INFLUENZA. 349 



mation, sometimes more so in low-lying districts than on the 

 hills. It may appear at all times of the year ; perhaps it is 

 more commonly seen in autnmn, winter, and spring than in 

 summer. Very often, however, its worst form is seen during the 

 hot summer months. Influenza has often appeared in foggy 

 weather, but outbreaks have occurred without such a coincidence, 

 and consequently no weight can be attached to this circumstance. 



Its spread is not influenced by the wind : sometimes it moves 

 against it. 



"The outbreak of 1766 and the American one of 1872 suc- 

 ceeded to two very similar climatic conditions. The winters 

 had been exceedingly severe and the summers unusually hot, 

 and earthquakes had been frequent." — (Law.) 



Similar climatic conditions have, however, not been followed 

 by an outbreak of influenza ; and as in the case of fogs, mildews, 

 &c., the coincidence can only be looked upon as accidental. 



Sudden changes of temperature appear to assist the develop- 

 ment of the influenza poison, and exposure to cold predisposes 

 the animal to the disease, but neither of these causes is suffi- 

 cient of itself to produce it. 



Ozone in undue quantity in the atmosphere has been supposed 

 to be a cause of influenza. This is, however, highly improbable, 

 as ozone, although an irritant to the mucous membrance of the 

 nose when applied in large quantities, has no effect on the 

 nervous system like the influenza poison. 



Predisposing causes. — Animals crowded together in damp, ill- 

 ventilated, and otherwise unhealthy situations, are generally the 

 first to suffer from influenza. In them it commits its greatest 

 havoc. Young horses are more predisposed than those of maturer 

 years ; still, the old suffer severely, and are often carried away. 

 Sex has no influence. Neglect of every description, as well as 

 bad food and over- work, by debilitating, render animals subject to 

 severe and early attacks of disease. But no amount of care will 

 exempt them from it, as it appears in the stable of the nobleman 

 as well as that of the poorest carter. The latter, however, expe- 

 riences it in its greatest intensity, and at a much earlier period 

 than the former. 



PATHOLOGY. 



The morbific agent absorbed into the blood gives rise to 

 febrile disturbance and depression of the nervous centres. The 



