I've asked about this before, but never really got an answer other than "who knows?" I should mention that this question was written while Saucy was very underweight, and that she has since gained weight and returned to her normal size, but I'm just waiting for it to happen all over again. My 4 year old Jack Russell b**** is currently weighing in at less than 4lb. Her normal weight is 6.5lb. She looks very thin, obviously... I can see her spine, hip bones, and every rib. She also seems slightly dehydrated, despite adequate fluids. She is eating, drinking, and behaving entirely as normal. Nothing in her environment or routine has changed. She has been regularly wormed and treated for all parasites since I got her three years ago. She has naturally had all shots and boosters, and is spayed. She is a very high energy dog, even for the breed. She started losing weight around 3 weeks ago, and it came off fast. This isn't the first time this has happened... the fourth in fact... around about once a year this happens, and just seems to resolve itself. My vet is at a complete loss, as is another vet who I took her to when I was staying with my parents last year. She has had general bloods done, bloods to check liver function, bloods to check thyroid function, bloods to check white blood cell count (the final one from THIS episode came back today). All came back normal. The vet says that she is happy and healthy and not to worry about it, as the other times, she has put the weight back on (usually the whole thing lasts for around 8 weeks). But I'm worried about the effects on her, particularly her heart. Does anyone have any idea at all what this could be? I'm considering full body ultrasound scan... what do you think? Thank you