You are working as a public health officer for the USDA doing pre- and post-slaughter meat inspection in a location that processes sheep. On this particular day you find yourself examining a flock, and as they paraded past you for a visual examination, you look for obvious wounds, nasal discharge, respiratory distress, poor body condition, and neurological signs.
The sheep all appeared fine except for one which is walking with a stiff, uneven gait. It's cut out and moved into the area reserved for "suspect" animals. A physical exam that includes a quick palpation of the head, neck, limbs, and topline leads you to a hard mass about the diameter of a golf ball over the thoracolumbar region and palpation of the mass elicits a lot of bleating and struggling from the sheep. The sheep is having difficulty supporting its weight with its hind limbs but the forelimbs appear unaffected. The sheep appears otherwise normal and you order it slaughtered for further inspection of the carcass. You find no gross abnormalities in the sheep except for a large, fibrous, coalescing mass surrounding L1 and L2 of the vertebrae. You incise into the mass and find a large abscess focused on L2 which has a hairline fracture of the spinous process. Further dissection reveals an abscess tract extending from the spinal canal and there is involvement of the spinal cord as well.
As you can see in the image above, the growth of this pus-filled abscess is actually not in the spinal cord proper, but rather in the space above it. Infection from the overlying region has been spread into the spece between the vertebral body (which has been removed in this specimen) and compressed the cord. Again, this is an example of a space-filling lesion, where pressure from a growing mass causes functional deficits. There is a good deal of inflammation here: note the hyperremia in the CT on the dorsal side of the cord.
The abscess is filled with pus, the liquefied remains of neutrophils. This inflammatory response has elicited a reaction in the CT surrounding the cord, in the form of fibroblast proliferation. The encapsulation of the infected area is easily seen at right: a thickened and fibrotic wall restricting the inflammatory response to a specific area.
A septic abscess like this one can form anywhere that bacteria can invade. In this case there may have been some minor injury to the skin (a bite from a coyote, a barbed wire penetration, etc.) that introduced bacteria past the protective barrier the skin provides. The subsequent infection's spread caused this problem. If the body is presented with an infective focus that rapidly gets out of control, a common response is to wall it off, as has happened here, and hope that its spread can be controlled enough for the immune system to deal with it. If this animal has managed to do that, it would have eventually removed the liquified debris and proliferated scar tissue into the place where it used to be.
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