185 



reach parasites or their eggs which are protected by overlying crusts 

 or matted wool. Observation has also shown that in certain cases 

 after one immersion in a recognised dipping fluid, some adult parasites 

 may survive, owing to the inability of the fluid to penetrate the hard 

 crusts or to the presence of an excessive amount of fat. Even when 

 using slightly alkaline fluids, which have an increased softening and 

 penetrating effect, the second dipping should not be omitted. In 

 cases where a flock has been badly infected for a considerable time and 

 a number of sheep show crusts of old standing, even two dippings cannot 

 be relied on to effect eradication, and a third dipping ten or fourteen 

 days after the second greatly increases the chances of a complete cure. 

 In the case of sheep with heavy fleeces special care is also necessary 

 before the interval between the first two dips is reduced or a third 

 dipping adopted, when an arsenical dip is being employed, owing to the 

 increased risk of poisoning ; this is especially necessary when a 

 proprietary dip is being used. 



The length of time the mites and their eggs can retain their vitality 

 apart from the sheep is of importance, for on it depends the possibility 

 of clean sheep becoming infected by means other than by direct 

 contact with other infested individuals. In crusts and wool kept at 

 room temperature in large jars the mites began to die in three or four 

 days and usually all were dead in sixteen days after removal from the 

 sheep, though in one instance a female was alive on the twentieth day. 

 When kept moist at 98° F. in an incubator, the mites were very active 

 for two or three days and eggs present in the crusts hatched. A day 

 or two later the larvae and most of the older stages died, very few sur- 

 viving more than six or seven days. In an ice-box at 32° F. all stages 

 were dead in four days, while in a desiccator in an atmosphere free 

 from water, at room temperature, they did not live longer than six days. 

 The majority of the parasites left the crust soon after removal from 

 the sheep and endeavoured to escape from the jars. In the incubator 

 a good number of the ovigerous females laid eggs, and occasionally this 

 w^as observed to occur at air temperature during the summer, but in no 

 case at a longer interval than two days after removal from the sheep. 

 The mites do not appear to live any longer upon shed wool or crusts in 

 the open. When wool and adhering crusts were pulled from a sheep 

 immediately after death and allowed to remain in an empty pen, 

 living acari could still be found in the crusts ten days later, but were 

 all dead fifteen days after removal. 



Eggs kept at air temperature hatched after having been removed from 

 the sheep for periods up to eight days. Batches of eggs stored for ten 

 or more days all failed to hatch. Several batches were also placed on 

 moist sheep faeces and returned to a sheep after ten or more days, 

 but in no case did any of the eggs hatch. In an ice-box the eggs 

 retained their vitality for ten days, but after twelve days all failed to 

 hatch. It is thus no longer possible to regard the eggs of this mite as 

 being capable of maintaining their vitality for long periods apart from 

 sheep. The details of ten experiments dealing with this point are 

 tabulated. Investigations showed that the mites infesting goats and 

 rabbits could not cause scab in sheep, and that P. communis var. ovis 

 could not infect goats, rabbits and other animals. 



As in Great Britain, great variations in the prevalence of sheep scab 

 in the course of the year are observable in South Africa, though they 



