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Just typed a long post and puter went off so will begin again)
So glad you've posted here
So sorry Monza is so poorly, it's such a worry. Having had old cats for a long time I've been in this situation a few times and it's never easy.
What I find helps is to feed little and often things they really like.
So, cat milk is great for building them up (not cows milk as many cats are lactose intolerant) giving some each day helped my cat Byron a lot.
Also, mashing the food up so that all gets eaten not just gravy or jelly. Pate and supermeat type foods are great too becaause they're easy to eat and mine have always loved those.
Oily fish is another thing which can help, things like salmon, tuna, mackerel, sardines and pilchards a few times a week because their sense of smell sometimes goes Schui suffered sinus problems and these are strong smelling and are healthy - fattening too.
Anthing they really like is better than nothing.
Also I found raising the dish so that it's easier for them to reach and feeding them with a spoon Schui loved this! he'd stay put and I'd take the food to him - nearly always worked even if he only ate bits, because getting them to eat can kick start the appetite.
Bits of cheese too if they like it can help.
Best of luck and gentle snuzzles to Monza