Black liver describes the melanin-affected livers of sheep and goats. Organs other than the liver may be affected, resulting in the name black organ disease. The pigment is melanin not lipofuscin. It is hypothesized that melanogenesis is induced in affected organs rather than it being absorbed as melanin from the gut. Melanin accumulation occurs progressively over time with affected organs turning grey and eventually black due to accumulation of melanin in liver cells. There is apparently no liver dysfunction.
Little is known about the condition, its pathogenesis or risk factors. Limited field observations in semi-arid regions of Queensland and New South Wales have implicated mulga trees and bore water as possible dietary risk factors for affected properties. In addition experimental studies have induced melanosis in pigs by feeding acorns which are rich in tannins.
The condition is reported intermittently in sheep, goat and cattle abattoirs that source stock from south west Queensland and north west NSW. Up to 1% of sheep and 1% of lines are affected. It has recently been reported in sheep from southern WA during extremely dry conditions. The condition is reported in cattle but is apparently much less common than in sheep and goats. The incidence is apparently reduced in very good seasons. It is rarely seen in other Australian states. Sheep in Norway, India and Iran have been reported with the condition.
In the live export process, it may be an incidental finding at necropsy in assembly depots or at sea. Its main significance is as a cause of condemnation of offal and carcasses when sheep are slaughtered in the Middle East and goats in Malaysia. In some sheep shipments, thousands of animals are affected and partly or wholly condemned. In some Middle East countries the whole carcass is condemned if three or more organs are discoloured.
Affected animals show no clinical signs and therefore cannot be diagnosed while alive. At slaughter the liver is grey to black and when cut, material like black grease sticks to the knife blade. If the liver is black, the kidneys and various lymph nodes may also be black. The lungs may be grey to black, and bones, tendons and artery walls off-white to grey. The black livers and lungs have a distinct metallic smell.
The very dark livers in animals dead for many hours, and the bronze coloured livers and black kidneys of animals dying of copper poisoning, should not be mistaken for melanosis.
Laboratory confirmation requires sections of liver, lung, kidney and aorta in buffered formalin for histology.
No treatment is available
There is insufficient knowledge about risk factors for the condition to allow identification of preventative measures. Avoiding sourcing sheep and goats from semi-arid areas of NSW, Queensland and possibly WA might be considered, particularly during very dry conditions.