474 DISEASES OF CATTLE. 



'When 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, nearl}' all die. Calves are much more likely to survive. The 

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

 which recover are not again attacked, as a rule, even after they be- 

 come adult. 



"When the infection i- disseminated beyond the permanently in- 

 fected district, the roads, pastures, pens, and other inclosures 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 if they 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 with one another. It is an indirect infection. The cattle 

 from the infected district first infect the pastures, roads, pens, cars, 

 etc., whence the susceptible cattle obtain the virus secondhand. 

 Usually animals do not contract the disease when separated from in- 

 fected pastures by a fence. If, however, there is any drainage or 

 washing by rains across the line of fence this rule does not hold good. 



The investigations made by the Bureau of Animal Industry demon- 

 strate that the ticks v.hich adhere to cattle from the infected district 

 are the only known means of conveying the infection to susceptible 

 cattle. The infection is not spread by the saliva, the urine, or the 

 manure of cattle from the infected district. In studying the causa- 

 tion and prevention of this disease, attention must therefore be 

 largely given to the tick, and it now seems apparent that if cattle 

 could be freed from this parasite when leaving the infected district 

 they would not be able to spread the malady. The discovery 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 

 plant, but it belongs to the lowest forms of the animal kingdom. 

 This very minute organism multiplies very rapidly in the body of the 

 infected animal, and in acute cases causes an enormous destruction 



