Very significant was the fact that the infection was not communicated directly from the Southern to Northern cattle but that the ground over which the former passed was infected by them, and that the infection was transmitted thence to susceptible cattle after a period of not less than thirty days had elapsed.
Of course a disease as striking as this, and which caused such enormous losses of cattle in the region invaded was fruitful in theories concerning its causation. The most widespread was the belief that pastures were infected by the saliva, urine, or manure of Southern cattle. There were not wanting keen observers who suggested that the disease was caused by ticks, but little weight was given to their view.
Various workers had described bacteria which they had isolated from the organs of the diseased animals, but their findings could not be verified. In 1889, Smith and Kilbourne discovered a minute, pear-shaped organism ([fig. 138]) in the red blood corpuscles of a cow which had succumbed to Texas fever. On account of their shape they were given the generic name Pyrososma and because they were usually found two in a corpuscle, the specific name, bigeminum. It is now generally accepted that the parasite is the same which Babes had observed the year before in Roumanian cattle suffering from hæmoglobinuria, and should be known as Babesia bovis (Babes).
By a series of perfectly conclusive experiments carried on near Washington, D.C., Smith and Kilbourne showed that this organism was carried from Southern cattle to non-immune animals by the so-called Southern cattle tick, Boophilus annulatus (= Margaropus annulatus) ([fig. 139]).
Of fourteen head of native cattle placed in a field with tick-infested Northern cattle all but two contracted the disease. This experiment was repeated with similar results. Four head of native cattle kept in a plot with three North Carolina cattle which had been carefully freed from ticks remained healthy. A second experiment the same year gave similar results.
Still more conclusive was the experiment showing that fields which had not been entered by Southern cattle but which had been infected by mature ticks taken from such animals would produce Texas fever in native cattle. On September 13, 1889, several thousand ticks collected from cattle in North Carolina three and four days before, were scattered in a small field near Washington. Three out of four native animals placed in this field contracted the disease. The fourth animal was not examined as to its blood but it showed no external symptoms of the disease.