Hi
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Sorry it's taken so long to respond.Folk are very thin on the ground these days
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I would have a look at this inform in the first instance
https://www.purina.co.uk/articles/cats/health/digestion/diarrhoea-in-cats. If he is a pig and still eating wet food it might be worth addibg a couple of tea spoons of tinned pumpkin. It's available from waitrose or asda and stores in the fridge once opened. I was advised to give this to my little girl when she had colitis and runny poos. It's fairly bland and cats seem to like it mixed in with their wet food.
Plain, cooked, unsweetened pumpkin can be healthy for cats, especially if they have digestive issues. However, you should avoid giving your cat pumpkin pie filling or canned pumpkin with added ingredients. most are 100% pumpkin.
Benefits
Fiber: Pumpkin's high fiber content can help with diarrhea and constipation.
Nutrients: Pumpkin contains vitamins A and C, potassium, and beta-carotene, which can support healthy eyes, skin, and coat.
Weight management: Pumpkin is low in calories and sodium, which can help your cat feel full without extra calories.
I wouln't mess about too much with his food for now and just hope he firms up
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