When you look at a carcass of an animal you'll notice 3 main types of changes:
Lesions, caused by the disease
Agonal Changes, happen before or during death
Post-mortem changes, happen between the time of death and the time you examine it
Post Mortem changes
If there are lesions they will be there because of autolysis or putrefaction.
Remember, autolysis is the work of the animal's own enzymes on the tissue.
Putrefaction is where micro-organisms act on the animals' tissue.
Autolysis can really get in the way of making a diagnosis and is the most destructive of post-mortem changes. It happens when the cell enzymes are released after death and this happens very quickly in the pancreas, intestines and kidneys.
Autolysed tissue can look paler than normal, the cut surface might be greasy and exude liquid, is fluid inside, and all tissue is affected.
In the first 24 hours, most of the putrefaction comes from bacteria and organisms already in the body. After 24 hours, outsider micro-organisms can invade.
Putrefaction causes tissue degradation. Enzymes from bacteria cause the break down of tissues.
Proteins are broken down producing ammonia.
Proteins that contain sulphur are also broken down and hydrogen sulphide is released. This gives off the rotten egg smells and greeny black discolouration of tissue. When iron joins with the sulphide, ferrous sulphide is produced and this is also green. To avoid the carcass being tainted, it must be bled out immediately after the animal dies.
This is when the muscles stiffen and contract. It is a breakdown of glycogen to lactic acid, then the muscle protein coagulates which shortens the muscle fibres.
It happens 2-8 hours after death.
Check out the previous pathology posts here.