Exercise 2 discussion

Clinical history and questions

Abattoir specimen from a 14 week-old pig (the other one looked the same).

Size and texture were judged to be within the normal range for the class of animal.

  1. Describe the abnormalities.
  2. What underlying disease process/es could produce these changes?
  3. Make a differential diagnostic list, in order of priority.
  4. With your preferred diagnosis in mind, what is the pathogenesis of these macroscopic changes?
  5. With your preferred diagnosis in mind, is there anything in particular that should you look for in the carcass from which this specimen came?

An interpretation of Diagnostic Exercise No. 2 – Roger Kelly

Essentially, the kidney shows diffuse, uniform pallor of the cortex, with very fine petechiae.

To make the cortex as pale as this, you have to reduce the concentration of the haem-containing oxidative enzymes in proximal tubular epithelium, which is normally very rich in them: they are brown in color, of course: hence the normal brown colour of renal cortex and liver.

To make the pallor as uniform as this, you have to either damage all the proximal epithelium with some toxic agent, or reduce vascular perfusion of all the proximal tubules. Hypoxic proximal tubular epithelium tends to shed oxygen-demanding cytochrome oxidases as a survival mechanism. About the only way you can achieve this vascular effect across the entire cortex is to damage all the glomeruli enough to significantly and uniformly reduce efferent blood flow from them to the cortex.

So, the differential diagnosis should include tubular nephrosis and glomerulonephritis, with things like amyloid discounted because of the age of the animal. Toxic tubular nephrosis is pretty rare in pigs (been recorded in oxalate poisoning, etc). If it was glomerulonephritis, it should be fairly acute, because we are given that the size and texture were normal (no scarring/shrinkage yet). Interstitial nephritis seems only to produce pallor as severe as this when it is fairly chronic (the organ would be uniformly tough); moreover, interstitial nephritis is rarely as perfectly uniform as this.

Pushed for time in the exam? Short answers to the questions should convey your grasp of the concepts:

  1. Diffuse, uniform cortical pallor with fine cortical petechiae.
  2. Acute/subacute inflammation; or degeneration,
  3. (a) Acute/subacute glomerulonephritis (b) tubular nephrosis (c) amyloidosis.
  4. Loss of proximal tubular cytochromes; plus microvasculitis/vascular degeneration to give the small haemorrhages.
  5. Search for necrotising skin lesions, particularly around the rear end, since that is often the preferred site for the cutaneous lesions of porcine dermatitis-noephropathy (PDNS), an important cause of glomerulonephritis in pigs. The cause of PDNS is uncertain.

Classical and African swine fevers can of course cause renal petechiae, and, while not producing classical glomerulonephritis, can cause enough glomerular vasculitis to produce this sort of picture, so they should also be on the list of differentials.