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What breed of cat is he? Is he an indoor/outdoor cat? Is he current on vaccines and has he been thoroughly dewormed? Was he tested foe leukemia and FIV as a youngster? Any other cats or dogs in house acting sick? When he flops down and acts exhausted, is he breathing heavily? And chance of toxin exposure? Has he ever be ill before? - Dr. C
That sounds very alarming. If he has any maine coon in him, I'd be really worried about heart disease as the cause of his lethargy and exercise intolerance. Heart disease can also cause GI symptoms and weight loss. With him being indoor/outdoor and not recently dewormed, he could also have a high intestinal parasite burden and that could cause weakness, vomiting and diarrhea as well as weight loss. I'd definitely recommend having him evaluated by an ER vet tonight if that is a possibility. Otherwise, he should be checked by your family vet first thing in the morning. Hope that is helpful, and please let me know if you have any other questions or concerns - Dr. C
I'm so sorry that he is so sick. The purring is also a cause for concern, as many times sick cats purr to try to comfort themselves. I hope you are able to find an emergency vet to help your kitty out tonight - Dr. C