TRACE ELEMENTS IN ANIMALS 127 



thyroid gland in higher animals. Cobalt is now known to be 

 essential for sheep and cattle, for without a sufficiency of this 

 element a serious disease which may prove fatal develops. Its 

 wide distribution in animals suggests that manganese may be 

 an essential element for animals as well as for plants, and there 

 is evidence that a deficiency of it is associated with a disease of 

 fowls known as perosis characterized by deformity of the leg 

 bones. The essentiality of vanadium for the blood of ascidians 

 has already been mentioned, but of the other elements, although 

 a number of them may be constantly present in the bodies of 

 man and the higher animals, nothing really definite is known 

 regarding their indispensability. 



Practically any trace element may produce poisoning if 

 administered or presented to the animal in too great excess. Two 

 diseases of animals, the causes of which have for long been 

 obscure, have in recent years been tracked down to the effects 

 of excess of two unexpected sources. One of these is the so-called 

 ' alkali disease ' affecting livestock in a number of the northern 

 United States, the other is the scouring of cattle, and to a less 

 degree of sheep, which occurs in England on certain land known 

 as 'teart' in Somerset and a small area of Warwickshire and 

 Worcestershire. The former disease has now been shown to be 

 due to an excess of selenium, the latter to an excess of molyb- 

 denum, in the pasture plants growing, and consumed by the 

 animals grazing, on the lands in which the two diseases are 

 respectively incident. 



In the following pages accounts will first be given of these 

 two diseases due to trace-element excess, after which the trace- 

 element deficiency diseases will be considered. 



1. Diseases due to Trace-Element Excess 



Alkali Disease (Selenium Poisoning). For many years a 

 disease affecting horses, cattle, pigs and poultry has been 

 known to occur in certain areas of the great plains of the Middle 

 Western United States. The symptoms of the disease include 

 loss of hair from the mane and tail of horses, from the switch of 

 cattle and from the body of pigs, and a marked change in the 

 growth of the horn of the hoof in all these animals which , when 



