The importance of Smith and Kilbourne s work can hardly be overestimated. It was the first demonstration that blood-sucking insects and ticks transmitted disease of any kind. Their findings led directly to the discovery that mosquitoes spread devastating human diseases such as malaria, yellow fever, and encephalitis. Later research showed why Longhorns from south Texas carried the disease without ill effects. Cattle first contracting the disease as calves do not become sick and develop an immunity that persists for as long as the Babesia remain in their blood.
Cattle remaining in the Texas fever area are continually reinfected, so they remain resistant.
Even if they leave the area, their immunity persists for several years, but they are carriers of the disease. In contrast, animals first infected as adults often die. Cattle anywhere north of south Texas had no chance to develop resistance to the disease as calves, and the Longhorns on the drives carried not only Texas fever but also the ticks necessary to transmit the disease. However, the cattle fever tick can be killed by even moderately low temperatures and cannot survive the winter more than a few hundred miles north of the mouth of the Rio Grande. So Texas cattle wintered in the North were no longer dangerous to native cattle.