Dry Eye Dx and Tx
Mites: Live and Let Live
BY KATHERINE M. MASTROTA, MS, OD, FAAO
I was delighted to lecture at the Westchester, Rockland, Putnam County Optometric Society. The topic was the few millimeters of anatomy known as the lid margin. Yes, two hours of continuing education devoted exclusively to this narrow, yet exquisitely important, strip of anatomy. Of course, it’s impossible to discuss the lid margin and its associated eyelash follicles/lashes without discussing Demodex. Demodex are small arthropods belonging to the subclass Acari, more commonly referred to as mites.
Mites exist in an array of habitats, and some even live as parasites on plants and animals. Approximately 48,000 species of mites have been described (Halliday et al, 2000). In almost every human adult, Demodex inhabit the eyelash and other hair follicles and sebaceous glands. Notably, mites are not found in the skin of newborns; follicles are thought to become colonized during childhood and early life by transmission from adult family members.
A Symbiotic Relationship
Most investigators consider Demodex as benign inhabitants of the cutaneous microflora, benefiting from human sebum. There is mounting evidence, however, that Demodex in humans, as in other species, have the potential to change status from benign commensals to parasites if the host environment facilitates their proliferation (Lacey et al, 2015). Invariably, newcomers to the notion of mite-inhabited hairs are quick to query how to eradicate bodily mite populations. But I ask: do “normal” populations of Demodex play an important, unknown symbiotic role?
Symbiosis is close and often long-term interaction between two different biological species. There are several classes of symbiosis: with commensalism, one organism benefits, and the other is neither harmed nor helped; with mutualism, both organisms benefit; and with obligate scenarios, both symbionts depend on each other for survival. A common symbiotic mutualistic relationship is the example of the clownfish and the sea anemone. The clownfish feeds on small invertebrates that otherwise have potential to harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the anemone. The clownfish is protected from predators by the anemone’s stinging cells, to which the clownfish is insensitive. Additionally, the clownfish emits a high-pitched sound that deters butterfly fish, natural predators of the anemone (Miller, 2016).
Other symbiotic relationships include associations in which one organism lives inside the other (endosymbiosis), such as lactobacilli and other bacteria living in the digestive track that benefit the host by their metabolic activity. It has been demonstrated that there are common patterns of gut microbiome composition evolution during life (Gerritsen et al, 2011).
Mutually Beneficial Demodex?
Interestingly, Demodex are host to their own bacteria, and it is unknown whether this is a symbiotic or parasitic pairing. The mammalian intestine is a stem cell niche—so is the hair follicle. Is there a purpose for Demodex in this regenerative niche that we do not recognize? Do Demodex or their associated bacteria play a role in maintenance of the follicle? Do they confer a mutualistic host benefit by ingesting harmful bacteria or other organisms in the follicular canal (Lacey et al, 2011)? Do they play a role in lid margin immunity? Perhaps, Demodex mites, like some cutaneous microbes, take on different roles depending on host status (Cogen et al, 2008), changing from commensals (or mutuals) to parasites as the host’s defenses are altered.
In my opinion, eradication of Demodex is most likely impossible. The goal for their population should be to maintain homeostatic balance. CLS
For references, please visit www.clspectrum.com/references and click on document #244.
Dr. Mastrota is Program Chair-Elect of the Anterior Segment Section of the American Academy of Optometry. She is a consultant or advisor to Allergan, B+L, Bio-Tissue, OcuSoft, Paragon Bioteck, and Shire and is a stock shareholder of TearLab Corporation. Contact her at firstname.lastname@example.org.