Development of Ticks. 771 



infectious ticks were introduced into the stables with the grass 

 or foliage from infected pastures. 



The introduction of the disease into localities which had 

 been free from the infection, may be brought about by infected 

 animals, and the more so as piroplasma may be present in 

 the blood of cattle years after their recovery from the disease. 

 In such cases the disease may be artificially produced with 

 the blood of such apparently healthy cattle. If ticks which are 

 suitable for the transmission of the virus are present in these 

 localities, they suck blood from the cattle introduced into that 

 locality, and pass the piroplasma to their progeny, which then 

 infect healthy animals. Affected or healthy cattle may carry 

 ticks about on their bodies, and these will continue to propa- 

 gate in the new locality, provided the requirements for their 

 development are favorable, where they may then infect fresh 

 animals. In both cases the pasture, formerly free of the in- 

 fection, becomes a new source of infection, in which the disease 

 may possibly recur from year to year. 



The exteusicn of the disease was strikingly observed in America, where in 

 the 60s and 70s of the last century it was introduced by cattle driven from Texas 

 to the north, to the states of Indiana and Illinois, whence it extended later in 

 a similar manner almost through the entire territory of the United States up to 

 Lake Michigan. After the trailing of the cattle from the south, as a rule in 

 from 30-50 days, which period corresponds to the cycle of development of the 

 Boophilus tick," the native cattle became affected with severe symptoms (Eeverley, 

 Smith & Kilborne). The disease was introduced into Australia by American cattle, 

 with the Boophilus ticks, which had been unknown there up to that time, and it 

 causes- there great loss from year to year. It also spread recently in Finland to 

 territories which had heretofore been free from the affection (Krogius & Hellen). 



Although the disease develops exclusively through infec- 

 lion by ticks, certain accessory conditions have an influence 

 on its appearance, and more so on its course. Thus as a rule 

 very young animals show a greater resistance against the in- 

 fection than older cattle. Poorly nourished, as well as gener- 

 ally weakened animals usually become affected with severe 

 manifestations; unfavorable weather conditions, especially 

 damp, cold weather, also exert an unfavorable influence on the 

 course of the disease. This explains the experience that the 

 affection is often observed in a severe form in the damp, cold 

 spring periods, and especially among improperly wintered ani- 

 mals of poor owners (Krageriid observed it repeatedly in such 

 a form in cows 12 to 14 days after parturition). 



Native cattle raised in affected territories usually become 

 affected under much milder sjmiptoms than animals brought 

 from uninfected localities. The greater resistance of the for- 

 mer may be explained by the supposition that they became 

 infected while young, and thereby acquired for their later lives 

 a relative immunity, which was still more increased by infec- 

 tions developing in the meanwhile. Such animals, _ however, 

 may also carry the virus in their blood in spite of their healthy 

 appearance, and this may sometimes multiply rapidly on ac- 



