Demodex may be beneficial to humans

Battling demodex on the lid margin is a regular occurrence for most ODs. Demodex,  more commonly referred to as mites, are small arthropods belonging to the subclass Acari. 

Mites exist in an array of habitats and live as parasites on plants and animals. Approximately 48,000 species of mites have been described.¹ In almost every adult human, Demodex folliculorum or Demodex brevis inhabit the eyelash, other hair follicles, and sebaceous glands. 

Other mite species, specific to each mammal, are similarly harbored by their host species. Mites are not found on the skin of newborns. Hair follicles are thought to become colonized by mites during childhood and early life by transmission from adults, similar to the process of acquiring other microbes (microbes include bacteria, protozoa, fungi, algae, amoebas, and slime molds). Microbe acquisition is a lifelong activity that begins the moment we are born. 

Previously from Dr. Mastrota:Using precision medicine for dry eye

Humans need microbes

Though babies develop in a sterile environment, a newborn emerges as a bacterial sponge picking up microbes that contribute to its health.

Microbes can be found in their greatest concentrations in the ears, nose, mouth, vagina, digestive tract, anus, and skin.² Like microbes, demodex mites are a natural part of the human microbiome and may serve a useful function.  

It is important to consider humans are not biologically self-sufficient-they must host microbes to avoid disease. For example, genes carried by bacteria in the gastro-intestinal tract allow humans to digest foods and absorb nutrients that otherwise would be unavailable.³

These fundamental microbes produce beneficial compounds, like certain vitamins and protective anti-inflammatories that humans cannot produce by themselves. For example, members of the gut microbiota can produce anti-inflammatory factors by enhancing cellular immune responses as well as generate Vitamins K and B.⁴

Related: What’s all the craze about demodex?

Symbiotic relationship important

Humans and demodex may have a symbiotic relationship. There are several classes of symbiosis: 

• Commensalism: one organism benefits, and the other is neither harmed nor helped

• Mutualism: both organisms benefit 

• Obligate scenarios: both symbionts depend on each other for survival

A common symbiotic mutualistic relationship is that of the clownfish and the sea anemone. The clownfish feeds on small invertebrates that otherwise have potential to harm the sea anemone. The fecal matter from the clownfish then 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.⁵

Is demodex mutually beneficial?

Most investigators consider demodex as benign inhabitants of the cutaneous microflora, benefiting from human sebum. Additionally, Demodex are host to their own bacteria, and it is unknown whether this is a symbiotic or parasitic pairing.  There is mounting evidence however that demodex in humans, as in other species, has the potential to change its status from benign commensals to parasites if the host environment facilitates its proliferation.⁶

Related: The role of lid hygiene in ocular surface disease

Demodex mites may take on different roles depending on host status, changing from a mutual relationship to a parasitic one as the host’s defenses are altered.⁷

Questions surround demodex

• Is there a purpose for demodex in the regenerative niche of the eyelash follicle that we do not recognize? 

•â€¯Does demodex or its associated bacteria play a role in maintenance of the hair follicle? 

•â€¯Could Demodex folliculorum be part of a natural cleaning system? 

•â€¯Does demodex confer a mutualistic host benefit by ingesting harmful bacteria or other organisms in the follicular canal?⁸

•â€¯ Does demodex play a role in lid margin immunity? 

• Does demodex defend us against other mite species?⁹

• Does demodex help drive tear production as inhabitants of the eyelid margin?

Eradication of demodex is most likely impossible and may be ill-advised.  The goal for those living with demodex should be to maintain homeostatic balance by using proactive rather than reactive lid hygiene.

References

1. Halliday RB, OConnor BM, Baker AS. Global Diversity of Mites. Nature and human society: the quest for a sustainable world: proceedings of the 1997 Forum on Biodiversity. National Academy Press. 2000 Mar:192-212.

2. Ackerman J. How bacteria in our bodies protect our health. Scientific American. 2012 May; 306(6):36.

3. National Institutes of Health (NIH). NIH Human Microbiome Project defines normal bacterial makeup of the body. Available at: https://www.nih.gov/news-events/news-releases/nih-human-microbiome-project-defines-normal-bacterial-makeup-body. Accessed 3/15/18.

4. Linares DM, Ross P2, Stanton C. Beneficial Microbes: The pharmacy in the gut. Bioengineered. 2016;7(1):11-20.

5. Miller A. Sea Anemones: Intricate relationship allows the other to flourish. AskNature. Available at: https://asknature.org/strategy/intricate-relationship-allows-the-other-to-flourish/#.Wqqt3pMwiu4. Accessed 3/15/18.

6. Lacey N, Russell-Hallinan A, Powell FC. Study of Demodex mites: Challenges and Solutions. J Eur Acad Dermatol Venereol. 2016 May;30(5):764-75.

7. Cogen AL, Nizet V, Gallo RL. Skin microbiota: a source of disease or defence? Br J Dermatol. 2008 Mar;158(3):442-55.

8. Lacey N, Ní Raghallaigh S, Powell FC. Demodex mites--commensals, parasites or mutualistic organisms? Dermatology. 2011;222(2):128-30.

9. Nicholls SG, Oakley CL, Tan A, Vote BJ. Demodex species in human ocular disease: new clinicopathological aspects. Int Ophthalmol. 2017 Feb;37(1):303-312.

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