Pathology (the study of how disease impacts organs to create the symptoms experienced by the animal) is pretty fascinating, because disease can look very different depending on whether you’re looking under the microscope or looking at larger structures at necropsy. One of our professors who teaches about digestive tract diseases likes to end his lectures by showing us some pictures from an interesting case he had on the pathology floor during the past week. Most of the time we approach diseases by learning what’s unique about them: what type of lesions form and where, and what breeds and ages are affected, for example. I really enjoyed working through our professor’s gross pathology cases, because instead of zeroing in the abnormalities, we had to start at the beginning: looking at the whole body, methodically checking every organ system.
It felt a little frustrating to look first at the lungs (normal), then the heart (normal), then the liver (also normal). But then, once we found something a little off—a smattering of small, whitish nodules spread across some loops of intestine and the kidneys—it was actually useful to know which organs were normal. Some diseases affect many organs, while other diseases primarily affect one body system. It turned out that this cat had the dry form of feline infectious peritonitis (FIP), in which macrophages attempting to control the mutated coronavirus form small granulomas on the surface of abdominal organs. Once our professor gave us the history of this cat, it became clear why he had withheld the information: if we had known this was a young cat, recently adopted from a stressful shelter environment with a history of upper respiratory signs and sporadic diarrhea, most us would immediately have FIP on our differential list, and the GI tract would have been one of the first places we looked.