My 6 mo. old kitten just passed from FIP, worried about our other rescue kitten,

my kitten died from FIP a few days ago, and I have a 4 month old kitten I've rescued that I'm worried about   Our 6 month old kitten passed away from FIP 5 days ago... it was heartbreaking. I had no idea he that sick. He had upper respiratory infections since we got him, but not knowing about FIP, i never asked the vet to check for it. He had zero contact with other cats since we got him and was never outside. We have one small dog that goes outside to the bathroom and comes back in. I don't know if it's possible she could have eaten feces of a cat that came into the yard at some point and then maybe drank out of the kitten's bowl and given him FIP...?   About a week before he passed, we found a stray kitten who is about 4 months old. Because she was a stray, we kept her isolated in her own room. We planned to do so for a couple weeks to make sure she wasn't sick. We also vaccinated her. She didn't have any contact with our kitten that passed away. We just tested her for any viruses that could cause FIP and she tested negative.   Long story short, I still have her in the room until I've sanitized the house from top to bottom. I'm even having the couches professionally cleaned. When is it safe to let her out of the room? And is it possible my dog could have brought the viruses that cause FIP inside the house? I'm so worried about this happening again...

Hello and I'm sorry that you lost your kitten to FIP.   FIP is actually a (different, human-safe,  cat-only) type of Coronavirus and can spread very easily; a surprising proportion of cats may have been exposed to it.  However, the symptoms only take hold in certain cases, when the virus mutates and seems to become active, leading to the disease we call FIP.  To answer your question about dogs, a dog may perhaps get the virus stuck in their fur and transfer it to a cat that way, but the same might be said of vegetation.  Dogs do not catch and directly spread the virus themselves.   Usually, Feline Coronavirus is thought to be spread via the faecal-oro route (via mouth and poo), in particular between Mother cats and babies.   Illness is not always a consequence of the virus.  Furthermore, there is actually a new treatment beginning to be used.   So, in summary, it may be premature to worry at this stage - however, you should definitely go and have a chat with your vet about the implications.

Lizzie Yoens has written an excellent article for our blog-site called FIP in cats, is there new hope?   It answers a few of your questions better than I have.

Finally, I want to say again how sorry I am for your loss.  If you don't know about The Ralph Site, some owners find that it helps at this time.  https://www.theralphsite.com/

Here is an article about Feline Corona Virus that answers your questions about cleaning.  https://www.biogal.com/blog/what-you-need-to-know-about-feline-coronavirus-fcov-fip-2/