When is coronavirus leading to FIP contagious?

Question

I adopted two middle-aged (or older?) cats from the same shelter 3.5 years ago. Both were healthy until recently when one developed FIP. Since I adopted them they have had no contact with other cats, aside from whatever happens behind the scenes at the vet's office.

According to this article from the Cornell Veterinary School, a cat can be exposed to the virus (with no symptoms) and develop FIP years later. So it sounds like it's possible that my cat came to me already infected and the disease only manifested now. (It's also possible that the other cat is a carrier who transmitted it to him without getting sick himself.)

When is a cat with future-FIP infectious? Only during an initial exposure to coronavirus (when my cat caught this), or from that time onward? Only after developing FIP? If a cat has been exposed to coronavirus in the past but has never shown symptoms of FIP, can he transmit that coronavirus to other cats?

I am trying to figure out the effect of this on my remaining cat, and whether I can safely adopt another cat to join him. There is no reliable test for FIP other than biopsies, so at best testing can tell us if he's been exposed to coronavirus (the test looks for the antibodies). But my biggest concern is whether he could be infectious. (If he's also infected then, sadly, there's nothing we can do about that. We can only test for coronavirus and coronavirus doesn't always lead to FIP from what I've read.)

Answer 1

Veterinary partner is a very reliable website that should be able to answer your questions on FIP

Your cat wont get FIP by being with an FIP + cat, it can however contract coronavirus. And yes as you mentioned, not all coronavirus cats will develop FIP.

From my "Common Diseases of Companion Animals" textbook:

80% to 90% of cats in catteries have antibodies to FECV and these cats shed virus intermittently.

That being said by continuously disinfecting and only having one/two cat(s) in your home you can eventually clear out FECV (most common household disinfectants work).

You can check your current cat what his current anti-body levels are for coronavirus if you're concerned about a new cat coming in as well.

An excerpt from veterinary partner on FECV:

An active infection lasts several weeks to a few months. Virus is shed in the infected cat’s stool during this period. If the cat is reinfected, virus sheds again for weeks to months. During this time, the cat may or may not seems at all ill. Some infected cats do not shed virus.

The mutation to a form of virus that can cause FIP is more likely to occur in an immune-compromised cat. Most cats with FIP are less than one year old (their immaturity is their immune compromise). Crowding is also an important source of immune-compromise. Another important factor in mutation is multiple exposure/infections. When cats keep getting reinfected, the virus they carry has more chance of mutating to a form that causes FIP.

I've only seen 3 cases of FIP in almost 4 years in practice, it's not very common.