Scabies question

Richard Speare Richard.Speare at jcu.edu.au
Sun Aug 6 00:21:18 EST 1995


On 2 Aug 1995 jjblanc at ibm.net wrote:

> Hi
> A friend of mine had scabies a few months ago. He recovered, but now he's 
down again.
> He's just had a baby (well, his wife has), and they're obviously paranoid 
about passing it on to her. 
> I'd be most grateful if anyone could answer these simple questions...
> Can the scabies bug remain dormant for months?
> Can it be transmitted by any means other than skin contact? (e.g. carpets, 
sofas, etc?)
> A combination of yes's and no's will do fine for an answer
> Thanks in advance!
> Julian Blanc.
> 
> 

A bit of background comment is needed first:

Scabies is a disease caused by the host's response to the parasitic mite,
_Sarcoptes scabiei_.  To clearly understand scabies, a very important
point is to separate the disease from the parasite.  If this distinction
is not made, understanding of the epidemiology and treatment of scabies
becomes confused. 

Sarcoptes scabiei, The Parasite
===============================

_Sarcoptes scabiei_ lives in tunnels in the most superficial layer of the
epidermis, the stratum corneum.  The female mite lays eggs in these
tunnels.  The eggs hatch into larvae either on the host or if exfoliated
with skin flakes, the eggs can hatch off the host. 

A study by Arlian et al. showed that of patients with scabies, mites could
be found on the floor of homes of about 40%.  Studies vary, but mites will
probably survive about 3 days in the right environment.  Off the host,
_Sarcoptes_ is very vulnerable to dehydration. 


Scabies, the Disease
====================

The disease scabies is due to the response of the host to the parasite. 
The major component of this response is an "allergic" reaction in the
dermis, the layer of skin below the epidermis.  This response is separate
from the mite (which is in the epidermis).  Hosts complain of symptoms and
signs due to the allergic response.  The host is therefore suffering from
its own responses to the parasite, and not from the effects of the
parasite itself. 

Experimental studies have shown that people usually have no symptoms of 
scabies until they have been infected for 1 month. However, once a person 
is sensitised to _Sarcoptes_, they often develop clinical signs very 
rapidly if reinfected.  In some cases, this may be within 6 hours of 
becoming reinfected.

This has implications for the epidemiology of infection with _Sarcoptes_
and its treatment.  In summary these are: 

1.   People can be infected with _Sarcoptes_ and not have
     clinical signs.  These asymptomatic people can transmit
     the parasite.

2.   In a person with scabies, since the clinical signs are
     due to an allergic reaction, the mites can be rapidly
     killed by treatment, but the clinical signs can take 2
     weeks or more to resolve.

3.   Mites can occur in the environment of a person with
     scabies, but they are shortlived.

4.   Current treatment recommendations often exclude the head. 
     This is a mistake as many infected people have mites on
     the head. 

5.   To be successful treatment has to be with an acaricide 
     to which the mite is sensitive; the acaricide has to be used 
     correctly - topical therapies have to cover the whole skin surface.

6.   Close contacts have to be treated simultaneously with the 
     patient.

 

So to answer the questions asked by Julian:

> Can the scabies bug remain dormant for months?

Depends what is actually being asked:  Off the host? No.

On the host?  Yes, a person may have no clinical signs and be 
infected with _Sarcoptes_.  If a human has become sensitised to 
_Sarcoptes_, they usually develop clinical signs of scabies soon after 
reinfection.  

> Can it be transmitted by any means other than skin contact? (e.g.
carpets, sofas, etc?)

Yes. But transmission directly from infected people is probably a much 
more significant route of infection.

What should Julian's friend do?

He and his family should have an effective treatment simultaneously.  The
best treatment currently is 5% permethrin cream applied to the whole body
including the head.  5% permethrin is a safe treatment for neonates. 

Bedding, clothes should be washed, the house cleaned thoroughly.  
Environmental sanitation for scabies has not been thoroughly researched, 
so all recommendations are empirical.  In instances of possible heavy 
contamination I usually use a surface insecticidal spray. Remember that 
infected people are the major source of _Sarcoptes_ so becoming paranoid 
about environmental sources is putting emphasis in the wrong place.

Rick Speare

Department of Public Health and Tropical Medicine
James Cook University
Townsville
AUSTRALIA

Phone:	-61-(0)77-212281
Fax:	-61-(0)77-715032
email:	Richard.Speare at jcu.edu.au





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