434 



DISEASES OF THE BLOOD. 



resemblance between louping-ill, Texas fever, tsetse fly disease, surra, 

 heart-water, yellow fever, and malaria. 



In the article previously referred to he summarises his conclusions 

 as follows : — 



One species only of tick, Ixodes rediiriiis, commonly known as the 

 grass tick, has been found to carry the louping-ill bacillus to the sheep. 

 It is easily recognised by the red body of the young females, the legs, 

 shield, etc., being dark brown. 



It lays its eggs, and undergoes its metamorphoses, in coarse 

 herbage, and after each change seeks a fresh " host " on which to 

 distend itself to a large size by suction of blood. 



In all stages grass ticks abstain from all food except when on a host. 



Fig. 203. — Female, tinder size. 



Fig. 204. — Headless female. 



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. 



