The mites are closely related to the ticks, and although none of them has yet been shown to be responsible for the spread of any disease their habits are such that it would be entirely possible for some to transmit certain diseases from one host to another, from animal to animal, from animal to man, or from man to man. A number of these mites produce certain serious diseases among various domestic animals and a few are responsible for certain diseases of men.
Face-mites. Living in the sweat-glands at the roots of hairs and in diseased follicles in the skin of man and some domestic animals are curious little parasites that look as much like worms as mites ([Fig. 19]). Such diseased follicles become filled with fatty matter, the upper end becomes hard and black and in man are known as blackheads. If one of these blackheads is forced out and the fatty substance dissolved with ether the mites may be found in all stages of development. The young have six legs, the adult eight. The body is elongated and transversely wrinkled. In man they are usually found about the nose and chin and neck where they do no particular harm except to mar the appearance of the host and to indicate that his skin has not had the care it should have. Very recently certain investigators have found that the lepræ bacilli are often closely associated with these face mites and believe that they may possibly aid in the dissemination of leprosy. It is also thought that they may sometimes be the cause of cancer, but as yet these theories have not been proven by any conclusive experiment.
In dogs and cats these same or very similar parasites cause great suffering. In bad cases the hair falls out and the skin becomes scabby. Horses, cattle and sheep are also attacked. The disease caused by these mites on domestic animals is not usually considered curable except in its very early stages when salves or ointments may help some.
Itch-mites. "As slow as the seven-years' itch" is an expression, the meaning of which many could appreciate from personal experience, for it certainly seemed to take no end of time to get rid of the itch once it was contracted. Just why seven years should have been set for the limit of the disease is not clear, for if the little roundish mites that cause the disease live for seven years on a host they are not going to move out voluntarily even if their seven-year lease has expired.
Fig. 18—Ornithodorus moubata, the Tick that Transmits Relapsing Fever. From Boyce's "Mosquito or Man."
Fig. 19—The follicle mite (Demodex folliculorum). (After Murray.)