482 OE^EBAL DISEASES. 



We may safely conclude that the infective agent of horse 

 sickness, like that of anthrax (p. 471) and tetanus (p. 527), can 

 live, multiply, and retains its virulence outside the animal body, 

 supposing tliat the conditions of climate are favourable to its 

 existence and development. 



The above statements warrant us in excluding worms, ticks, and 

 hloud sucking flies from the list of producers of this disease. 

 Arguing against the malarial character of horse sickness, Wilt- 

 shire points out " that there have been instances of great mortality 

 from it in one district, while an adjoining one, separated only by 

 a narrow river, has been almost entirely exempt." 



The effects of moisture and heat are well marked in this 

 disease. " Horse sickness occurs mostly in low-lying parts of the 

 country, independently of the fact that the general locality may 

 be considerably elevated above the sea-level. Thus, Johannes- 

 burg, which is about 6,000 feet above the sea-level, is frequently 

 subject to its baneful influence. . . . It is commonly observed 

 to occm" during periods when the air has become humid and has 

 been associated with a high daily temperature. . . . Deep 

 kloofs or gullies where vegetation is abundant and the ground, 

 below the undergrowth, moist, are especially dangerous to sus- 

 ceptible animals " (Edingtun). Dry, high pastures at an altitude, 

 say, of 6,000 feet above the level of the sea, are as a rule free 

 from the disease, unless they have been contaminated by the 

 introduction of affected stock ; the original home of this organism 

 appearing to be moist and low-lying lands. Many of the South 

 African highlands which were formerly exempt, have become in- 

 fected, apparently by the introduction of diseased horses. We are 

 at present unable tO' say how long the microbes of horse sickness 

 would continue to exist in a dry and cold locality, in which, we 

 may take for granted, they would be more or less exotics. Eding- 

 ton's remark as to the prevalence of horse sickness at Johannes- 

 burg, shows that any preventive effect which altitude above the 

 level of the sea may have on horse sickness, is due to increased 

 cold and dryness. In almost all cases, frost stops the progress 

 of an outbreak of horse sickness. 



Practical observations and the elaborate bacteriological researches made by 

 Edington, leave no doubt that this disease is caused by a microbe •which 

 unfortunately has not been isolated, up to the present. In this respect, 

 Edington occupies the same position as Pasteur did with regard to rabies. 

 We have, however, sufficient proof to reasonably assume that this organism, 

 whatever it may be, becomes developed into a virulent condition on grass and 

 other herbage, under the combined influence of heat and moisture. It is 

 probable that the microbe of horse sickness (supposing that such an organism 

 exists) forms toxins which exert a paralysing action on the vaso-constrictor 

 nerves, and thus causes dilatation of the blood-vessels, with consequent in- 

 ovease in the amount of plasma exuded into the tissues, and have also a 



