THE PROBLEM OF HORSE SICKNESS. 73 



A. Result of Injections. 



Ordinary Tzaneen 



Virus Virus 



per cent. per cent. 



Not reactin.!.,' to the injection (non-reactors • . 20 28 



Reacting with a Dunkop to the injection (Dunkop ] ] 



recoveries) . . . . . . . . 18 ^^ 24 , 



Reacting with a Dikkop to the injection (Dikkop 1 i 



reco\eries) .. .. .. .. 18/ 19' 



Deaths from Dunkop . . . . . . 26 ) 10 • ^q 



,, Dikkop . . .. .. 18)"'^"^ 19/ ■'^ 



Abstract. 



Total survi\ors from injection . . . . • • 56 71 



Dunkop form produced in . . . . . . 44 34 



Dikkop .. ,, .. .. ..36 3S 



It is thus quite clear that, in the first place, a certain per- 

 centage of animals do not react, and that this figure varies with 

 difterent viruses. We can, therefore, speak of weak and strong 

 viruses, Tzaneen being a weak virus, and ordinary a strong 

 virus. This conclusion is also supported by the percentage of 

 mortality caused by these viruses, the weak one killing 29 per 

 cent., and the strong one 44 i^er cent. This point in itself is 

 particularly interesting, seeing the Tzaneen strain broke down 

 the immunit}' given by the ordinary virus in the case of the 

 mule just referred to, and in many others. The failure of a 

 certain percentage of animals to react can be explained in two 

 wa} s — firstly, a naturalh" acquired immunity to the virus, and 

 secondly, their individuality. It can be readily understood that, 

 in experimenting on so many animals collected from all parts 

 of the Union, we must expect to come in contact with horses 

 that have salted as a result of natural infection on the veld, and 

 that their immunity is sufficiently strong to afford protection 

 against one injection of virus : there may also be horses not sus- 

 ceptible to horse-sickness. 



A discussion on the question of the individualitv of an 

 animal Avould take up too mucli of our time, and is rather out- 

 side of the scope of this paper, but experiments have definitely 

 proved that it is a factor which cannot be ignored. It will be 

 sufficient to say now, that although a horse may not react to a 

 virus at one moment, yet when re-iniected with the same virus 

 at a later date, death may result. This is particularlv the case 

 with an attenuated virus, as, for instance, the Tzaneen virus. 



The second point of interest that arises from the above 

 table is in connection with the percentage of cases of Dikkop 

 produced by the injection of virus as compared with the cases 

 of Dunkop. With ordinary virus more horses die of Dunkop 

 than of Dikkon. With the Tzaneen virus the reverse 

 is the case. I mentioned before that the appearance of 



