Thanks for responding, everyone. NRD - sorry to hear you're going through it too!
My vet was not concerned either; in fact I had called after I got their message about the fecal test results and probably sounded like an idiot. But, they basically told me that all I need to really do is make sure the doo is picked up immediately and wash my hands a lot. They also suggested that I could use baby wipes to wipe his rear and paws when he's coming in to the house. That really didn't sound like enough to me, since reading up about Giardia on the internet makes it seem as though this parasite is the next superbug.
I did get some insight from another person on a different forum who went through an infestation 5x with the same dog when it was a puppy. She told me that she just vacuumed a lot, and every time he went poop she would pick it up and spray it with a bleach solution (she used 50/50!). She said the grass died but it grew back. Her other dog, two cats and two birds never caught it.
So far he's been on the meds for 2.5 days, and he's doing a lot better. The cats are not showing any signs (though I know I'm not in the clear, by any means). I'm also thinking that because my kitties are older (5 and 7) and in good health, they likely have a strong enough immune system to fight off an infestation should they happen to ingest some of the cysts. I hope so, anyway. In the mean time, I've been making sure to keep their boxes super clean. That with the daily vacuuming, surface disinfecting and cleaning Riley when he comes inside is about the best I can do, I guess. If I don't see them start showing signs of being infected, then when I take Riley's sample in for the follow up test I'll bring on of the girls' samples too to test and make sure.
From what I've read there isn't a lot known about the spread of Giardia between animals and humans. Some groups think it's possible, others do not. Some groups even think that dogs cannot pass it to cats. I think it's best to err on the side of caution.
Also, giardia is the parasite in water that makes you have to boil it before you can drink it - when humans get infected by them it's known as "Montezuma's Revenge" or "Traveler's Diarrhea". I have actually gotten sick from Giardia before, about two years ago on my honeymoon to Mexico. I was sick for about a week when we came home and finally admitted that I would need to see the doctor for meds because I just wasn't getting any better. I don't remember what she prescribed, but I was on it for about a week and after a couple days I was already better. I know it's a common thing, I just don't want it in my house to further infect my furbabies!!