4 :''.: t'l ,•'■ " , ' TNTECPUCTORY 



causes may produce the same disease. For example, acute indi- 

 gestion may be caused by a change of diet, watering the animal 

 after feeding grain, by exhaustion and intestinal worms. 

 Usually, but one of the animals in the stable or herd is affected. 

 If several are affected, it is because all have been subject to 

 the same condition, and not because the disease has spread from 

 one animal to another. 



Specific Diseases. — The terms infectious and contagious are 

 used in speaking of specific diseases. Much confusion exists in 

 the popular use of these terms. A contagious disease is one that 

 may be transmitted by personal contact, as, for example, in- 

 fluenza, glanders and hog-cholera. As these diseases may be 

 produced by indirect contact with the diseased animal as well as 

 by direct, they are also infectious. There are a few germ dis- 

 eases that are not spread by the healthy animals coming in direct 

 contact with the diseased animal, as, for example, black leg 

 and southern cattle fever. These are purely infectious diseases. 

 Infection is a more comprehensive term than contagion, as it 

 may be used in alluding to all germ diseases, while the use of 

 the term contagion is rightly limited to such diseases as are 

 produced principally through individual contact. 



Parasitic diseases are very common among domestic animals. 

 This class of disease is caused by insects and worms, as for ex- 

 ample, lice, mites, ticks, flies, and round and flat worms that 

 live at the expense of their hosts. They may invade any of 

 the organs of the body, but most commonly inhabit the digestive 

 tract and skin. Some of the parasitic insects, mosquitoes, flies 

 and ticks, act as secondary hosts for certain animal micro- 

 organisms that they transmit to healthy individuals through the 

 punctures or the bites that they are capable of producing in the 

 skin. 



Causes. — For convenience we may divide the causes of 

 disease into the predisposing or indirect, and the exciting or 

 direct. 



