31 



is generally not influenced by the latter ; the longer the interval between 

 the two, the surer a reaction will result. This applies also to in- 

 adequate virus. 



Rksumk. — Our present knowledge concerning immunity in horse- 

 sickness can be resumed as follows : — 



(1) Recovery from an attack of horse-sickness causes immunity ; 



this immunity is complete to a virus adequate to the one which 

 caused the disease (Ordinary strain). 



(2) Immunity may vary according to the virulency of a strain 



which can be influenced by passage from animal to animal. 

 (This apphes to the Tzaneen strain.) 



(3) Immunity obtained by one strain of virus does not protect all 



the animals against a different strain of virus. 



(4) Serum of immune animals, hyperimmunised by either strain, 



has protective quahties against virus of the same and of 

 different strains. 



(5) Recovered animals do not retain the infection in their blood. 



Blue-Tongue in Sheep. 



This disease is due to a filtrable ultravisible micro-organism, which 

 in vitro preserves its virulency for several years. Blood of a sick animal 

 acts as virus. A sheep which has recovered from an attack of blue- 

 tongue is considered immune ; there are no definite data at our disposal 

 to show how long the. natural acquired immunity lasts, but it is generally 

 admitted that " salted " sheep contract the disease in a lesser degree. 



Immunisation. — There exists two methods of immunisation, viz., the 

 first introduced by Spruell,* consisting of an injection of adequate serum and 

 virus in appropriate quantities either simultaneously or mixed at the time 

 of injection ; the second one is the vaccination introduced by the writer of 

 an attenuated virus obtained by passing it through a number of 

 generations. 



Sheep which recover as a result of either inoculation are immune. 



The following data refer principally to my own observations, where 

 not otherwise noted : — Mortality from vaccination is almost nil. The 

 sheep show visible symptoms of blue-tongue in a shght degree ; the 

 intensity varies in various flocks and under adverse conditions (rain, for 

 instance). 



Character op Immunity. — The following note explains the nature 

 of the immunity : — 



A sheep which recovered from a 20 c.c. injection of virus reacted to 

 a subsequent injection of 30 c.c. virulent blood injected 33 days later 



* '■ Cape Colony Agricultural Joumfjl," 



