61() SCABIES — SCAB — MANGE. 



while it is not at all improbable that an entirely new infection has 

 accidentally been introduced by birds or other animals. Certain 

 authors of high standing scout the idea that birds can introduce an 

 infection of scab, but there is no reason why birds should not do this, 

 and there are some reasons for believing that they do. It has been 

 noticed on the Experiment Station of the United States Bureau of Agri- 

 culture, for instance, that crows delight in perching on the backs of 

 scabby sheep and picking at the scab ; while so doing it is only natural 

 that small tags of wool would adhere to their feet, and thus scatter scab. 



Delafond's experiments show that psoroptic mange is most trouble- 

 some amongst thin, ill-nourished, weakly animals, whilst robust sheep 

 in good condition may be cured simply by attention to cleanliness and 

 abundant feeding. 



In America this disease causes extremely heavy pecuniary loss- 

 second only in importance to that produced by hog cholera. It has 

 also interfered very seriously with the export of American sheep. 



The course of the disease is affected by the time of year and sur- 

 rounding conditions. In autumn and winter, when sheep are in con- 

 tinual contact in a moist, warm atmosphere within the folds, the 

 disease makes rapid progress. Young, weak, closely-inbred animals, 

 and those with long, coarse wool, most quickly succumb. Unhealthy 

 surroundings, damp, and bad ventilation favour the disease. Pure or 

 mixed bred merino sheep suffer severel3^ In summer the animals are 

 generally shorn and live in the open, and the disease then usually 

 diminishes or may even be arrested. 



A study of the life history of the scab parasite is necessary in 

 order to determine several important jaoints of practical value, such 

 as the proper time for the second dipping, etc. 



The female mite lays about fifteen to twenty-four eggs on the 

 skin, or fastened to the wool near the skin ; a six-legged larva is 

 hatched ; these larvffi 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 writings of different authors, a fact which is probably to be ex- 

 plained by individual variation, and by the conditions under wJiich 

 the observations 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 pair- 

 ing 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 larvse moult and 

 the fourth pair of legs appears ; this fourth pair is always present 



