Acari multiply rapidly and live their entire life on the body of the host.
A new generation is produced in about fifteen days. Gerlach has estimated
the natural increase in three months at 1,000,000 females and 500,000
males. Scab and mange are exceedingly contagious diseases.

[Illustration: FIG. 68.--Sheep scab.]

_Common sheep scab_ is caused by that specie of mites known as the
_Psoroptes communis var. ovis_ (Fig. 67). Any part of the body may become
affected. The bites of the mites greatly irritate the skin, and the animal
scratches, bites and rubs the part in its effort to relieve the intense
itching. The skin becomes inflamed and scabby, the wool is pulled and
rubbed out, and the fleece becomes ragged (Fig. 68). By pulling wool out of
the newly infested area, or collecting skin scrapings and placing this
material on black paper in a sunny, warm place, the mites may be seen
crawling over the paper. This method of diagnosis should be resorted to in
all suspicious cases of skin disease, and before the disease has developed
to any great extent.

The mite that most commonly causes _mange in cattle_ is the _Psoroptes
communis var. bovis_. It may invade the skin in the different regions of
the body, but it is in the regions of the tail and thighs that the first
evidence of the mange is noticed. The animal rubs, scratches, and licks the
part. The itching is intense. The hair over the part is lost and the skin
appears inflamed, thickened, moist, or covered with white crusts. Cracks
and sores may form in the skin. The examination of scrapings from the
inflamed skin should be practised in order to confirm the diagnosis.

_Mange in horses_ may be caused by either psoroptic or sarcoptic mites.
_Psoroptes communis var. equi_ seems to be the more common parasite. The