Exercise 9 discussion

Clinical history and questions

Serosal and mucosal surfaces of a segment of the right dorsal colon of a thoroughbred yearling, which was killed because of persistent ill-thrift and worsening exercise intolerance.

Additional necropsy findings included extensive suppuratives bronchopneumonia.

  1. Describe the abnormalities.
  2. What underlying disease process/es could produce these changes?
  3. Preferred pathological diagnosis?
  4. Most likely cause?
  5. Discuss the relationship (if any) between the lung and colon lesions.

An interpretation of Diagnostic Exercise 9 – Roger Kelly

Colon of thoroughbred yearling


The serosal surface of the colon is elevated by smooth cream-coloured semiconfluent nodules of variable size; the smaller ones tending to form chains aligned circumferentially around the bowel.  There are areas of haemorrhage, hyperaemia and oedema associated with some groups of nodules.

The mucosal surface is extensively and deeply ulcerated, most ulcer margins being reddened and raised.

Interpretation (basic processes):

There is always some degeneration involved in ulceration, but this is likely to be secondary to either inflammation or to neoplasia.

Likewise, hyperaemic nodularity can be due to either productive inflammation or to neoplasia, and again any degeneration is likely to be secondary (primary degenerative processes rarely produce focal increase in volume).

Of course the age and history strongly favour inflammation over neoplasia, so inflammation is the preferred primary disease process.  This quantity of exudate and degree of damage clearly did not occur over a few days, so the inflammation could be designated chronic, although the oedema and hyperaemia suggest that there is an acute component.  Hence chronic active inflammation.

Morphological diagnosis:

Chronic active ulcerative colitis


Rhodococcus equi is the most likely cause, given the association with extensive suppurative bronchopneumonia and the age of the animal.  In passing, it could be mentioned that yearlings are less susceptible to this infection than are younger animals.


R. equi is often recoverable from faeces of normal horses and foals, so association of R. equi colitis withR. equi pneumonia has been held to be the result of infectious exudate being swallowed and acting as a high-dose inoculum which is necessary for infection of the alimentary tract.  Nevertheless, cases of R. equi enterocolitis have been recorded in the absence of R. equi pneumonia, so it seems that primary infection of the bowel can occur.  Conversely, about half of the cases of R. equi pneumonia are unaccompanied by enterocolitis, so there is probably a lot more to learn about the pathogenesis of this infection.

With the increase in iatrogenic and infectious immunosuppression in humans, there has been a corresponding increase in reported R. equi infections in man; often these are severe and generalised, as one might expect.