Richard Speare Richard.Speare at
Mon Mar 17 08:15:13 EST 1997

Dear Claire

> 1.  Is it true that there are known to be strains of the scabies mite
> resistant to malathion &/or other treatments?

Yes.  _Sarcoptes scabiei_ has been reported to be resistant to lindane,
crotamiton, and just recently we found permethrin resistant mites.  There
are no reports of resistance to malathion.  It is not know how widespread
resistance is in the mite population. 

> 2.  Is anything conclusive known about how long you have to be
> touching skin for the scabies mite to transfer from person to person?
> If I shake hands with someone (touching for a few seconds), can they
> get it from that much contact?  Or does it take longer than that for
> a mite to get going?
> (Note that a common belief, apparently shared by the manufacturers
> of the treatments, is that either (a) "prolonged" or "intimate" skin
> contact, or (b) sharing bedding, are necessary to transmit the mite.
> Frankly, this is certainly false, or I never would have caught the thing
> in the first place.)

No work has been done as far as I am aware on how much contact is needed
to acquire scabies.  However, in the right circumstances the mite could be
acquired by very brief contact.  The right circumstances for transmission
are a mite on the surface of the skin of an infected person in the area
that is touched by someone else.  I suspect the "intimate and prolonged
contact theory" has arisen because in most infected people, mites on the
skin surface are rare.  Also remember that if the chance of acquiring one
mite is low, the chances of acquiring a male and a female mite (it takes 2
to tango!) is even lower.  Hence if a person is infected with scabies, the
odds of acquiring scabies from them is increased by a lot of close
contact.  However, don't forget that unlucky instance of fleeting contact
at the right moment which may result in infection.  The exception to this
is a person with crusted scabies. In these cases the skin has thousands of
mites, and many of these mites come off with the skin flakes shed by these
people. People with crusted scabies are a major infection risk and rapidly
infect those in contact with them. 

> 3.  What's the minimum temperature for washing clothes, that will
> definitely kill the mite?

_Sarcoptes scabiei_ is a very sensitive creature.  I'm sure any sort of
washing will kill it. I would expect one cycle in a washing machine in
cold water would kill all mites. However, no work has been done on this. 
Any heat over 37C would increase the efficacy of this killing. 
> 4.  How long can the mite survive away from the human body?  (for
> instance, on clothes that can only have a cool wash, or on upholstery
> etc. that can't be washed and would have to be "quarantined".)

The mites are fragile.  They die rapidly from dehydration.  For example,
in an airconditioned room (RH of 65%?) I would be surprised if mites could
survive 12 hours.  In ideal circumstances with a higher RH (close to
saturation) a mite could survive 3 days, but it would be getting fairly
feeble by this time. Studies on the homes of infected patients have found
mites in about 40%. However, most people infected with scabies mites have
few mites and so the number in the environment must be low.  Also, the
studies did not try assessing whether these mites were viable and capable
of causing infection.

> 5.  Why is it believed that the mite can't live on the faces or
> scalp of adults?  

I have been unable to find the origin of this myth.  Mites do occur on the
face and neck in humans, just like in other mammals.  I suspect the myth
has arisen due to therapeutic inadequacies.  Perhaps the story went like
this: the only therapies available in the early days (e.g., benzyl
benzoate) was very irritant on the face, so the original prescribers said
don't use it on the face because it is too irritant.  When people asked
others "why not treat the head?", I suspect they then said because mites
don't occur there, rather than admit they were prescribing inadequate
treatment. One of the causes of treatment failure is not treating the
whole epidermis, and this definitely includes the head, the finger nails,
the external genitals and the natal cleft. Now less irritant treatments
are available, there is no excuse not to treat scabies adequately. 

> Is it possible that the resistant strains (if there
> are such things) are becoming so partly by changing their habitat?
> (All treatment instructions agree that it isn't necessary to treat
> the face or scalp except in babies.  So the evolutionary pressure is
> for them to learn to live there it seems to me.)

We had an interesting case of a lady who had masses of scabies mites on
the head and few on the body. She had had several treatments with lindane
on the body, and none on the head. 80% of the mites on the head were
resistant to lindane on testing in the lab.  20% were sensitive; possibly,
a population responding to selection pressure. 
> 6.  Is there any chance that the mite can live in ears, nose, anus
> or vagina and survive the treatment in that way?  If not, what is it
> about those environments that prevents it from doing so?

_Sarcoptes scabiei_ lives in the top layer of the epidermis, eating
keratinised skin cells. I would expect to find it in any epidermis that
has a keratinised layer.  Ears .. yes, but probably only the outer parts;
nose, anus, vagina - not inside, the stratum corneum in these sites is not
keratinised enough. 
> 7.  Is the mite mobile away from the human body, or does it basically
> just sit there waiting for another host to come along?  

When the mites are out of the skin, they are very mobile. As you know they
have 8 legs (apart from the larval stage which has only six).  The front 2
pairs are walking legs, and they have suckers which allow them to attach
to a wide range of surfaces, and to walk upside down on inverted surfaces. 
When active they probably can cover about 1 cm in 5 minutes, but I have
never timed them.  I don't know if _Sarcoptes scabiei_ actively searches
for a host, but the constant movement would probably increase a mite's
chances of coming across a host.  They respond to cues when walking that
would increase their chances of finding a host. 

> For instance,
> could it crawl through sheets and onto the mattress, and then back again
> after we change the sheets?  Could it crawl through socks and onto shoe
> linings, and then back again later?

Possibly, but remember they are fragile and unless your sheets are damp,
they are not going to find the environment favourable.  If you are having
problems curing yourself of scabies, the answer is usually elsewhere than
in the environment; i.e, yourself or other people.  Worried about
environmental contamination? Use an effective scabidice properly on
yourself, use an insecticidal surface spray on the floor, change your
sheets, wash your clothes, and you should have killed the mites on
yourself, and probably reduced to zero any chance of reacquiring scabies
from the environment.

> 8.  Is anything known about the relative vulnerability of the mite at
> different points on its life cycle?  (For instance, if eggs are more
> likely to survive than larvae or adults, it would presumably make sense
> to re-treat _after_ any surviving eggs have hatched and _before_ the
> larvae become adult and lay some more.)

The eggs are probably less susceptible to scabicides than the other
stages.  Eggs take about 3 days to hatch, and egg to laying adult
female is probably about 10 days at a minimum. Recommended treatment
schedule is to repeat treatment in 7 days.  This would kill any stages
that survived the initial treatment as eggs, and if all adults had been
killed on the first treatment, there would be no more eggs laid. 
> 9.  Itching increased over the week following the second treatment with
> Derbac (active ingredient: malathion).  Is it possible that this is due
> to reaction to the treatment?  or would that result (gradual increase
> over days) _always_ be because the mites are still there?  I realise
> that reactions to the treatment are possible, but I would expect such a
> reaction to flare up shortly after treatment and then _decrease_ gradually
> in the following.  But maybe that's not the case.

I think the UK is the only place where malathion is licensed to treat
scabies in humans. Organic phosphates can be toxic, although malathion is
one of the safest.  Allergies can occur to these compounds.  The rash
could have been an allergic one, but most likely it was due to inadequate
therapy.  The rash is a systemic one, and is not necessarily
topographically associated with the mite; i.e., mites and rash don't occur
together on the body.

The body's response to _Sarcoptes scabiei_ causes most of the signs and
symptoms people complain about.  Apart from local skin crusting, the
reaction to mites is an allergic one.  When mites are successfully
treated, papules and itching may take 1-2 weeks to resolve.  And some of
the more severe manifestations, e.g., nodules, may take months to go.  In
the usual case of scabies with itchy papules, treatment makes little
difference for 2 days or so, and then the itch gets less, and is gone
after a week or so. The papules may take 2 weeks to go completely.  People
who scratch their lesions have big problems!  Scratching exacerbates the
disease. I have seen the odd enthusiastic scratcher keep their scabies
rash going for months after successful treatment.  But on the other hand
the mites may not be eradicated.  Managing treatment in "scratchers" is a
difficult problem. 

> 10.  Is malathion noticeably toxic to humans?  What about permethrin
> or benzyl benzoate - how do they compare?  I know that Quellada used
> to be made with lindane which is now considered a Bad Thing.

Malathion is toxic to humans, but is the least toxic of the
organophosphates.  Be wary of applying it too frequently.  Some malathion
is absorbed.  Permethrin is far less toxic and absorption though the skin
is about 2% of that applied.  Permethrin 5% is safe for all ages, and
can be applied to the head.  Lindane is toxic when used to excess. 
Poisonings, some resulting in death, particularly in the elderly and in
infants has resulted in its being phased out. 

> 11.  Any more useful advice...  including other sources of information
> on the net.

Information on scabies on the net is pretty scarce really.  On these
topics, the valuable in-depth info is rarely found on the Net. Try the
very informative paper by Ian Burgess (1994) _Sarcoptes scabiei_ and
scabies. Advances in Parasitology 33:235-292. The Net, however, is
immensely valuable in making contact with experts that you are unaware of.

Some Net sources on scabies:

1. Scabies on ACTM Gopher

gopher:// and follwo path "JCU Academic Departments /
Australasian College of Tropical Medicine / Tropical Parasitology /
Parasitic Arthropods / Scabies". 

Other sites have nice photomicrographs, and superficial info, but
lack details.

Comments on history:  If the head and neck are not treated, the treatment
is inadequate, and the mites may not be eradicated if mites are living in
the areas that are not treated. 

Try retreating yourself using permethrin 5% (Lyclear) and applying it to
all areas including head, neck, external genitalia, under nails, natal
cleft.  Repeat in 7 days. 

Rick Speare

Department of Public Health and Tropical Medicine
James Cook University

Phone:	-61-(0)77-225700
Fax:	-61-(0)77-225788
email:	Richard.Speare at

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