432 



DISEASES OF THE BLOOD. 



Etiology. Depressing and weakening influences of all kinds have 

 been blamed for producing the disease, but the general consensus of 

 opinion points in the direction of infection with micro-organisms carried 

 and introduced into the sheep's system by the common sheep tick or 

 " grass tick " {Ixodes reduvius). The following remarks on, and illus- 

 trations of, this parasite are from an article by Mr. Wheeler, of Alnwick 

 {Vetcrinanan, Vol. LXXIII., No. 867, p. 141). 



Life History of the Grass Tick. Sheep ticks (which must not be 

 confused with the sheep-ked, or keb, a wingless six-legged fly, universal 

 on sheep everywhere) are allied to the spiders. They pass through four 

 stages of existence: the egg — the six-legged larva — the eight-legged 

 pupa — and, finally, the eight-legged adult male or female. 



In each of the three stages of larva, pupa, and adult female, all 



Fig. 200. — Adult male of the grass 

 tick. Length, ^th of 1 inch. 



Fig. 201.— Adult female. Length, |th of 1 inch. 



species of ticks attack some " host " or animal, either beast, bird, or 

 reptile, to which they attach themselves by the " rostrum " or beak, and 

 become greatly distended by suction of the host's blood. When replete 

 they fall to the ground — if a larva or pupa, in order to undergo its 

 metamorphosis to the next stage of its existence, and afterwards seek a 

 fresh host ; if an adult female, to lay its eggs amongst herbage. The 

 adult male is not capable of distension by suction, though it equally 

 attaches itself to a host. 



After undergoing metamorphoses, grass ticks, with the exception of 

 males, are light in colour, soft and lethargic, and remain concealed for 

 some time while recovering strength before seeking a fresh host. 



Professor Neumann alludes to the fact that a fresh host is sought by 

 ticks three several times during their existence. 



