INFECTIOUS DISEASES OF CATTLE. 435 



skin they inoculate tlie cattle and Texas fever breaks out a week or 

 more thereafter. That ticks may and do produce Texas fever had been 

 suspected for many years in various parts of the country. A definite 

 proof was not offered, however, and the experiments carried on by the 

 Bureau of Animal Industry were the first to demonstrate this impor- 

 tant fact. It is not within the seope of this work to describe the steps 

 by which this conclusion was reached, nor the experiments made in 

 this direction. These will be found in the forthcoming report. There 

 are some important facts in the disease itself cleared up by these 

 experiments which require mention here. 



The so-called period of incubation. After the young ticks have 

 attached themselves to cattle the fever appears about ten days there- 

 after iu midsummer. When the weather is cool, as in autumn, this 

 period may be a little longer. The actual period of incubation may be 

 shorter than this, for if blood from a case of Texas fever be injected into 

 the blood-vessels of healthy cattle the fever may appear within five days. 

 When cattle graze upon pastures over which Southern cattle have 

 {>;i-.-ed, the time when the disease appears varies within wide limits. 

 When the animals have been put upon pastures immediately after 

 Southern cattle have infected them with ticks, it may take from thirty 

 to sixty days, or even longer, before the disease appears. This will be 

 readily understood when we recall the life history of ticks. The South- 

 ern cattle leave only matured ticks which have dropped from them. 

 These must lay their eggs and the latter be hatched before any ticks 

 can get upon native cattle. The shortest period is thus not less than 

 thirty days, if we include ten days for the period of incubation after 

 the young tick has attached itself to native cattle. When the infec- 

 tion of pastures with ticks has taken place early in the season, or when 

 this is coldj the period will be much longer because it taTtes longer for 

 the eggs to hatch. 



If native cattle are placed upon pastures which have been infected 

 .some time before with ticks, the disease will appear so much sooner 

 because the young ticks may be already hatched and attack the cattle 

 at ouee. It will be evident, theretore, that the length of time elapsing 

 bet ween the <ujpoliit of native cattle on infected fields and the appear- 

 ance of the disease will depend on the date of original infection and on 

 the weather, whether cold or hot. When native cat tie are placed upon 

 fields on which young ticks are already present they will show the lever 

 iu thirteen to fifteen days if the season be hot. 



The fever appears before the ticks have matured. In fact they are 

 still small enough to be overlooked. In any case very careful search 

 should be made for them in those places upon which they prefer to 

 locate, the thighs, escutcheon, and udder. After the acnte stage of the 

 fever has passed by the ticks begin to swell np and show very plainly. 

 (Plate xi.iv. Figs. 6 ami 7.) 



Preretttion. So far as our experiments have gone they indicate that 



