South African Horse Sickness, etc. 563 



Race horses that receive no green fodder very rarely suffer. 

 Horses that are corralled (in kraal) at night escape. 



The hot season is the season of greatest prevalence, the disease 

 beginning in November and proving especially fatal from the end 

 of December to the first of March. It appears in a modified form 

 until May when the first frosts appear. 



Though the disease can be fatally inoculated by transferring 

 the blood from one horse to another, it is the rarest possible 

 occurrence to have it propagated in this way. It can be absolutely 

 prevented therefore by attention to the diet. 



The alleged cause of horse sickness is an ultra-microscopic 

 organism which enters the system with the moist, dewy food. 

 Edington, who discovered a cryptogam, has found it in the 

 blood in all his necropsies of horses dying of horse sickness. 

 Why this should be no longer infecting when dried does not 

 clearly appear. It has been alleged that the disease has gradu- 

 ally extended to the higher grounds which were formerly 

 free from it, and the introduction of diseased or infected horses 

 has been advanced as the cause, but in the unfenced state of the 

 veldt and the former abundance of wild animals this should 

 have ensured such extension long ago, if it is really a permanent 

 one. The deadly prevalence of the malady in particular areas in 

 given years, and its entire absence from such localties in others 

 may explain the instances of apparent extension. The dryness 

 and cold of winter is the factor which usually extinguishes the 

 poison in a given district. We have as yet no absolute proof of 

 a progressive acclimatisation of the germ in a colder and drier 

 region. Wittshire observes that it will prevail on one side of a 

 narrow ' river, while the other at an equal elevation is practically 

 free from it. There is no mention of shade which might have 

 explained such a difference in the growth of microbes on the 

 right and left banks. 



Such limitations, together with the activity of the infection in 

 damp seasons, and during damp hours of the day, and its inac- 

 tivity in dry air and vegetation, would strongly suggest a microbe 

 which is conveyed in the body of some invertebrate, but this 

 appeals to be nonessential because Edington has cultivated 

 his virus in vitro and inoculated its products on horses so as to 

 secure immunity. 



