28 VETERINARY PARASITOLOGY 
vitality on a cast-off scab for a period ranging from 
one to two months, providing that the scab is kept in 
a moist situation. 
The parasite is quite visible to the naked eve, es- 
pecially when viewed against a dark background, and 
if exposed to the rays of the sun. 
The length of the adult male parasite is about $ milli- 
metre, or +; inch, while the female is rather larger. 
Crows and other birds which perch on the backs of 
sheep have often been accused of spreading the disease, 
and there is no reason to believe that this theory is 
incorrect. 
Symptoms.—Psoroptic mange may attack sheep of 
any age, but the members of the flock which usually 
show the earliest and most serious symptoms are lambs 
and yearling lambs, with fine, supple skins. 
Some breeds of sheep seem particularly predisposed 
to scab, though lack of condition plays a far greater 
part. 
Unlike sarcoptic mange, it is the parts most thickly 
covered with wool that become attacked, such as the 
neck, withers, back, and sides. The first symptom 
noticeable is intense pruritus, and the animals constantly 
scratch and rub the affected areas. The fleece becomes 
matted, much stained from scratching, and in many 
places it is torn out in the attempts of the animal to 
allay the itching. These symptoms are especially 
evident if the flock is driven, when the sheep so affected 
constantly halt and rub the bare patches. If one of 
these is gently scratched with the finger, the animal 
evinces signs of great satisfaction by nodding the head, 
smacking the lips, and wriggling the hinder parts of the 
body. 
If the fleece is parted at this stage, the underlying skin 
will be seen to be intensely inflamed and covered with 
numerous yellowish-white papules. These go through 
