DOMESTIC ANIMALS 287 



mule is gradually but surely crowding out the ox as 

 a work animal. With abundant cheap pasturage it 

 costs but little more to raise a two-hundred-dollar 

 mule than a forty-dollar ox, and it is most surprising 

 that the subject of mule breeding is not receiving 

 more attention in all of these Southern countries. 



At the South and in the tropics cattle imported 

 from the North are attacked by the serious disease 

 variously known as Texas fever, tick fever, or 

 splenic fever. It is caused by a minute parasite 

 of the red blood corpuscles that is very similar to the 

 one causing malarial fevers in man. In the case of 

 the cattle disease, however, the parasite is carried, 

 not by mosquitoes, but by certain species of cattle 

 ticks. Native Southern cattle have all become im- 

 mune to this disease although the parasite is still 

 present in their blood, and hence they are constantly 

 infecting the ticks which bite them. The presence 

 of even a few of these ticks on imported Northern 

 animals will quickly induce the disease. A method 

 of artificial immunization has, however, now been 

 devised which consists in inoculating the animals to 

 be treated with fresh blood serum taken from an 

 animal that is known to be immune. The toxine 

 contained in this serum induces a slight attack of the 

 fever, which renders the animal immune to future 

 natural tick inoculations. By previously immuniz- 

 ing young Northern cattle in this way they can be 

 taken South or to the tropics with the loss of not 

 over 10 per cent and frequently of less than 5 per 

 cent, while without this treatment the loss would 

 run from 50 per cent to 90 per cent. It is now, 



