DISEASES OF CATTLE 327 



fever is concerned, and, in fact, to all external parasites. There- 

 fore, it is just as important to eradicate the cattle ticks for reasons 

 other than those associated with Texas fever as it is to exterminate 

 lice, fleas, and other vermin. Furthermore, cattle ticks, aside from, 

 the losses sustained by their purely parasitic effects, are the greatest 

 menace to the profitable raising and feeding of cattle in the South, 

 because they are an obstacle to cattle traffic between the infected and 

 noninfected districts. 



The So-called Period of Incubation. After the young ticks 

 have attached themselves to cattle the fever appears about ten days 

 thereafter, in 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 south- 

 ern cattle have passed, the time when the disease appears varies with- 

 in wide limits. When the animals have been put upon pastures im- 

 mediately 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 southern 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 infection of pastures with ticks has taken place 

 early in the season, or when this is cold, the period will be much 

 longer, because it takes longer for the eggs to hatch. 



If native cattle are placed upon pastures which have been in- 

 fected 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 once. It will be evident, therefore, that the length of 

 time elapsing between the exposure of native cattle on infected fields 

 and the appearance of the disease will depend on the date of original 

 infection, and on the weather, whether cold or hot. When native 

 cattle are placed upon fields on which young ticks are already pres- 

 ent, they will show the fever in 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 pre- 

 fer to locate the thighs, escutcheon, and udder. After the acute 

 stage of the fever has passed by, the ticks begin to swell up and show 

 very plainly. 



Prevention. It is generally accepted that if southern cattle are 

 entirely free from that species of tick known as Margaropus (Boo- 

 philus) annulatus, they can be allowed to mingle with the most sus- 

 ceptible animals without danger. Furthermore, it has been learned 

 from the study of the life history of the cattle tick and from the fact 

 that this tick infests pastures only transiently, never permanently, 



