74 THE PROBLEM OF HORSE SICKNESS. 



Dikkop is connected both with virus and the individiiahty of 

 the horse. With a very virulent virus more Dunkop will result ; 

 the animal has less time to put up a defence. With a \veaker 

 virus a Dikkop appears ; the incubation period being- longer, the 

 animal can put up a defence. 



B. Result of Tests. — The majority of the horses that 

 survived the injections just referred to were later tested on 

 their immunity, using- both the same virus as that utilised for 

 injection, and, in addition, fresh viruses. These latter viruses 

 were obtained in many Avays : some from horses or mules that 

 contracted horse-sickness or died from horse-sickness in various 

 parts of South Africa, others from immunised mules that died 

 of breakdowns or showed relapses, and others, again, from 

 naturally salted horses and mules that showed aaimiaaniiiqs or 

 died. Finally, compound viruses were obtained either by mix- 

 ing two or more viruses or l\v injecting various horses with 

 dJilerent strains, thus gradually amalgamating the viruses until 

 all were present in one horse. 



The actual numl)er of tests carried out on each horse, and 

 the kind and c|uantity of virus used, varied to a considerable 

 extent. .Some horses were onlv tested by the injection of I or 

 2 c.c. virus, whereas others received as many as ten or twelve 

 different injections, of wliich perhaps three or four consisted of 

 2 or 5 c.c. virus each, whilst the remaining eight or nine injec- 

 tions consisted of io,ooo c.c. each fhvperimmunisatiiMiV In 

 other words, no horses received as a test less than an amount 

 sufficient to kill over i,ooo horses, whereas others received suffi- 

 cient to kill over to million animals. Out of t'lie original num- 

 ber of 1,078 horses used for injection, 670 were suljsequently 

 tested, and the following figures represent some 1.7.^8 tests, an 

 average of roughly three tests per horse. The individual figures 

 are: 1,187 tests carried out on the 494 horses that survived the 

 injection of Tzaneen virus, and 551 tests on the 176 horses that 

 survived the original injection of ordinary virus. 



( i) A!iiiii(i!s that Failed to React to tJic Original Injection. 



Immune to . Tzaneen 

 Ordinary \'irus 



per cent. per cent. 

 Again failed to react to any tests . . - . 20 11 



Showed one or more Dunkop reactions and recovered ^1 94 ^"' ^^ 



Dikkop ,, ,, ,, 15) " If. I 



reactions and finally died of Dunkop 481 c^ 44) ^ 



Dikkop 81 ' 12 1 



Abstract. 



Total survivors from tests • . . . . . 44 44 



Dunkop form produced by tests in . . . . 57 dl 



Dikkop ,, ,, ,, .. ..23 28 



