I'm very sorry you lost your cats I do think though if it had been FIP that you would have noticed the symptoms, it makes them really obviously poorly. not the sort of thing you might have missed.

Dry FIP can affect the internal organs, and causes changes in the eyes, and neurological symptoms. Wet FIP causes the build up of masses of horrible liquid, causing swelling and vomiting/ Cats will fail to thrive, and be undersized and generally sickly looking. I lost a little cat to wet FIP a long time ago, and she was very small compared to other cats her age.

Has your vet given you any explanation at all as to why you have lost your two?

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