Background:
Observation of the arachnid, Demodex
folliculorum, has been reported since 1840. This hair follicle mite is the
only metazoan organism commonly found in the pilosebaceous components of the
eyelid of humans. Coston "opened the eyes" of ophthalmologists when he described
22 patients with demodectic eyelid signs and symptoms.
Pathophysiology:
D folliculorum (all stages) is
found in small hair follicles and eyelash hair follicles. In all forms, immature
and adult, it consumes epithelial cells, produces follicular distention and
hyperplasia, and increases keratinization leading (in eyelashes) to cuffing,
which consists of keratin and lipid moieties. Demodex brevis (all
stages) is present in the eyelash sebaceous glands, small hair sebaceous glands,
and lobules of the meibomian glands. Adults and immature forms consume the gland
cells in all of these loci and, when infestations are heavy, can affect the
formation of the superficial lipid layer of the tear film coacervate. Demodectic
mites produce histologically observable tissue and inflammatory changes,
epithelial hyperplasia, and follicular plugging.
Infestation of the eyelash hair follicle results in easier epilation and more
brittle cilia. These mites also serve as vectors of infective elements and
interrupt tissue integrity. They have been implicated in meibomian granulomas
and are associated with certain dermatologic changes. All reported histologic
sections of lid follicles infested with D folliculorum show distention
and thickening. Coston claims that less than one half of the specimens he
observed showed perifollicular lymphocytic infiltration.
Follicular inflammation produces edema and results in easier epilation of the
eyelashes. It also affects cilia construction, and lashes are observed to be
more brittle in the presence of demodicosis. Madarosis (loss of lashes) is
associated with abundant mites, the loss of eyelashes as a result of
intercellular edema in the hair shaft, and loss of hair resiliency. Although
epithelial hyperplasia associated with follicular plugging is often encountered,
dermal changes seldom extend beyond the perifollicular epidermal area. Once
believed to be mite excreta, this plugging is now known to be epithelial
hyperplasia with interspersed layers of lipid. The formation of a collar of
tissue around the base of the lashes is observed clinically. This occurs
significantly more often in follicles infected with D folliculorum. The
epithelial hyperplasia is hypothesized to be most likely a product of the
abrasive action of the mite's claws.
Accumulation of waste material of the follicle mite may occur in affected
follicles or sebaceous glands. Electron micrographs of the mite surface and
feces show bacterial, viral, and rickettsial elements. Specific reports have
revealed that both species pierce epithelial cells and consume cytoplasm. Only
D brevis has been observed with channels burrowed to the germinal
epithelium in the sebaceous glands.
Demodex species-induced pathologic changes have been implicated in
dry eye conditions. When follicular plugging involves the meibomian gland (D
brevis) or the gland of Zeiss (D folliculorum or D brevis),
reduction of the superficial lipid layer of the tear film occurs. The effect of
D brevis on the meibomian structure has been implicated in chalazion
formation. Chalazia are granulomatous inflammation of the meibomian glands, made
of an organized core of epithelioid cells and histocytes surrounded by
fibroblasts, lymphocytes, and plasma cells. These defense cells encircle
particles too large for normal macrophages to engulf. D brevis has been
observed in the center of these meibomian granulomas. Lid infestation by the
Demodex species may or may not accompany dermatologic changes of the nose,
the cheek, or the forehead.
D folliculorum has been suggested as a factor in pityriasis
folliculorum. This dermal inflammation manifests itself as a diffuse erythema of
the affected areas; scaly, dry skin; and, in certain cases, rosacealike lesions.
The dry skin cycle described by Ayres is initiated when the demodectic mite
plugs the follicle and reduces the sebaceous outflow, which leads to scaling as
well as rough and dry skin texture. Sebaceous outflow is further reduced when
patients inadvertently decide to apply facial cream. The mite flourishes in this
environment of oily additives, leading to an increase in the population of the
mites and a continuation of the dry skin cycle.
Frequency:
Mortality/Morbidity:
Madarosis (loss of lashes) may result
from untreated demodicosis.
Race: No racial predilection has been observed.
Sex: Infestation by these parasites is equal in males and
females. Infestation is correlated to the number of sebaceous glands but not to
the density of the hair follicles.
Age: Post reported that D folliculorum was observed
in 84% of the sample population with a mean age of 61 years and in 100% of those
older than 70 years.
