Hi
Same thing happened to me 2 weeks back, found 1 flea on Ruby's belly and couldn't find any after that but I didn't want to take any chances so I brought Advantage 4-10kg, as my Ruby weighs 5.5kg, haven't seen any since.
Ruby tends to have a good itch from time to time but trust me if she had fleas I would see them, must be her long hair or excitement
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I would get the 4-10kg as your little yorkie weighs 4.5kg which is over 4kg.
Advantage has got the best reviews in the UK so I would stick to that brand.