A. The diarrhea may be not related to Giardia this time. If it returned immediately after the neutering it may be related to anesthesia and to the drugs that were given; in this case the diarrhea should be self-limiting and resolve in few days. Otherwise other differentials are possible at this stage like other type of bacterial or parasitic infection, dietary related, gastroenteritis of other type (like inflammatory bowel disease). The link with the neutering may be only incidental in these cases.
I am happy to consult and discuss with you this problem further or I would recommend to discuss with your veterinarian how to recognize the different causes of diarrhea and give appropriate treatment.
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