Having heard the story the Microbiologist asked “might this be mites, in particular Demodex follicularum?”
“Oh, it might be”, replied the doctor, “what might I do about it?”
OKAY! No more “mite/might” jokes!
What is Demodex follicularum?
Demodex follicularum is an arachnid, a bit like a spider, having eight multi-jointed legs and being an invertebrate (having no spine). Demodex are tiny, less than half a millimetre in length so you usually need a magnifying glass of some kind to find them.
Before you start thinking that their “ain’t no mites on me” be warned, 80-90% of us have these little critters on our skin as part of our normal flora and they inhabit the hair follicles and associated sebaceous glands. They feed on the skin cells and the oils produced by the sebaceous glands. (Can you feel yourself starting to itch already!?)
The pathogenic role of Demodex is a bit controversial and the evidence for it causing infections is based upon the observation that treating makes symptoms improve or go away… not very scientific really but it lends some support to a causative role. It is suspected that symptoms are more likely to occur when the density of mites in the skin increases.
The main clinical manifestations of Demodex infection are:
- Folliculitis – inflammatory papules and pustules on the face causing pruritus (itching)
- Rosacea – a common chronic skin disorder associated with facial flushing and acne-like inflammatory papules and pustules
- Chronic blepharitis – inflammation of the eye lid margins and eye lashes often presenting with styes (hordoleums)
How is Demodex folliculitis diagnosed?
The trick to diagnosing Demodex folliculitis is to think about it. Especially if a patient has folliculitis which does not respond to specific antibiotic therapy targeted against the most common bacterial cause, Staphylococcus aureus.
The presence of Demodex is diagnosed by either taking a skin scraping from the affected area or sending a hair follicle for microscopy. A cut piece of hair does not work as you must have the follicle attached as this is where the mite lives; the hair has to be pulled out roots-and-all!
The skin or hair follicle is dissolved in 3-5% potassium hydroxide (KOH) on a glass slide, to dissolve the keratin in the associated skin, and then looked at under a light microscope using the x10 or x40 lenses. KOH is usually available in the lab as it is used to prepare slides in the same way for examination for fungal skin infections.
The only problem with this test is that the presence of Demodex does not confirm that it is causing the clinical problem as Demodex mites are present normally in 80-90% of the population. It has been suggested that a comment about how many mites are present in the sample may be helpful (>5/cm2) but no one has yet proposed a definite “cut-off” value for what is normal and what is abnormal.
I personally think there is a value in looking for Demodex in patients before treatment is given because comparison can be made in before and after treatment specimens.
If Demodex is present before treatment and absent after treatment AND the patient’s symptoms get better, then it is likely that Demodex was the cause. In contrast, if Demodex is present before treatment and absent after treatment BUT the patients symptoms do not get better, then it is very unlikely that Demodex was the cause and an alternative diagnosis should be considered.
How is Demodex treated?
Demodex folliculitis can resolve spontaneously however if it is particularly troublesome then active treatment may be indicated.
Conventional treatment commonly involves antiparasitic drugs such as topical 1% Ivermectin cream OR PO Ivermectin OR PO Metronidazole.
Less conventional but gaining in supportive evidence is the use of tea tree oil, a naturally occurring oil from the tea tree (Melaleuca alternifolia) found in Western Australia. Tea tree oil has been shown to be very potent against Demodex in a laboratory setting however the doses used would be pretty unpleasant to put on inflamed skin and would irritate the eyes as well. Further work is being done to try and work out exactly what the lowest concentration of tea tree oil is required to kill the mites. I know that many Dermatologists do recommend that patients with rosacea use a facial cleanser with tea tree oil in it as it can help improve the rosacea whilst being unlikely to cause any harm. I would agree with this.
As for our patient, samples of their skin were sent to the microbiology laboratory who confirmed the presence of lots of Demodex mites. The patient was given Ivermectin cream and having been shown pictures of the Demodex mite (which horrified them!) they used the cream avidly. Their symptoms eventually resolved and they started to use a face wash containing tea tree oil to prevent the mighty mites from coming back.