Dog's Breath Smells Like Dead Animal. Kidney disease leads to high urea levels, causing breath smelling like urine and smelling like rotten fish. The ensuing blockage then causes discomfort, which can prompt your dog to clean himself excessively, fouling his breath.
By contrast, if you’ve been asking, “why does my dog’s breath smell like poop or pee?”, it might be a sign of kidney disease—or it could just be a sign that your dog has recently eaten feces. Older dogs develop tumors in the mouth or digestive system; Bad breath can be caused by a dog’s diet, ingestion of different foods or trash, mouth or dental disease, disease of the upper respiratory tract.
If your dog’s breath smells like they just ate a dead animal, this could be a sign that they have dental issues such as gum disease, tooth decay or an infection.
Some dogs eat just about anything that may include other animal’s feces, garbage, compost, mulch, and dead animal carcass. This is because the liver is no longer able to properly filter out toxins so stinky sulfur compounds, such as thiols, are released into the lungs. Dogs use this to identify each other. The fluid in the glands can be near odorless, musty smelling or smell like feces or even dead things.