HAIR-FOLLICLE MITE 



347 



The wormlike appearance of the adult mites is due to the great 

 elongation of the abdomen which is marked by numerous fine 

 lines running around it. The beak is short and broad, and the four 

 pairs of legs, all similar, are short, stumpy, three-jointed append- 

 ages. The female mites are .35 to .40 mm. 

 long (about ^^ of an inch), while the males are 

 a little smaller. 



The nmltiplication of these mites is slow. 

 The eggs hatch into tiny six-legged larvae in 

 which the legs are mere tubercles. It requires 

 four moults to bring the larvse to sexual 

 maturity. 



In most cases these parasites cause no incon- 

 venience whatever and their presence is not 

 even suspected. In Europe a large proportion 

 of people are said to be infected, but in Amer- 

 ica, according to Riley and Johannsen, there is 

 reason for believing that the infection is far 

 less common than is usually supposed. Since 

 generation after generation may be produced 

 on a single host the infection is potentially 

 indefinite in its duration. When the mites 

 become numerous in the hair follicles or 

 sebaceous glands they sometimes cause " black- ^^^'^ 

 heads " by causing a fatty accumulation to be produced, but 

 they are not the only or even the usual cause of "black-heads." 

 The skin disease known as " acne " has also been attributed to 

 these mites, but probably erroneously. Follicle mites have been 

 suspected also of spreading leprosy. 



The method of transmission of the mites to another host is 

 not definitely known but it is probable that the adults wander 

 on the surface of the skin at times, and may then be transmitted 

 by direct contact or by towels, as are itch mites. In dogs, 

 where the follicle mite, possibly a different species, causes a very 

 severe and often fatal form of mange, transmission from dog 

 to dog takes place in a very irregular manner, and there are 

 frequent instances cited of infected dogs associating for a long 

 time with uninfected ones without spreading the disease. Ex- 

 periments with transmission of the canine follicle mite to man 

 have invariably failed. Little is known about treatment of 



Fig. 144. Hair- 

 follicle mite, Demo- 

 dex folliculorum. X 

 200. (After Meg- 



