320 *^o^ Pf*^- 



its virulence even after 14 days. The symptoms are caused by 

 the development of vesicles on the walls of the milk ducts 

 (Lienaux & Hebrant). 



The tenacity of the virus is apparently similar to that of the virus of sheep 

 pox. It resists drying for several weeks, but is destroyed at a temperature of 

 57.5° in five minutes (Power). On the other hand it is not destroyed by ■ — 180 

 degrees, and its virulence is not greatly influenced by the action of glycerin, as 

 when mixed with it and kept in a dark place the virus retains its virulence from 

 8 to 10 months. 



Natural Infection. In healthy herds the infection usually 

 occurs through vaccinated persons. This has been proven by 

 the positive results of inoculation experiments and also by the 

 observations that the disease attacks quite frequently large 

 numbers of cows shortly after the vaccination of children. For 

 instance the disease is usually observed among cows in the 

 spring, at which time the vaccination of children is customary, 

 when the dissemination of the virus is given a very favorable 

 opportunity. 



Infection usually takes place during milking, when the virus 

 is easily transmitted from the hands of the milkers to the 

 superficially injured skin of the udder.. The disease is also 

 frequently transmitted from animal to animal by the milkers 

 during milking, but contaminated straw, food and the stable 

 floor may play a part as intermediate carriers of the virus, and 

 this infrequent mode of infection has been observed in young 

 stock, steers and oxen. In all probability the infection is not 

 transmitted through the air. 



True human pox may also in all probability be trans- 

 mitted to cattle, although the results of artificial inoculation 

 do not prove it (see p. 300). As a matter of fact in the 

 eighteenth century and at the beginning of the last century, at 

 which periods human pox still occurred" frequently in an epi- 

 demic form, the disease was also observed with great frequency 

 among cows. 



Sjrtnptoms. The disease usually commences after an incuba- 

 tion period of 4 to 7 days, with moderate febrile symptoms (rise 

 in temperature o£ 1/2-! degree, diminished appetite, irregular 

 rumination, weakness, etc. ) . In many cases however these prod- 

 romal symptoms are so mild that they pass unnoticed. During 

 milking it is noted that the udder is sensitive, the milk is 

 thinner, of a lower specific gravity and coagulates more quickly. 

 On the slightly warm and swollen teats, as well as on adjoining 

 parts of the udder, hard nodules in size ranging from that of 

 a lentil to a pea appear on the second to third day, which change 

 inside of 1 to 2 days into bean-sized vesicles, filled with a clear, 

 transparent lymph. The vesicles are reddish or bluish, or 

 yellowish-white, of a mother-of-pearl or metallic luster, some- 

 times lusterless, depending on the thickness of the skin,^ and on 

 its color. On the body of the udder the vesicles are uniformly 



