334 PRINCIPLES OF VETERINARY SCIENCE 



making a close examination of the skin little areas of redness and 

 scabs will be seen. The mites may be seen by examining skin 

 scrapings with a hand lens, or by placing this material in a warm, 

 dry bottle. Later, after the mites have increased in number, 

 scabs appear over a large part of the body, and the wool drops 

 out leaving large bare areas. Emaciation and weakness soon 

 follow, and unless assistance is given the sheep they will die in 

 two or three months. 



No time should be lost in dipping the whole flock as soon as a 



case of scab is found. The lime-sulphur and the tobacco juice 



dips give best results. The dipping solution should be warmed 



to the temperature of the body and the sheep dipped only on 



•e***^^. a warm day. To insure success the 



sheep should be redipped after ten days. 



The Texas fever tick (Margaropus 



annulatus) is a parasite of cattle that is 



indirectly responsible for the destructive 



disease known as Texas or tick fever. 



The life history of the Texas fever tick 



Fig. 85. — Mature Texas ■ • , ,• m u j; n j? 



fever tick with pile of freshly is very interesting. The full-grown fe- 

 laid eggs. male tick is reddish-brown in color. 



When she becomes engorged with her 

 host's blood, she drops to the ground. In from three to five days 

 she has secluded herself and begins laying eggs. The eggs are 

 ovid in shape, brown in color, covered with a varnish-like sub- 

 stance, and often number 4000 (Fig. 85). In warm weather 

 and under favorable conditions the eggs hatch in about three 

 weeks. The progeny of the eggs are so small as to be hardly 

 visible. They have only three pairs of legs, are asexual, and re- 

 semble little spiders. They crawl over the ground or upon grass 

 and twigs to wait until a passing animal comes along. As soon 

 as they find a host they attach themselves and begin sucking 

 blood. After a week they moult twice when they are provided 

 with four pairs of legs and are sexually mature Texas fever 

 ticks. 



These little spider-like forms are really larvae. They carry 

 the protozoa (Piroplasma bigeminum) which cause Texas fever. 

 The protozoa enter the red blood corpuscles of cattle which break 

 up and liberate their coloring matter or hemoglobin. The adult 

 ticks do not carry the protozoa from one animal to another, but 



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