Fig. 204.—Headless female.
In all stages grass ticks abstain from all food except when on a host, and they are endowed with extraordinary powers of fasting until a host is found.
Ticks soon die of drought where there is no good harbourage among rank vegetation.
Judging from analogy, it is probable—
That the bacillus can only be obtained from a diseased sheep, and inserted by the tick into another sheep.
That ticks convey the bacillus through their eggs to their offspring, as well as retain it through their metamorphoses.
That there is no danger in removing sheep from foul ground to cultivated lowlands, but that the disease is easily imported from one hill farm to another.
Strong and fat animals are nearly as susceptible to attack as weakly ones.
If the land is once free of disease, it can only be re-imported by diseased sheep, or ticks taken from them.