INFECTIOUS DISEASES OP CATTLE. 463 



When the infection is disseminated beyond the permanently infected 

 district, the roads, pastures, pens, and other inclosures are dangerous 

 for susceptible animals until freezing weather. The infection 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 when 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 with each other. It is an indirect infection. The cattle 

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

 etc., and the susceptible cattle obtain the virus second hand 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 demon- 

 strate that the ticks which adhere to cattle from the infected 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 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 cf investigation which have yielded and arc 

 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 

 of red corpuscles in a few days. How it gets into the red corpuscle 

 it is not possible to state, but it appears that it enters as an exceed- 

 ingly minute body, probably endowed with motion, and only after it 

 has succeeded in entering the corpuscle does it begin to enlarge. 

 Plate XL VII, fig. 4, illustrates an early stage of this blood parasite. 

 The red corpuscle contains a very minute roundish body which is 

 stained blue to bring it into view. The body is, as a rule, situated 

 near the edge of the corpuscle. Fig. 5 illustrates an older stage in 



