288 African Horse-Sickness. 



conductivity of the serum is always below the normal average. Similar 

 changes are also present in immune horses. 



The course of tlie acute and peracute horse-sickness is usu- 

 ally unfavorable, inasmuch as horses, especially those brought 

 from non-infected territories, frequently succumb almost with- 

 out e^xception, while the subacute form runs a much milder 

 course. 



The disease is distinguished from anthrax by its slower 

 course, and especially by the absence of the pronounced, acute 

 enlargement of the spleen; from piroplasmosis, by the absence 

 of icterus, by the edematous swellings, the severe inflammation 

 of the left half of the stomach and by its occurring only at cer- 

 tain periods of the year. Microscopical examination of the blood 

 in the diseases mentioned gives positive results, however the 

 presence of piroplasmas does not exclude horse sickness. 



Prevention. The treatment attempted up to the present 

 time has generally proved ineffective. In order to prevent the 

 disease, horses are in the summer driven to high pastures, which 

 are known to be non-infectious, and in suspected places they 

 are allowed to pasture only in day time. By persistent breeding 

 of immune horses and mules it will probably be possible to 

 produce resistant breeds in the infected localities (Rickmann). 



Immunization. Eecovery from the disease reduces the sus- 

 ceptibility of the animals, but does not entirely eliminate it, 

 as even comparatively immune ("salted," "gezouten") horses 

 may become affected by a virulent infection (Theiler). Never- 

 theless this acquired immunity protects the animals from 

 natural infection, in about two-thirds of the cases, and when 

 they do become affected the course of the disease is much milder. 



Immunity may be produced artificially in various ways, but 

 only in mules without great losses. In these animals immuniza- 

 tion has been applied in practice in South Africa for several 

 years, but it must be repeated annually, as the immunity gradu- 

 ally vanishes. 



Koch's Method of Immunization. Koch obtained a serum from 

 "salted horses" which were injected subcutaneously and intravenously 

 with increased doses (up to 2000 ce.) of virulent blood at intervals of 

 from 3 to 4 weeks, which produced passive immunity on being injected 

 into healthy horses without exerting a hemolytic action, as was the 

 case with sera prepared by other methods. Further experiments showed 

 that by repeated injections of virulent blood and then of serum horses 

 may be actively immunized. The quantity of virus and serum must be 

 proportioned in such a way that the animals become affected by the 

 disease without dying of it. 



It was found most effective to make an inoculation of 0.01 ce. of virus 

 and 100 cc. of serum for the first injection, tlie dose of the virus to be increased 

 to 0.05 and 0.2 ee., at the same time reducing the serum to 50 ee. and finally 

 continuing the inoculations with the pure virus (0.5, 1.0, 2.0 ec). The serum is 



