STATE HYGIENE 613 



There are places where horse sickness is very virulent 

 and stabling does not afford complete protection, but as 

 a general rule the statement may be accepted. 



If grass were the cause of introducing the virus into the 

 body, experimental infection by the digestive canal should 

 be easy ; as a matter of fact it is extremely difficult, a dose 

 of virulent blood many thousand times greater than that 

 required for hypodermic inoculation is required to produce 

 infection. 



Inhalation has been held by many as the chief source of 

 infection, and nosebags soaked in a disinfectant are fre- 

 quently employed for horses that have to go out at night, 

 the nosebag acting, it is supposed, as a filter. Again this 

 theory is not supported by experimental inquiry, even the 

 intra-tracheal injection of the virus is not always followed 

 by the disease. It is more in accordance with our know- 

 ledge of the disease, to regard the fine skin of the muzzle 

 when grazing as a likely seat of attack by a blood-sucking 

 insect, and the nosebag on horses working at night may act 

 by protecting the skin of the muzzle. 



The insect theory explains how the disease may travel 

 over the Continent, and why it is worse some years 

 than others. It also explains why the wet season and 

 proximity to water are unfavourable conditions, why alti- 

 tude gives relative protection, and further, why the disease 

 ceases quite suddenly a few days after the first general 

 frost. 



The disease consists essentially in the outpouring of 

 serum into the subcutaneous tissues of the head and 

 throat, or the effusion of serum into the lungs, thorax, and 

 pericardial cavities. There are other lesions, stomach and 

 bowel for instance, but those that kill early and certainly 

 are the effusions into the pericardial sac and lungs. 



The mortality is high especially from the chest form, 

 while when the head and throat become affected the chances 

 of recovery are better ; should recovery occur the animal 

 is practically immune, though second attacks have been 

 observed. 



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