Applying our recently modified sampling method, we detected Demodex
in 100% (n
32) of patients presenting CD at eye lashes, and in 22% (n
23) of those with clean lashes.19
In a randomly selected general population (n
206), the incidence of Demodex
on lids and the nose is 4% in those younger than 19 years old, 30% in those between 20 and 80 years old, and 47% in those older than 80 years old.20
However, no research has convincingly demonstrated whether a minimal number of mites sampled must be present in order to produce symptoms.
Consistent with our earlier report,19
we noted that the Demodex
count in all seven patients receiving daily lid hygiene with baby shampoo never reached zero in a period between 40 and 350 days—that is, beyond one life cycle (fig 2). These results, together with those published earlier,2,4
strongly imply that the conventional treatment cannot eradicate Demodex
, a notion also supported by the finding that 50% baby shampoo did not kill Demodex
in vitro (table 1). It should also be noted that 75% alcohol and 10% povidone-iodine, agents known to kill most microbes, could not kill Demodex
in vitro (table 1). Because previous incubation with 50% baby shampoo or 10% SDS, detergents dissolving oil, before 10% povidone-iodine still could not kill Demodex
(table 1), we suspect that Demodex
is resistant to povidone-iodine. Although 4% pilocarpine gel was used to treat Demodex
it did not kill Demodex
in 150 minutes (table 1).
tested 45 different agents and found only a few could kill Demodex
within several minutes—absolute alcohol, ether, xylol, benzene, Danish (sulfur-containing) ointment, dill weed oil, and caraway oil. We also noted that 100% alcohol killed Demodex
in 3.9 (1.2) minutes. Furthermore, 100% of dill weed oil and caraway oil—that is, essential oils, also killed Demodex
in 14 (8.3) and 4.4 (2.3) minutes, respectively (table 1). Unfortunately, these agents are not amenable for clinical use because of their intrinsic toxicity (irritation) to the eye.
We were encouraged by TTO’s killing effect, which resembled that of caraway oil and exhibited a clear dose dependent relation (table 1). Although the killing effect of TTO on lice is thought to be mediated by anti-cholinesterase activity,22
it cannot explain its killing effect on nits. TTO might cause contact dermatitis and allergy,23,24
but the dermal LD50 exceeds 5 g/kg in rabbits, and undiluted TTO does not produce phototoxic effects on the skin of hairless mice.25
TTO patch test does not cause adverse reactions.26
These results encouraged us to use TTO to treat ocular Demodex
. TT shampoo is commercially available for treating head lice without an adverse effect.
Although both effectively cleaned CD, TTO, but not baby shampoo, stimulated Demodex to migrate from the CD buried inside the skin (figs 3 and 4). However, if daily lid scrub were not followed, CD returned in 1 week (fig 4). Because mechanical agitation to the lash following TTO scrub was sufficient to stimulate Demodex to migrate out to the skin, we advocate daily lid scrub following each office lid scrub with TTO. Because neither 50% baby shampoo nor TT shampoo, of which the concentration of TTO is less than 10%, could kill Demodex in vitro (table 1), we believe that home lid scrub acts by arousing Demodex to move out and eliminates them before mating.
Using the proposed scrub regimen, the Demodex
count was dramatically reduced in 1 week and reached zero in 1 month without recurrence in seven of nine patients (fig 5). The two patients in whom treatment failed were older, started with a higher Demodex
count, and rebound in an interval of 2–3 weeks. Norn,21
and our recent studies19
have noted that patients with a higher Demodex
count tend to be older, suggesting that Demodex
infestation, if untreated, tends to be worsened with age because of continuous propagation by mating. We thus speculate that higher Demodex
infestation produces more CD inside and outside the lash follicle, and with more resistance to TTO. If this interpretation were correct, we predict that it is easier to treat Demodex
at an early stage. Alternatively, the failure might be caused by re-infestation by mites migrating from other places of the body or from the spouse. Because the rebound count was less than the previous one, we speculate that continuous lid scrub might eventually eradicate Demodex
Lid scrub with 50% TTO did generate variable degrees of irritation in some patients, but could be minimised if caution is exercised to avoid spilling into the eye. We were encouraged by the finding that patients’ symptoms were relieved, ocular surface inflammation was resolved, and the lipid tear film stability improved in these patients (in preparation). Collectively, these findings strongly suggest that ocular Demodex infestation might indeed be pathogenic and warrants further controlled studies.