The present study was undertaken to determine the incidence of infestation with Demodex folliculorum, a skin mite parasitizing man, and to investigate those factors which influence its occurrence. These factors were the personal hygiene, sex, age and skin condition of the human host. A detailed view of the habitat of this parasite is also included so that the controversy surrounding its role as a possible pathogen or vector of disease could be explored in light of the current knowledge. A number of sites of infestation were sampled by a number of different techniques. The primary sample site was the nasal area of the face which was examined by slit lamp magnification after chemical application to partially expose the mites. A secondary site involved the eyelash follicles from which the lashes were epilated and examined for mites under the microscope. The 441 subjects examined in the nasal area were tabulated by age, sex and skin condition and the incidence of infestation was computed for each of five age groups. There was a definite increase in incidence of Demodex with the advancing age of the host and the normal and oily skin conditions appeared to be much more involved than the dry skin condition. A similar increase in incidence with advancing age was apparent in the 143 subjects whose eyelashes were examined. The results from both areas exhibited a statistical correlation between the age and skin condition of the human host and the presence of the mites. The explanation of the increase in incidence with the advancing age of the human host is based on the development and secretory rate of the sebaceous glands of the human host. Past studies have shown that Demodex lives in and around the sebaceous glands of the skin and feeds on sebaceous material and cellular debris. The face appears to be the habitat of optimum occurrence which corresponds to the distribution of sebaceous follicles and glands whose size and rate of secretion are greater that those over most of the body. The role of Demodex in the formation of ingrown hairs on the neck is discussed as a possible, new clinical entity. The influence of the host’s personal hygienic habits on demodectic infestation remains rather obscure. Although the accumulation of sebaceous material would tend to favor the maintenance of mites, the reservoirs of the sebaceous follicles of the face in which the mites are found are not affected by normal soap and water cleansing. Inhabitation of the eyelash follicles may be enhanced by ineffective cleansing, but more study is needed to determine the validity of this contention. The finding of selective sites of infestation on the human host led to speculation on the possibility of physiological races of Demodex. Although it is difficult to determine the validity of reports by earlier workers, there appears to be a decreasing trend in the incidence of Demodex folliculorum since its discovery over a century ago.

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