Studies of a naturally occurring molybdenum-induced copper deficiency in the yak.

The clinical signs of a disorder known locally as "shakeback disease" in yaks in the North of the Qing Hai-Tibetan Plateau of China were defined: they included emaciation, unsteady gait, a 'shivering' back and deprived appetites. Coat colour was not affected. The mineral composition of soil and forages, and samples of blood and hair from yaks in affected ranches in this region were compared with those of 20 samples from unaffected areas. The mean concentration of molybdenum (Mo) in soil and forage was 4.85+/-0.21 and 4.96+/-0.25 microg/g (dry matter), respectively; the mean copper (Cu) to Mo ratio in the forage was only 1.34+/-0.36 compared to 8.12+/-1.31 for unaffected areas. The mean concentrations of Cu in blood and hair from the affected yaks were 0.29+/-0.17 and 3.51+/-1.12 microg/g, respectively, compared with 0.85+/-0.24 and 6.42+/-1.21 microg/g, for controls. The highest value for Cu in the blood of affected yaks was 0.58 mug/g and the lowest 0.03 microg/g (the corresponding highest values for unaffected yaks were 1.03 mug/g, the lowest 0.56 microg/g). There was a hypochromic microcytic anaemia and a low level of ceruloplasmin in the blood. The Cu deficiency in yaks was most severe during pregnancy and lactation, but oral administration of copper sulphate prevented and cured the disease. We conclude that "shakeback disease" of yaks in this region is probably caused by a secondary Cu deficiency, mainly due to the high Mo content in soils and forage.