Veterinary questions? Ask a Vet for Answers ASAP
You're through to Dr. Edele, a UK based vet.
Poor Mylo! I have some questions to help me best advise you.
When did the diarrhoea start for Mylo?
Is Mylo still with his mum?
How is Mylo behaving in himself?
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What are you feeding him?
Is his sister affected?
Ok, at their age they are naturally being weaned from mum, so mixing the food like you are is perfect. Some of the upset may be related to diet change. As they're both happy and playful in-between times it's unlikely to be illness. If would also be a good idea to worm both of them (panacur is a good product that you can get from your vets) as parasites are common in young kittens and can cause the signs you're describing.
That's loads for them which's really good, they have excellent appetites. Cat milk isn't essential for them at this point but you can feel free to keep offering them a little.
At your health check, your vet will likely worm the kittens again for you. Luckily worming products for kittens are safe and Panacur (fenbendazole) can be administered every 2 weeks until they're 12 weeks of age. If they start to become dull or anything before then, you can try to bring your appointment forward but from what you've described, it's unlikely to be related to an infection.
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Have a good day :)