Exercise 14 discussion

Clinical history and questions

This bulky surgical resection was removed from the deep subcutis caudal to the elbow of a middle-aged collie dog.  The owners didn’t know how long the mass had been there under the animal’s long coat.

The surgeon sent the specimen to the pathology laboratory in a bucket, carried by a student who was babbling something about galloping melanoma.

  1. Describe the abnormalities
  2. What basic disease processes might be present?
  3. Pathological diagnosis (give other possibilities in order of preference)?
  4. What diagnostic laboratory procedures should now be carried out?

An interpretation of Diagnostic Exercise 14 – Roger Kelly

Description:  This large specimen (scale not provided) has been extensively cut about, and the cut surfaces reveal a complex matrix of red tissue alternating with pale (presumably fibrous) tissue in which are embedded numerous small black sharply demarcated granules.  These tend to protrude from cut surfaces and some have been released and are apparently lying free.  The only normal-looking tissue is some fat on the lower left side of the mass.

Interpretation:  A mass as bulky as this can only be due to either chronic inflammation or to neoplasia, with associated secondary degenerative changes of course.  A neoplasm with such dense black pigmentation would have to involve melanocytes, and to be as widely infiltrative is this, it would have to be malignant.  Unlike some carcinomas, malignant melanomas do not often stimulate a florid fibrous tissue accompaniment, so the diagnostic tide turns toward chronic proliferative inflammation.  All we need now is an invasive black agent, and this comes in the form of the so-called dematiaceous or pigmented fungi.

Preferred gross diagnosis:  Extensive deep dematiaceous fungal granuloma, preferred over invasive melanoma.

Appropriate diagnostic laboratory procedures:  An immediate unstained crush preparation, viewed wet under a coverslip, would probably reveal the coloured fungal elements, and a fresh chilled tissue block supplied to the laboratory would provide the microbiologist with the opportunity to dig out some granules for culture after the block had been surface-sterilised by a brief dip in boiling water.  In this case. the causal organism Curvularia sp. (probably geniculata) grew rapidly on Sabaroud’s medium; the pigment developing in the older central part of the colony.

The histological appearance is of course pretty spectacular and characteristic.  Have a look at www.doctorfungus.org/thefungi/Curvularia.htm for description and images of these fungi.

Comment:  It turned out that this animal had received prolonged treatment with glucocorticoids for chronic arthritis, so it may well have had reduced defenses against this sort of infection.