140 TRACE ELEMENTS IN ANIMALS 



long time, but the first scientific description of it appears to have 

 been recorded only some 14 years ago (Stewart, 1932). It was 

 recorded for Australia at about the same time (Bennetts, 1932). 

 Dunlop, who has specially studied the disease, thus (1939) de- 

 scribed the symptoms : ' In most cases the symptoms are noticed 

 in the lambs at birth. Some lie recumbent swaying their heads 

 and make spasmodic efforts to rise and obtain milk from their 

 dams. Such attempts frequently result in the lamb rolling over on 

 its side, kicking vigorously with the hind legs. Others may be able 

 to stand, but often the hindquarters swing about and eventually 

 fall over drawing the rest of the body to the ground. Attempts 

 to walk are characterized by incoordination of movement, a 

 swaying gait, staggering and finally collapse.' Blindness may 

 sometimes occur. In some cases the symptoms at birth may be 

 slight and develop later. Dunlop states that the mortality of 

 affected lambs is almost 100 per cent, so that adult animals 

 showing symptoms of swayback are rare. However, according 

 to Innes and Shearer (1940) mildly affected animals may survive 

 and when later used for breeding may give birth to healthy 

 lambs. Post-mortem examination of affected animals shows that 

 lesions of the brain occur, the white matter disintegrating leaving 

 cavities filled with a clear fluid or jelly. The degeneration may 

 extend down the spinal cord. These lesions arise while the lamb 

 is still in utero. 



Investigations show that the disease is not hereditary, nor 

 could any evidence be obtained to suggest that it was due to 

 infection by micro-organisms. For example, no transmission of 

 the disease results from inoculating new-born healthy lambs 

 with extracts from affected tissues. All the facts, including the 

 localization of the disease to certain limited areas, point to the 

 disease being nutritional in origin. The coincidence of the disease 

 in Derbyshire with soils containing lead might suggest that the 

 disease results from lead poisoning, but this opinion has no 

 experimental evidence to support it and is by no means generally 

 held. 



Dunlop describes a large-scale experiment carried out in 

 Derbyshire designed to test the theory that swayback is a 

 nutritional disease. In this experiment 1800 ewes distributed 

 over fifty farms were used. Four groups of 300 ewes each, the 



