474 DISEASES OF CATTLE. 



When cattle from other sections of the country are taken into the 

 infected district they contract tliis disease usually during the first 

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

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

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

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

 come adult. 



"When the infection is 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 ju-t 

 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 hy 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 which adhere to cattle from the infected district 

 tire the only known means of conveying the infection to susceptible 

 cattle. The infection is not spread Ijy 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 

 ira-gely given to the tick, and it now seems apparent that if cattle 

 could be freed from this parasite when leaving the infected district 

 tJiey would not be able to spread the malady. The discovery of the 

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

 a very important part in determining the methods that should be 

 :;(lopte<l in preventing its spread. It established an essential point 

 and indicated many lines of investigation Avhich have yielded and are 

 still likely to yield very important results. 



Nntivre of the dhcdHc. — Texas fe\-er is caused by an organism which 

 lives within the red l)h)od corpuscles and l)i-eaks them up. It is there- 

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

 I>;K:teria but to the protozoa. It is not, in other v.-ords, a microscopic 

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

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

 infected animal, and in acute cases causes an enormous destruction 



