430 DISEASES OF CATTLE. 



therefore be largely given to the ticks, and it now appears probable 

 that if the cattle could be freed from this parasite when leaving the 

 infected district they would not be able to cause the malady. The dis- 

 covery of the connection of the ticks with the production of the disease 

 is so recent that it is impossible to predict at this time the influence 

 which it may have in preventing its spread. It establishes an essen- 

 tial point, however, and indicates many lines of investigation which 

 are likely to yield 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 therefore 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 corpus- 

 cles 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 exceedingly minute body, 

 probably endowed with motion, and only after it has succeeded in enter- 

 ing the corpuscle does it begin to enlarge. Concerning the more detailed 

 description of this micro-organism we must refer the reader to the 

 forthcoming special report on Texas fever. We shall simply delay in 

 this place to describe its main characters. Plate XLIII, 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 the growth of the parasite, in fact the 

 largest which has thus far been detected. It will be noticed that there 

 are usually two bodies in a corpuscle. These bodies are in general 

 pear shaped. The narrow ends are always toward each other when two 

 are present in the same corpuscle. If we bear in mind that the average 

 diameter of the red blood corpuscles of cattle is from. - 4 -oVo to -^Vo inch, 

 the size of the contained parasite may be at once appreciated by a 

 glance at the figures referred to. 



The various disease processes which go on in Texas fever, and which 

 we may observe by examining the organs after death, all result from 

 the destruction of the red corpuscles. This destruction may be ex- 

 tremely rapid or slow. When it is rapid we have the acute, usually 

 fatal, type of Texas fever which is always witnessed in the height of the 

 Texas-lever season; that is, during the latter weeks of August and the 

 early weeks of September. When the destruction of corpuscles is 

 slower a mild, usually non-fatal, type of the disease is called forth which 

 is only witnessed late in autumn or more rarely in July and the early 

 part of August. Cases of the mild type occurring thus early usually 

 become acute later on and terminate fatally. 



* The investigations from which the facts in the remainder of this article are taken 

 will be published in full in a forthcoming report of the Bureau of Animal Industry 

 byDrs, Theobald Smith and F. L. Kilborne. 



