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Since he is not showing any signs of upper respiratory infection, I agree that the likelihood of a "kitty cold" is lower on the list. Since loss of appetite is a vague clinical sign, it may help to narrow down the possibilities with some more information. What kind of office does he live in and does he go outside? Does he eat things he shouldn't or play with string, rubber bands, etc?
Has he had any vomiting or is it just a lack of appetite?
Is he drinking and urinating normally?
Is he drinking and urinating more or less than normal?
What is his activity level like? Is he lethargic or depressed?
Thank you for the detailed history. That certainly helps. While its possible that he picked up a viral infection from the feral kitty, the only way to know would be to have him s=tested and if he is vaccinated appropriately, then this is less likely. While the presence of momma cat and litter of kittens in his space may have disrupted his routine, I think its important to rule out a medical condition before we chalk up these changes to the new officemates alone. Cats normal have a body temperature that is warmer than a person's so this may be why he feels warm. But the only way to know for sure if he has a fever is to take his temperature. The best way to do so is with a digital thermometer (these can be purchased at any drug or grocery store) and take his temperature. In cats the best way is to do this rectally, you can use vaseline, KY or a similar product for lubricant to make it easier for him. It will probably take 2 people to take his temperature and some cats will not allow this so be careful if he begins to growl or get angry. normal temperature for a cat is 100.5-102.5 so if he is higher than 102.5, then he has a fever. Fevers can be cyclical and can come and go over time so checking his temperature a few times a day will give you a better idea of his status. Temperature aside, the best recommendation I can make for Jake is to have him examined by your veterinarian. There he can be tested for viral diseases and blood work can be performed to make sure his organs are functioning normally since he is a senior cat and this may be result of age-related illness that happened to coincide with the new mommy's arrival but is not related to it. If nothing can be found, then your vet can prescribe an appetite stimulant to jump start Jake's appetite.