34 



There'does not yet exist any practical metliod of immunisation, and 

 the observations made on the immunity refer to animals recovered from a 

 disease transmitted to them by injection of virulent blood. 



Character of Immunity. — Oxen, sheep, and goats which have 

 recovered from an attack of heartwater can be injected with large 

 quantities of virulent blood adequate to the virus which produced immunity. 



Five cattle were hyperimmunised to the extent of 1,000 to 3,000 c.c. 

 virulent blood in one operation, and eight sheep from 50 c.c. to 400 c.c. 

 in one injection were hyperimmunised in this way. 



This hyperimmunisation was carried out in various intervals. No 

 breakdowns occurred. The longest interval between two hyperimmuni- 

 sations was in cattle, seventeen months, and in sheep, eighteen months. 



Conclusion. — Immunity against heartwater when tests were carried out 

 with the adequate virus was complete concerning the quantity of virus, 

 and, concerning time, at least for eighteen months. 



Experience with Inadequate Virus. — Eight sheep, all immune and 

 hyperimmunised to a particular strain of virus called the Sjamboks Kraal 

 Virus, which were injected with virus of a different strain obtained from 

 Komatipoort, showed the typical heartwater reaction, from which 1 died. 

 SpreuU* injected 3 sheep and 2 goats previously immunised by virulent 

 blood injection and then exposed to natural infection with 5 c.c. blood 

 of a sick animal into the jugular vein; 1 sheep sickened and recovered, 

 1 goat sickened and recovered, and 1 died. 



To judge from Spreull's statement, the virus must have been inade- 

 quate to the one with which immunity was produced. The experiment 

 of the same man proved that immunity obtained by the inoculation of a 

 virulent blood did not protect against natural infection (tick infection). 

 These facts are probably also due to a virus of a different strain. 



Conclusion. — The immunity obtained from one particular strain of 

 virus can be broken by an inadequate strain. {Strain of a 

 different locality.) 



Experience in Practice.— This is only hmited and refers specially 

 to a lot of cattle which were the recoveries from the experiments and 

 had been hyperimmunised to various extents ; they were exposed as 

 controls in connection with an experiment of a different nature two 

 years after immunisation. 



There were altogether exposed eight animals immune against heart- 

 water. In no case could with certainty a breakdown due to the tick 

 infection be noticed, although the control animals contracted this disease. 

 Dixon,t in his experiments with goats which had been immunised by 



* "Cape Agricultural Journal," Vol. XXIV, No. i, 1904. 

 t " Cape Agricultural Journal,'' Vol. XV, No. 12, 1899, 



