Now as I am sure you can appreciate, diarrhea and weight loss can be triggered by so many issues. Just to note, I would note that if he had appetite loss and diarrhea all at once, then those combined (low intake, high output) would have been the cause of that severe sudden weight loss. As well, it likely caused dehydration and when severe in pets so small, we can see severe metabolic and electrolyte changes which can be fatal for them.
Otherwise, in regards ***** ***** triggers, common causes will be bacterial, viral, parasitic, protozoal, secondary to IBD or cancer, obstructions in the gut, or related to severe allergic reactions to something they ate. With all these in mind, I would note that there are a number of viruses that can cause these signs but also affect the rest of the body and therefore induce severe signs like we have seen here. As well, there are a lot of bacterial agents that could trigger what you have seen as well. Some can spread into the blood stream to cause a severe sepsis. Otherwise can produce toxins which can lead to toxic shock and sudden death. And in regards ***** ***** cancers, we can see a situation called “tumor lysis syndrome” where the tumor outgrows its blood supply, dies, and releases toxins that can again cause severe illness and death. So, we have a lot to consider on those fronts for what we have seen here but these would be our suspicions for Barney’s severe and sudden signs.
Please take care,