Causes and Spread of Infection

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ESSEX HEALTH PROTECTION UNIT
Part of the
INFECTION CONTROL GUIDELINES
CARE HOMES
Issued January 2004
Revised April 2006 and December 2007

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ESSEX HEALTH PROTECTION UNIT
INFECTION CONTROL GUIDELINES
FOR CARE HOMES
SECTION B – INFECTION, ITS CAUSES AND SPREAD
1. The Causes of Infection
An understanding of commonly encountered mi
cro-organisms is essential for good
infection control practice. Micro-organism
s that cause disease are referred to as
pathogenic organisms. They ma
y be classified as follows:
Bacteria
are minute organisms about
one-thousandth to five-thous
andth of a millimetre
in diameter. They are susceptible to a
greater or lesser ext
ent to antibiotics.
Viruses
are much smaller than bacteria and
although they may survive outside the
body for a time they can only grow inside cells
of the body. Viruses are not susceptible
to antibiotics, but there are
a few anti-viral drugs available which are active against a limited number of viruses.
Pathogenic Fungi
can be either moulds or yeasts
. For example, a mould which
causes infections in humans is
Trichophtyon rubrum
which is one cause of ringworm
and it may also infect nails. A common yeast infection is thrush caused by Candida
albicans
.
Protozoa
are microscopic organisms, but larger
than bacteria. Free-living and non-
pathogenic protozoa include
amoebae and paramecium. Ex
amples of medical
importance include:
Giardia lamblia
which can cause an ent
eritis (symptoms of
diarrhoea).
Parasites
Worms
are not always microscopic in size
but pathogenic worms do cause infection
and some can spread from person to pers
on. Examples include: threadworm and
tapeworm.
Ectoparasites
i.e. headlice and scabies.
Prions
are infectious protein particles. Ex
ample: the prion causing (New) Variant
Creutzfeldt-Jakob Disease (vCJD).
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2. The Spread of Infection
One feature that distinguishes infection from
all other disease is that it can be spread,
i.e. one person can ‘catch’ it
from another or via a vect
or (e.g. crawling or flying
insects). There are various means by wh
ich micro-organisms can be transferred from a
reservoir to susceptible individuals.
The modes of spread of infection can be classified as:
Direct Contact
Direct spread of infection occurs when one pers
on infects the next by direct person-to-
person contact (e.g. Chickenpox
, Tuberculosis, sexually trans
mitted infections etc.).
Indirect Contact
Indirect spread of infection is said to o
ccur when an intermediate carrier is involved in
the spread of pathogens e.g.
fomite or vector.
A
fomite
is defined as an object, which
becomes contaminated with infected
organisms and which subsequently trans
mits those organisms to another
person. Examples of potential fomite
s are bedpans, urinals, thermometers,
oxygen masks or practically
any inanimate article.
Crawling and flying insects
are obvious examples of
vectors
and need to be
controlled. Insect bites may cause infe
ctions such as malaria in areas where
malaria carrying mosquitoes live.
Hands
The hands of health and social care workers
are probably the most
important vehicles
of cross-infection. The hands of residents c
an also carry microbes to other body sites,
equipment and staff.
Inhalation
Inhalation spread occurs when pathogens ex
haled or discharged into the atmosphere
by an infected person are inhaled by and infe
ct another person. The common cold and
influenza are often cited as examples, but it
is likely that hands and fomites (inanimate
objects) are also important in t
he spread of respiratory viruses.
Ingestion
Infection can occur when organisms capable of
infecting the gastro-intestinal tract are
ingested. When these organisms are excr
eted faecally by an infected person,
faecal/oral spread is said to occur. Organi
sms may be carried on fo
mites, hands...
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