I am very sorry to hear about your girl's weakness, loss of appetite, and previous diagnosis of dehydration.
With her symptoms, young age and great response to intravenous fluids, I am concerned that she is suffering from Addison's disease. Addison's disease, which is a poorly functioning adrenal gland, can lead to lots of vague symptoms including weakness, loss of appetite and sometimes diarrhea and vomiting. Dogs may appear fairly normal on examination and on routine blood tests, other then showing signs of dehydration, so they can be difficult to diagnose.
These dogs cannot handle stress at all because their adrenal gland doesn't produce cortisone when stressed and their electrolytes can be off too if their adrenal gland isn't controlling that normally either. We can see physical collapse in severely affected dogs.
It is more commonly seen in female, young adult dogs.
Testing is an ACTH response test to check adrenal gland function and checking electrolyte levels.
Treatment is steroid replacement therapy and electrolyte replacement.
Many dogs respond very well to fluid and electrolyte therapy, but will relapse quickly if proper medical treatment doesn't follow.
If you want to read more about Addison's disease here is a link to a webpage with accurate information: http://www.vetmed.wsu.edu/cliented/addisons.aspx
I highly recommend screening bloodwork including a complete blood count and biochemistry profile to look for other problems too, but if things look relatively normal other than mild changes in electrolytes and evidence of dehydration then an ACTH response test is indicated.
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