486 DISEASES OF CATTLE. 



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

 f)eriod may be a little longer. The actual period of incubation may 

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

 the blood vessels of healthy cattle the fever may appear within five 

 days. Wlien cattle graze upon pastures over which southern cattle 

 have passed, the time when the disease appears varies within wide 

 limits. TVlien the animals have been put upon pastures immediately 

 after southern cattle have infected them with ticks, it may take from 

 30 to 60 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 must be hatched 

 before any ticks can get upon native cattle. The shortest period is 

 thus not less than 30 days if we include 10 days for the period of 

 incubation after the young ticks have attached themselves to native 

 cattle. "V\Tien the infection of pastures with ticks has taken place 

 early in the season, or when it is cold, the period is much longer, 

 because it takes longer for the eggs to hatch. 



If native cattle are placed upon pastures which have been infected 

 with ticks some time before, 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 between 

 the exposure of native cattle on infected fields and the appearance 

 of the disease depends 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 present, they will show the 

 fever in 13 to 15 days if the season is 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 f-er them in those places which they prefer — the 

 thighs, escutcheon, and udder. After the acute stage of the fever has 

 passed the ticks begin to swell up and show very plainly. (PI. 

 XL VI, figs. 6 and 7.) 



Prevention. — It is generally accepted that if southern cattle are 

 entirely free from that species of tick known as Margaropus annu- 

 latus they can be allowed to mingle with the most susceptible animals 

 without danger. Furthermore, it has been learned fi-om the study 

 of the life history of the cattle tick and by observation that this tick 

 infests pastures only transiently, never permanently, and will not 

 mature except upon cattle or equines, that its extermination is 

 possible, and that the disease it causes may be prevented. Therefore 

 the various methods with these results in view should be directed 

 toward the destruction of ticks on cattle as well as their eradication 

 from the pastures. 



