Fig. 253.—Left-hand figure shows adult male parasite of common sheep scab, dorsal view; right-hand figure, ventral view of same; top figure shows head of female; bottom figure, leg of female. All greatly enlarged. (Salmon and Styles, Annual Report, U.S.A. Bureau of Animal Industry, 1897.)
Experience has shown that in some cases apparently healthy sheep have become infected in places where no sheep have been kept for four, eight, twelve, or even twenty-four months. The conditions underlying this infection are not thoroughly understood. Possibly some of the eggs have retained their vitality a long time and then hatched out; possibly the vitality of the fecundated female has also played a rôle; 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 Agriculture, 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 troublesome 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 surrounding conditions. In autumn and winter, when sheep are in continual 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 severely. 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 points 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 larvæ 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 explained by individual variation, and by the conditions under which 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 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 larvæ moult and the fourth pair of legs appears; this fourth pair is 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 larvæ may moult when three to four days old, and become mature; after pairing, a second moult takes place, lasting four to five days; a third moult follows immediately, then eggs are laid and the adults die; in some cases there is a fourth moult, but apparently without any further production of eggs. Accepting Gerlach’s estimate of fifteen days as an average for each generation of ten females and five males, in three months time the sixth generation would appear and consist of about 1,000,000 females and 500,000 males.
Fig. 254.—Adult female parasite of common sheep scab. (Both greatly enlarged.) (Salmon and Stiles, loc. cit.)
Several practical lessons are to be drawn from these figures: Firstly, 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; secondly, 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 develops; thirdly, 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; fourthly, as dipping is not certain to kill the eggs, the sheep should be dipped a second time, the time being selected between 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 are required for the entire cycle, the second dipping should take place after the seventh day, but before the fourteenth day; allowing for individual variation and variation of conditions, the tenth, eleventh, or twelfth day will be the best time to repeat the dipping.