This has the potential for being serious when the onset and general health of the mule is considered. There can be a number of causes... we've a neighbor that has mules (Mammoth Jacks) who lost a 12 year old jack last year. Seems the mule had congestive heart failure but death was preceded by about 3 weeks of diarrhea. It caused severe dehydration which contributed to the animals death. I imagine (we have horses) that the malady can be caused by other, less severe things, but the recommendation is to seak the help of a qualified (large animal) veterinarian... Good luck!