470 DISEASES OF THE SKIN 



lyre or horseshoe head, provided with a broad, scissors-like masticatory 

 apparatus, a mobile anterior trunk, arul three articulated tentacles 

 (Fig. IGy), It has an oblong or worm-shaped, wrinkled abdomen, with 

 eight triple-jointed, clawy, thick, short feet attached to the thorax. 

 The elongated oval larvae have only six legs (Fig. 168). The eggs are 

 spindle-shaped, and the larvae escape from the eggs, almost fully devel- 

 oped. The exception is that the newly born larvse have only six feet 

 (Fig. 168). _ 



These parasites are found in the hair-follicles and sebaceous glands, 

 their head toward the bottom of the sac, and the caudal end toward the 

 external opening (Fig. 170). Their number may be large in a sebaceous 

 gland, even as many as 200 have been found there. They generally average 

 from twenty to thirty. The larvse and eggs lie beside the parasite. The 

 irritation of the parasite, causes the follicles and sebaceous glands to become 

 dilated and the subsequent atrophy of the hair papilla causes falling out and 

 permanent loss of hair and where there is a great number of parasites 

 present, pus is produced and a purulent inflammation of the perifollicular 

 and periglandular tissue ensues, with the formation of pustules. The re- 

 lapses, after the disease appears cured, may be due to temporarily encysted 

 units. It may be that these parasites may have been in some inac- 

 cessible place, such as the eye, ear, or prepuce. In very severe cases 

 with numerous pustules of the sebaceous glands, death may occur 

 from general septic infection. The secbaeous glands are also de- 

 stroyed, causing aeneous pustules. The demodex acne shows itself 

 in certain preferred parts of the body, especially the head, throat, 

 neck, and paws, but it may extend over the entire body. It is not very 

 easily transmitted from one animal to another, as has been proven by the 

 attempts made by Weiss, Martemucci, and others, who were unsuccess- 

 ful. The writer had four dogs, two adults and two pups, and kept them 

 for months with a bitch that was badly affected with acari, but failed 

 to get any signs of transmission. When the animals have been experi- 

 mentally inoculated Haubner reported a case in which twenty-four hours 

 after inoculation there were all the marks of the disease and the sac 

 contained eggs and larvse. A second inoculation cured itself spontane- 

 ously. Other observers have had similar experiences. It seems that to 

 propagate the disease the animal must have a certain predisposition, the 

 nature of which has not been satisfactorily explained. Young dogs when 

 exposed to infection seem to take it more readily than older dogs, but 

 whether it is congenital is a question. The natvu-al method of contagion 

 is the contact of healthy dogs with diseased ones, but there is no doubt 

 that some individuals ai-e immune. 



Clinical Symptoms and Course. — These are distinguished by a pustu- 

 lar and sc^uamous form of eruption, but both forms are apt to be united 



