INFECTIOUS DISEASES. 



By llTsu Smi'i'KN Huiukkopkk, M. 1>., Vet. 

 [Kcvlsid by A. Kichhoru, 1). V. S.] 



GENERAL DISCUSSION. 



An infectious disease may be defined as any malady caused by the 

 introduction into the body of minute organisms of the ve<retable or 

 animal kingdom ^vhich have the i)ower to multiply indelinitely and 

 set free certain peculiar poisons which are chiefly responsible for mor- 

 bid changes. Nearly all diseases of animals for which a definite 

 cause may be attributed are caused by bacteria ; such are tuberculosis, 

 anthrax, blackleg, lockjaw, and others. There are some diseases, as, 

 for instance, Texas fever and rabies, which are caused by a minute 

 aniuuil parasite known as protozoa, while others again, like lumpy 

 jaw and aspergillosis, are caused by fungi. Besides there are infec- 

 tious diseases in which the causative agents have never been success- 

 fully isolated, as they are so small that they can not be detected by the 

 aid of the most powerful microscope, and accordingly they are termed 

 as ultravisible viruses. TTog cholera, foot-and-mouth disease, small- 

 pox, and others belong to this group. 



Bacteria may be defined as very minute unicellular organisms of 

 plantlike character. They multiply either by simple division or by 

 spore formation, the latter usually taking place when the conditions 

 p>ertaining to the growth of the bacteria become unfavorable. The 

 spores are much more resistant to destruction than the bacteria which 

 produce them. 



Another group of parasites producing disease is known as protozoa. 

 These are more comj>lex than bacteiia, and their artificial cultivation 

 is also much more dillicult than is the case with the bacterial parasites. 

 Of the representatives of this group, causing disease in animals, are 

 the trypanosomes, which are the causative factoi-s of dourine and 

 .surra, and the piroplasma, which induce Texas fever in cattle and 

 malaria or biliary fever of horses. There are also di.sease-producing 

 fungi which are responsible for certain affections in horses; among 

 these the most important aie mycotic lyni|)h;mgitis, or sjiorotiMchosis, 

 and streptotrichosis. 



The introduction of the infection may take place in various ways. 

 The most frequent metliod is by ingestion. Further, the entrance of 



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