418 VETEEINAEY HYGIENE 



Hasmoglobinuria, the adult female passes the organism 

 mto the egg, and the larvse resulting from this infected 

 egg are alone capable of producing the disease. 



In Pyroplasmosis of the dog the infection is passed 

 through the egg, larvte, and nymph, but only the adult can 

 communicate the disease, the larva and nymph being 

 innocuous. 



In East Coast Fever and Heart-Water another variation 

 is observed. Infection is taken up either in the larva or 

 nymph stage, and communicated by either the nymph or 

 adult. Unlike the above-named diseases, the virus cannot 

 pass through the egg. 



Ticks are essentially blood-sucking parasites, they live 

 on blood and that only. They are capable of taking in a 

 relatively enormous supply so that, for example, a seed or 

 larval tick which before feeding is almost too small to see, 

 after feeding is swollen and very evident. Adult ticks may 

 gorge themselves to such an extent as to measure an inch 

 in length, three quarters in width, and weigh one-tenth 

 of an ounce. 



The drawings shown in Pig. 175 are to scale, and give a 

 good idea of the normal size of the insect at different 

 stages of development, both in a fasting and gorged 

 condition. Long periods of fasting, weeks and months, 

 may be experienced without the tick dying. 



A full-grown tick is not at once developed from an egg, 

 but passes through four stages of development, and these 

 are interesting from a pathogenic point ; the life stages 

 are : 



1. The Egg. 



2. The Larva or seed-tick, possessing 6 legs. 



3. The Nymph, possessing 8 legs. 



4. The Adult or sexual tick. 



It is the larvae which first attack the animal, having 

 previously crawled up stems of grasses or bushes, and 

 there waited patiently for weeks or even months for a host 

 to appear. In some species of ticks the whole of the sub- 



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