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Hello, how are you? I am a veterinarian in Argentina, I would also investigate the possibility of an infectious disease, even if you are vaccinated, FELINE INFECTIOUS PERITONITIS. REGARDS.
The digit lesion is very severe, and likely the only way to improve quality of life would be to amputate. However, additional lesions will develop in the near future (especially since 2 digits are already affected).
To obtain a diagnosis, you could also consider a fine needle aspirate or biopsy under anesthesia.
In my opinion, even without additional diagnostics, this is very suspicious for lung-digit syndrome and the cat’s prognosis is guarded to poor.
Amputation of the digits will temporarily improve the quality of life, but the cat may have mobility issues postoperatively.
Prior to removing them, I would recommend radiographs of the rest of the feet to rule out other lesions. If other lesions are present, my recommendation would be pain management of euthanasia, unfortunately.