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I.— DISEASES DUE TO ULTRAVISIBLE VIRA. 



A., — Horse-sickness, Blue-tongue, Heartwater. 



Horse-sickness is due to a filtrable ultravisible organism, which in 

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

 acts as a virus. A horse or a mule which has recovered from an attack of 

 this disease is known to have acquired immunity (salted) ; it is generally 

 understood that immune animals do suffer from relapses (aanmanings), 

 from which as a rule they recover. 



The following notes will demonstrate the nature of the immunity 

 acquired, and the cause of the relapses. Tt will be advisable to record 

 the observation in chronological order, as this will best explain the develop- 

 ment of our knowledge concerning the immunity in this disease. 



Immunity in Mules.* — The method of immunising mules consists in 

 the simultaneous injection of virus (2 c.c.) and serum (average dose, 300 

 c.c). The serum is obtained from horses or mules which have recovered 

 and which have been hyperimmunised. This process consists in the 

 intra] ugular transfusion of blood from the sick into the immune animal, 

 averaging in amounts to 10 litres, transfused in two to four operations in 

 intervals of 6-18-24 hours. About three to four weeks after infusion, 

 the serum of the infused animal is fit for use. The virus with which the 

 experiments were started was collected from a horse in Pretoria, which 

 had contracted the disease spontaneously. As nearly all our initial experi- 

 ments were carried out with this virus, and later it became necessary 

 to distinguish it from vira collected from other animals, it was called the 

 " Ordinary Virus." Once the fact had become established that through 

 the simultaneous injection of virus and a corresponding dose of serum 

 immunity could be obtained, it had to be decided whether this immunity 

 was complete, both concerning quantity of virus and time it lasts. 

 The former could naturally easily be settled, whereas the latter has not 

 yet found a definite solution. 



The best proof for immunity was the process of hyperimmunisation. 

 The statistics at our disposal show that up to January, 1907,t the total 

 number of 295 mules were tested with the Ordinary Virus. Amongst 

 this number were 41 mules which were hyperimmunised twice, 13 mules 

 three times, 4 mules four times, and 2 mules five times. Tt must be 

 remembered here that the infusion for one hyperimmunisation averages 

 8 to 10 htres. The observation made was, that in no instance did the 

 infused animal show any reaction typical for horse-sickness; indeed 

 usuallv not even a rise of temperature due to the huge infusion of sick 

 febrile blood was noticed. " The animals -used for hyperimmunisation had 



* Annual Report, Gov. Vet. Bac, Transvaal, 1903-04. 

 tAnnu3,l Report, Gov, Vet. Bac, Transvaal, 1907-08, 



