THE PROr.LIiM OF HORSE SICKNESS. 77 



conferred by Tzaneen virus. In fact, after we had collected 

 all the different strains of virus from all parts of the country, 

 we found that practically any virus could break down the im- 

 munity given by any other virus. Accordingly, although 

 ordinary virus can be spoken of as a strong virus, yet it has 

 practically no greater " breaking power " than any other virus. 



6. Some special consideration merits the degree of immunity 

 apart from the quality. A horse that reacted to the minimum 

 test dose of at least 2,000 fatal doses of ordinary virus cannot 

 be infected with horse-sickness if we use the same virus as a 

 dose of a million times or even more. The horse will not show 

 the slightest reaction. It produces, nevertheless, as a response 

 to this injection, anti-bodies. This is the principles of hyper- 

 immunisation. Notwithstanding the presence of anti-l)odies in 

 the blood-stream, the horse can contract the disease again, when 

 a virus of a higher generation of the same strain is used, and 

 likewise, such horses exposed to natural conditions ma}" con- 

 tract the disease as well. For protection, therefore, the degree 

 of immunity does not seem to come into consideration, Inu the 

 quality, a point wdiich has already been brought out. 



The Soliitio)i of ihc Frohlcui is the Discovery of a Cure or a 



Method of Prevention. 



I shall deal with the cure first. Under natural conditions 

 about 90 per cent, of all sick horses die. The disease is usually 

 only detected in its final stage, or at a time when the destiny of 

 the horse is already settled one way or another. It is an old 

 maxim in medicine, *' To cure the disease, remove the cause." 

 The removal of the cause means in all infectious diseases the 

 destruction of the parasites. This is done by so-called specifics. 

 Only for a few organisms of the protozooic kingdom are speci- 

 fics known, vis., the plasmodium of the malarial fevers, which 

 in the merozootic stage can be attacked by quinine ; the group 

 of Babesia, which yield to the aniline dye trypan-blue ; and the 

 spirochsetes, which are susceptible to organic arsenic com- 

 pounds, in particular salvarsan, as are also some of the trvpano- 

 somes. It was the merit of Ehrlich to have shown how the 

 action of these drugs can be explained. Based on his lateral 

 chain theory, he assumes that in the micro-organisms are certain 

 chemo-receptors, which fix the chemical compound, and through 

 the bridge so formed acts the toxophoric group of the com- 

 pounds, viz., the arsenic. Most of the chemical compounds 

 which could be utilised as parasiticides act principally on the 

 cells df the body of the host ; they are, in Ehrlich's language, 

 organotropic, and hence damage the host. To find " para^itio- 

 tropic " drugs is the aim of modern chemotherapy. Thanks to 

 the assistance of Ehrlich, I have been able to try quite a number 

 of these drugs on horse-sickness, in particular, arsenophenvlgly- 

 cin, salvarsan, novoflavin, and some aniline dyes. I am 



