612 VETEEINAEY HYGIENE 



How infection is produced is still a debated point, but 

 considerable evidence has been accumulated to show that it 

 must be due to some blood-sucking insect which is generally 

 absent during the day and present at night. Bearing in 

 mind the habits of the mosquito, this theory is not difficult 

 to understand, in fact Theiler, the greatest authority in 

 South Africa on veterinary bacteriology, believes that some 

 variety of mosquito is responsible. The mosquito is pos- 

 sibly merely the simple carrier, as is the case with the 

 Tse-tse fly, but where the mosquito obtains the poison is 

 still a mystery. 



The disease is essentially one of the hot and wet months, 

 November to April ; it is usual to look for its disappearance 

 a few days after the first general frost in winter, and 

 excepting in some places where it is enzootic nothing more 

 is seen of it for some months. 



The influence of dew and wet grass has been much 

 debated ; the Dutch farmer implicitly believes in it, but as 

 a cause it does not stand the test of experimental inquiry. 

 Even the dew has been collected and used for inoculation, 

 but with no results. On the mosquito or blood-sucking 

 insect theory we can understand why dew has been asso- 

 ciated in the minds of the Dutch with the cause of this 

 disease. The insect is perhaps entirely nocturnal, it 

 objects to buildings, preferring the open, and it is in the 

 open left out grazing all night that horses get most severely 

 infected ; dew has nothing to say to it. 



As Theiler points out there is no means of explaining 

 how the disease could come in the dew, and even if it were 

 possible the first sun would kill the virus on the grass, for 

 as he has pointed out a few hours' drying at ordinary 

 temperature is sufficient to kill it. 



The influence of shelter is remarkable, horses in stables 

 enjoy comparative immunity provided they are stabled 

 before sundown and not taken out again until after sunrise. 

 Nor is it necessary that the stable should be closed; we 

 have seen extraordinary protection afforded by stables with- 

 out doors and with very incomplete walls. 



Digitized by Microsoft® 



