352 DISEASES OF CATTLE, SHEEP, GOATS, ETC. 



of the liver by these parasites is said to cause the animal to take on 

 flesh with greater rapidity. On account of this phenomenon, it 

 has been stated that in an early day in European countries sheep, 

 to be fattened quickly, were purposely infested with the liver fluke. 

 Such a practice is extremely dangerous, owing to the great danger 

 of infecting the pastures and other members of the flock. 



As previously stated, it has been argued that parasites do not 

 injure the host; but such opinions were offered in an early day, 

 when but little was known of the various forms and degrees of 

 parasitism. It is now a well demonstrated fact that parasites are 

 injurious in at least three ways : First, as direct agents of loss from 

 the animal economy. Second, as carriers of the micro-organisms 

 of contagious and infectious diseases. Third, as carriers of other 

 forms of parasites. In the first instance, must be considered the 

 effect produced by the extraction of nutrition from the animal by 

 the parasite the loss of vitality by the production of sores, and 

 the loss of energy due to the irritation and worry caused by the 

 parasites. At first thought, it would not seem that the small amount 

 of the animal fluids consumed by these little insects would have 

 any appreciable effect on the host. When we consider, however, 

 the amount of blood consumed by a single mosquito, fly or louse, 

 and then multiply this by thousands, it will be seen that the 

 amount of nourishment extracted by them is enormous. The det- 

 rimental effect on the host is all the more apparent when the slow- 

 ness of preparation of these fluids for the building up of the tis- 

 sues is taken into consideration, as well as the need of the fluids 

 consumed by the parasite for the growth and strength of the ani- 

 mal, the laying on of fat, etc. It will, therefore, be seen that the 

 drain is immense, and that the presence of parasites is no insigni- 

 ficant matter. The loss by the degeneration of tissues manifested 

 in its formation of sores, ulcers, scabs, etc., making a heavy drain 

 on the animal economy, is also of great importance, and is a source 

 of heavy drain on the animal, in addition to the loss of nutrition. 

 The irritation caused by parasites, especially on nervous animals, 

 or on sensitive parts of the animal, is often of great injury, as is 

 illustrated in the attacks of the botflies of the horse, sheep and ox. 

 The irritating effect of the common horse and horn flies upon 

 horses and cattle in pastures in the summer time is often an impor- 

 tant factor in the prevention of the laying on of flesh. 



It has been stated that as the biting lice live on the cast off scales 

 of the skin, hair, or feathers of fowls, they are an advantage rather 

 than a disadvantage. The irritation these parasites cause by crawling 

 about the skin is frequently considerable, as indicated by the host 

 scratching and rubbing itself on the fence, posts, etc., and in the case 

 of fowls dusting themselves in ashes or dust. 



As carriers of contagious and infectious diseases, parasites play 

 an important part. Although comparatively little is known on this 

 point, enough information is at hand to justify the statement that 

 in all probability a large number of contagious diseases are contracted 

 and disseminated through the medium of parasites. It is now known 



