CHAPTER IV 

 ACCIDENTAL OR FACULTATIVE PARASITES 



In addition to the many species of Arthropods which are normally 

 parasitic on man and animals, there is a considerable number of those 

 which may be classed as accidental or facultative parasites. 



Accidental or facultative parasites are species which are normally 

 free-living, but which are able to exist as parasites when accidentally 

 introduced into the body of man or other animal. A wide range of 

 forms is included under this grouping. 



ACARINA 



A considerable number of mites have been reported as accidental 

 or even normal, endoparasites of man, but the authentic cases are 

 comparatively few. 



In considering such reports it is well to keep in mind von Siebold's 

 warning that in view of the universal distribution of mites one should 

 be on his guard. In vessels in which animal and other organic 

 fluids and moist substances gradually dry out, mites are very abund- 

 antly found. If such vessels are used without very careful prelimi- 

 nary cleaning, for the reception of evacuations of the sick, or for the 

 reception of parts removed from the body, such things may be readily 

 contaminated by mites, which have no other relation whatever to 

 them. 



Nevertheless, there is no doubt but that certain mites, normally 

 free-living, have occurred as accidental parasites of man. Of these 

 the most commonly met with is Tyroglyphus siro, the cheese-mite. 



Tyroglyphus siro is a small mite of a whitish color. The male 

 measures about 500^1 long by 250^ wide, the female slightly larger. 

 They live in cheese of almost any kind, especially such as is a little 

 decayed. "The individuals gather together in winter in groups or 

 heaps in the hollows and chinks of the cheese and there remain 

 motionless. As soon as the temperature rises a little, they gnaw 

 away at the cheese and reduce it to a powder. The powder is com- 

 posed of excrement having the appearance of little grayish microscopic 

 balls; eggs, old and new, cracked and empty; larvae, nymphs, and 

 perfect mites, cast skins and fragments of cheese, to which must be 

 added numerous spores of microscopic fungi." Murray. 



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