3fi 



Eedwater or Texas Fever. 



This disease is due to the introduction of Piroplasma higeminum into 

 a susceptible beast naturally by the blue ticks, artificially by inoculation 

 of blood containing this parasite. The blood kept in vitro retains its 

 infectivity only for a hmited number of days (fourteen days). Animals 

 which have recovered from an attack of this disease are known to be 

 immune. An immune Africander ox injected with various amounts of blood 

 from cattle actually suffering from redwater, in intervals and varying in 

 quantity to the total extent of 8 litres, showed no breakdowns of immunity. 



In connection with the immunisation of oxen against rinderpest, 

 country-bred oxen were injected with large doses of blood, which 

 undoubtedly contained Piroplasma higeminum — no accurate records were 

 kept about the reaction, but judging from results, no breakdowns occurred. 



English CAXTLE.^Of 10 Enghsh heifers, which were all immunised 

 against redwater and had shown Piroplasma higeminum in the blood, 6 

 showed reactions and rare Piroplasma higeminum when injected with blood 

 containing this parasite. Of 6 English heifers recovered from an attack of 

 redwater brought on by injection of blood, when exposed to natural 

 infection, 2 died. Smith and Kilborne* also, in their experiments came 

 to the conclusion that one attack of Texas fever does not completely 

 protect against a succeeding exposure to new infection. 



Conclusion. — The immunity conferred by an injection of virulent blood 



can be broken by inoculation, which is, as a rule, not accompanied by 



severe symptoms or death. Immunity conferred by injection of virulent 



blood does not protect completely against natural infection. The 



immunity thus conveyed varies in the various breeds of animals ; 



it is better and almost complete in South African born and bred cattle; 



it is not so good in imported cattle. 



The Blood of Immune Animals. — Animals recovered from redwater 



retain the infection in the blood and such blood remains infective when 



injected into susceptible animals. The progeny of the blue ticks feeding 



on such immune animals likewise transmit the disease. The blood of an 



immune animal exposed to tick infection remains so during the whole 



life of such an animal and probably so in an animal not exposed to tick 



infection. There is an observation recorded from America,f where the 



blood of such an immune animal proved to be infective after twelve 



years. It has been observed that the injection of immune blood causes 



in the majority of animals only a shght reaction during which Piroplasma 



higeminum is found in the blood, and this fact has been made use of as 



a preventive inoculation. The immunity obtained in this way protects 



the greatest num.ber of animals against heavy infection and deaths from 



redwater when exposed to redwater veld. {Vide foregoing conclusion.) 



* U.S.A. Bureau Animal Industiy, 8th and 9th Annual Eeports. 



t Sehroeder and Cntton, 22nd Annual Keport, Bureau of Animal Industry, 1905. 



