Hi Laura.
I am going to assume that this has started recently, and that all the usual and most of the unusual things have been ruled out.
Has anyone considered a neurological condition such as a head injury, or an early brain tumor?
There is a condition called psychogenic polydipsia, which produces incessant drinking. It usually is a developmental problem, starting early in life. It is very uncommon and not easily confirmed by any diagnostic tests.
Animals cannot seem to resist drinking, and diagnosis is usually made after exhausting other possibilities. I have seen this in a young cat, and after the initial fascination with the behavior, it became pathetic to watch.
I mention this as it possible for a brain lesion from injury or neoplasm to mimic this kind of behavior.
I hope this is of help to you and your team..
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