INFECTIOUS DISEASES OF CATTLE. 481 



southern Europe, Central and South America, Australia. South 

 Africa, and the West Indies. 



"VVTien cattle from other sections of the country are taken into the 

 infected district they contract this disease usually during the first 

 summer, and if they are adult animals, particularly milch cows or fat 

 cattle, nearly all die. Calves are much more likely to survive. The 

 disease is one from which immunity is acquired, and therefore calves 

 Avhich recover are not again attacked, as a rule, eren after they 

 become adult. 



When the infection is disseminated beyond the permanently in- 

 fected district, the roads, pastures, pens, and other inclosure^ are 

 dangerous for susceptible animals until freezing weather. The infec- 

 tion then disappears, and cattle may be driven over the grounds or 

 kept in the inclosures the succeeding summer and the disease will not 

 reappear. There are some exceptions to this rule in the section just 

 north of the boundary line of the infected district. In this locality 

 the infection sometimes resists the winters, especially Avhen these 

 are mild. 



In regard to the manner in which the disease is communicated, 

 experience shows that this does not occur by animals coming near or 

 in contact Avith each other. It is an indirect infection. The cattle 

 from the infected district first infect the pasture^s, roads, pens, cars, 

 etc., and the susceptible cattle obtain the viinis secondhand from 

 these. Usually animals do not contract this disease when separated 

 from infected pastures by a fence. If, however, there is any drain- 

 age or washing by rains across the line of fence this rule does not 

 hold good. 



The investigations made by the Bureau of Animal Industry demori- 

 strate that the ticks which adhere to cattle from the infeeted district 

 are the only known means of conveying the infection to the bodies of 

 susceptible cattle. The infection is not spread by the saliva, the 

 urine, or the manure of cattle from the infected district. In studying 

 the causation and prevention of this disease, attention must there- 

 fore be largely given to the ticks, and it now seems apparent that if 

 cattle could be freed from this parasite when leaving the infected dis- 

 trict they would not be able to cause the malady. The discovei7 of 

 the connection of the ticks with the production of the disease has 

 played a very important part in determining the methods that should 

 be adopted in preventing its spread. It established an essential point 

 and indicated many lines of investigation which have yielded and are 

 still likely to yield very important results. 



Nature of the disease. — Texas fever is caused by an organism which 

 lives within the red-blood corpuscles and breaks them up. It is there- 

 fore simply a blood disease. The organism does not belong to the 

 bacteria but to the protozoa. It is not, in other words, a microscopic 

 16923°— 12 31 



