4'22 BLOOD DISEASES, 



in July, the second may appear in September, and perhaps be 

 of greater virulence. 



When the disease is induced by inoculation, experiments have 

 proved that it has no contagious properties, — in fact, that it is not 

 transmitted by cohabitation, but by the intermediation of the 

 cattle tick. But, as already stated, it seems to be concluded 

 that preventive inoculation has so far been a failure, for the 

 effect produced in the body of an animal by the destruction of 

 the red corpuscles equal in amount to all those circulating in the 

 body at any given time should make much more impression than 

 any method of inoculation is likely to do. Not being, so far as 

 at present known, a ptomaine-producing organism, like bacilli, 

 &c., this is as might be expected. Yet such an attack not only 

 does not always prevent a second attack, but may not pre- 

 vent death during a second attack. Aside from the difficulties 

 attending the production of immunity, under any circumstances 

 the difficulties of preparing a " vaccine " according to the method 

 hitherto practised are at present insurmountable. The micro- 

 organism cannot be cultivated. The infection of ISTorthern cattle 

 might be prevented by careful destruction of the ticks upon 

 Southern cattle before their removal to Northern territories, or 

 by prohibiting their migration during hot weather. These 

 suggestions are not, however, made by tlie reporters, but they 

 attach great importance to the protection of animals taken south 

 into permanently infected pastures. They state that it is 

 probable that if calves be taken they may, without treatment 

 of any kind, survive the infection upon southern pastures, and 

 become gradually insusceptible. But in case of animals more 

 than twelve to eighteen months old the first attack might be 

 fatal ; and, perhaps, the simplest way of dealing with older cattle 

 is to endeavour to induce immunity by exposing them to infection 

 at some specified time in autumn. In the latitude of Washing- 

 ton it was found that the most convenient time is the middle of 

 September. In more northerly latitudes the exposure should be 

 correspondingly earlier. Cattle exposed in this way invariably 

 take the fever, but the mortality is very small. Such animals 

 may die of a second attack during the succeeding summer, but 

 a mild exposure during the following autumn may furnish a 

 sufficient protection. 



Another method of inducing the disease is the injection of 

 blood from infected cattle, v/hich generally induces — if practised 



