Gray-black or green discoloration of the rectal mucosa is occasionally observed at slaughter and is an economic concern because affected tissues are no longer suitable for processing and are devalued or discarded. The descending colon and rectum are most often affected, and the condition is known as melanosis coli or pseudomelanosis coli. The pathogenesis is not fully known but appears to be related to oxidative injury. The condition often occurs in pigs consuming nonstandard rations, so a dietary component appears likely.
Microscopically, pigment-laden macrophages are present in the lamina propria, and the severity of pigment accumulation correlates with the degree of discoloration observed.
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Wilberts BL, Schwartz KJ, Gauger PC, Wang C, Burrough ER. Evidence of oxidative injury in pigs with melanosis coli. Vet Pathol. 2015;52(4):663-667.
