Primary differential

I know that title sounds like something out of Star Trek, but it's not. When fledgling veterinarians are learning how to diagnose disease - we are taught a "problem-based" approach. As in - list the problems your patient has - then list differential diagnosis.

An example: an un-vaccinated 5 month old puppy comes in with loss of appetite, vomiting, and bloody diarrhea.

Simple enough, the problem list is: anorexia, vomiting, bloody diarrhea.

The next step is formulating a differential diagnosis list i.e. what could be causing these signs based on the age, sex, vaccination status, etc of the patient.

In this puppy, my differentials would be as follows: parvovirus, GI parasites, foreign body, GI intussusception, dietary indiscretion, HGE, etc - in that order. When formulating a differential list (Ddx in our parlance), it should be done in order of most likely to least likely.

So, this was what went through my mind when I was presented with a 13 year old dog at 3am on Tuesday night. He belonged to a technician from one of our local DVMs. He had been going downhill since the previous Thursday. No vomiting or diarrhea. He was just progressively more lethargic and depressed. He had woken up the owner that night crying in pain and then he vomited twice. Bloodwork at his vet on Thursday had been unremarkable. He was on tramadol for pain control.

When I examined him, he was extremely depressed, dehydrated (his gums were dry), with a moderately distended, painful abdomen. Rectal exam revealed no abnormalities other than scant loose stool. He also had a fairly rocking heart murmur.

Since he'd just had bloodwork 4 days previously, I elected to start with abdominal xrays. It only took one glance to diagnose an abdomen full of fluid. Ultrasound confirmed this. I tapped the fluid for cytology (slide for microscopic examination). I saw peritonitis - gobs and gobs of neutrophils. Bacteria were surprisingly scarce - although here and there, I found intracellular rod bacteria. Certainly not as many as I've seen with previous septic abdomens. There were also occasional clumps of large, irregular, darkly staining, multi-nucleated cells that looked like this:



So, veterinary students and vets alike and anyone else: what would your primary differential be? What other differentials should you consider?