DISEASES OF SHEEP 531 



against which scabby sheep have rubbed. Even after observing the 

 precautions here given it is not possible to absolutely guarantee that 

 there will be no reinfection, but the probabilities are against it. 



Life History of the Scab Parasite. A study of the life history 

 of the scab parasite is necessary in order to determine several im- 

 portant points of practical value, such as the proper time for the 

 second dipping, etc. 



The female mite lays about 15 to 24 eggs on the skin, or fast- 

 ened to the wool near the skin ; a six-legged larva is hatched ; these 

 Iarva3 cast their skin and become mature; the mites pair and the 

 females lay their eggs, after which they die. The exact number of 

 days required for each stage varies somewhat, according to the writ- 

 ings of different authors, a fact which is probably to be explained by 

 individual variation, and by the conditions under which the obser- 

 vations and experiments were made. Thus Gerlach, in his well- 

 known work (1857), estimates about fourteen to fifteen days as the 

 period required for a generation of mites from the time of pairing 

 to the maturity of the next generation. He divides this time as 

 follows : Under ordinary conditions the eggs hatch in three to four 

 days, although two authors allow ten to eleven days for the egg 

 stage; three or four days after birth the six-legged larvae molt and 

 the fourth pair of legs appear; this fourth pair are always present 

 when the mites are two-thirds the size of the adults ; when seven to 

 eight days old the mites are mature and ready to pair; several (three 

 or four) days are allowed for pairing; another generation of eggs 

 may be laid fourteen to fifteen days after the laying of the first 

 generation of eggs. Without going into all of the other observations 

 on these points, it may be remarked that the eggs may not hatch for 

 six or seven days ; the six-legged Iarva3 may molt when three to four 

 days old, and become mature; after pairing a second molt takes 

 place, lasting four to five days; a third molt follows immediately, 

 then eggs are laid and the adults die ; in some cases there is a fourth 

 molt, but apparently without any further production of. eggs. Ac- 

 cepting Gerlach's estimate of fifteen days as an average for each 

 generation of 10 females and 5 males, in three months' time the 

 sixth generation would appear and consist of about 1,000,000 fe- 

 males and 500,000 males. 



Several practical lessons are to be drawn from these figures: 

 First, it is seen that the parasites increase very rapidly, so that if 

 scab is discovered in a flock, the diseased sheep should immediately 

 be isolated; second, if new sheep are placed in a flock, they should 

 either first be dipped, as a precautionary measure, or they should 

 at least be kept separate for several weeks to see whether scab de- 

 velops; third, since the chances for infection are very great, the 

 entire flock should be treated, even in case scab is found only in one 

 or two animals ; fourth, as dipping is not certain to kill the eggs, the 

 sheep should be dipped a second time, the time being selected be- 

 tween the moment of the hatching of eggs and the moment the next 

 generation of eggs is laid. As eggs may hatch between three and 

 seven, possibly ten or eleven, days, and as fourteen to fifteen days 



