... we recently lost 2 chocolate lab puppies. With our first puppy we were told that two parvo tests came back negative. He was later diagnosed with a congenital liver disease (by a different vet). His symptoms included vomiting (clear fluid), no appetite and very lethargic... just like parvo symptoms. So, with the vet telling us that it was a liver disorder, we ruled out parvo and got a second puppy (from the same liter). After being with us for around 2-3 weeks this puppy starting showing the same symptoms. I then began to think that every puppy in this liter must have the same congenital "liver disease". While talking to still another vet she explained how she was not always convinced by traditional parvo tests. I assume she was referring to blood test and rectal swabbing. Being very frustrated at this point I agreed to a post mortem parvo test. After a few weeks we got the results... positive for parvo! Since this puppy was extremely healthy and happy when we first brought him home, I feel confident that the first lab puppy must've had parvo... even though all tests came back negative. Our yard in surely infected with the virus (from the first puppy) and this is what got the second puppy.

Well, today we purchased a 2-3 year old yellow lab from our local Humane Society's rescue program. He was essentially a stray when the local animal service picked him up and he had no tags. So there's no way of really knowing what regimen of shots he's had in his life. My question is this... do you think this older dog will be okay in our yard since 2 other puppies have been infected there somehow? This lab is big, fat, strong and very happy. We want him to stay that way! Any help or advice would be appreciated!
Thank you!