Subject: IFST: Toxin-Producing E.Coli Food Poisoning and its Prevention

From: [email-masked] (J Ralph Blanchfield)
Date: Fri, 13 Sep 1996 23:16:32 GMT

Hello Everyone,

Herewith the IFST Position Statement on Toxin-Producing E.coli Food
Poisoning and its Prevention, dated 14 September 1996. 
It is also being posted on four food-related mailing lists, for the
benefit of those who subscribe to only one of them. Usual apologies to
those who subscribe to more than one of them and will therefore
receive more than one copy.

Regards

Ralph

TOXIN-PRODUCING E.COLI FOOD POISONING AND ITS PREVENTION

The Institute of Food Science & Technology, through its Public Affairs
and Technical & Legislative Committees has authorised the following
Position Statement, dated 14 September 1996, prepared by its
Professional Food Microbiology Group.

SUMMARY

Foodborne  illness  caused  by  verocytotoxin-producing  E.coli (VTEC)
-- sometimes referred to as enterohaemorrhagic E.coli (EHEC) -- was
first recognised in the early 1980s.  It does not appear to be a very
common illness but it is now regarded as an important pathogen because
serious infections particularly in children may result.   The O157-H7
serotype is the predominant cause of human infections but other
serotypes have also been implicated.   In comparison with for example
Salmonella, numbers of cases appear to be low but they seem to be
increasing.  Infection may produce a mild diarrhoea, or a severe or
fatal illness.  The infective dose may be very low,  i.e. less than 10
cells.

Cattle appear to be the main source of infection, most cases being
associated with the consumption of undercooked beefburgers and similar
foods, or raw milk.  However,  other foods have also been implicated.
The mechanism of transmission in the food chain is not fully
understood but the contamination of meat from intestinal contents at
slaughter is probably an important factor.  More research is needed on
the incidence of VTEC in cattle and meat products.

Control of VTEC illness in humans requires good slaughterhouse hygiene
and heat treatment of raw meat and milk.  VTEC is destroyed by heat;
adequate cooking of meat  (70°C for 2 minutes internal temperature)
and pasteurisation of milk will protect consumers from infection from
these sources.   Hygienic food handling and good chilled storage
conditions are essential and should ensure that other foods do not
become contaminated.

For most foods, testing for VTEC is not an effective control strategy
and the  setting of end product specifications  is not appropriate.
Contamination rates in suspect foods are thought to be low, so the
chance of isolating the bacteria from a few samples of a batch of food
is small.  Also, there is little agreement on the best method to use
for routine testing.  The widely used standard methods for detection
and confirmation of E.coli are not appropriate as many VTEC strains
grow poorly or not at all at 44 °C.  However, extensive research on
isolation techniques is being carried out and various commercial kits
have become available recently.

BACKGROUND

Escherichia coli is a common organism found in the intestinal tract of
man and animals and, like some other potential pathogens, it is also
readily found in damp, ambient temperature environments e.g. soil,
vegetation, moist or wet areas in factories, untreated water etc.
There are many types and strains of E.coli, a few of which are
potentially pathogenic.   Different strains may cause illness by
different infective and toxin-producing mechanisms.  Symptoms vary
according to the strain of E.coli encountered, and the resistance of
the individual to such illnesses.  Infants, young children, elderly
and sick people are generally more susceptible to E.coli (and other)
infections than healthy older children and adults.

VTECs produce   one or more verocytotoxins  (VT)  which are similar
to  a  toxin  produced  by  Shigella dysentenae type 1.    The
pathogenicity of these strains is not fully understood, but it
involves adhesion to and colonisation of the intestinal tract and the
production of powerful toxins which act on the colon. The infectious
dose is not known, but it may be very low i.e. less than 10 cells.
Clinical symptoms range from mild diarrhoea to severe bloody diarrhoea
(haemorrhagic colitis), and in some sufferers include haemolytic
uraemic syndrome (HUS) and kidney failure, which can be fatal.  In
some outbreaks a high proportion of cases develop HUS which is the
most common cause of renal failure in children in the UK. In adults,
rarely, the infection may progress to cause thrombotic
thrombocytopaenic purpura, a condition like HUS but with neurological
complications. Most VTEC infections in the UK and USA arise from
strains of serotype Ol57:H7, but other serotypes can produce VT and
some have been shown to cause illness. Because o157:H7 is the only
serotype routinely looked for, it may be that its predominance is
partly owing to this fact. There is no specific treatment for VTEC
infections at present and recovery from symptoms within two weeks is
usual, although the excretion of organisms is frequently longer.

OUTBREAKS

VTEC illness was first recognised early in the 1980s and reports of
outbreaks and cases are now increasing rapidly; there have been
well-publicised incidents in many parts of the world. A large outbreak
in the USA (1993) recorded 732 affected people: undercooked hamburgers
were the cause [CDC (1993)].  An outbreak of bloody diarrhoea in a
refugee camp in Malawi affecting 20,000 people is thought to have been
caused mainly by VTEC; some cases were probably due to Shigella
[Paquet et al (1993)].  Evidence from stored sera in the Netherlands
has indicated that VTEC may have been the cause of HUS since at least
1974 in that country [Chart et al (1991)].

A provisional UK figure of 656 cases was reported for 1994. Cases tend
to peak in the summer and children under 4 years have the highest
infection rates.  There seems to have been a particular problem in NE
Scotland, but the reason is not clear.

In July/August 1996 there has been a very large outbreak in Japan. Up
to 26 August 1996, 9,578 people have been affected, many of them
school children.  Eleven people are reported to have died and 20-30
are critically ill. Cases have occurred in 40 Prefectures and several
food sources have been implicated. An urgent investigation programme
is under way, and consumers and food handlers are being instructed in
safe food handling practices.

FOOD SAFETY

The foods believed most likely to be contaminated are raw meats,
particularly beef, and raw milk. There is a need for more information
on the incidence of VTEC in cattle and meats.  Studies have shown
considerably different isolation rates, partly because the material
studied and methods used have been different. It is thought that the
source of the contamination is usually faecal material transferred
onto carcasses at slaughter, or into milk at milking. The consumption
of raw milk, cream and cheeses made from raw milk is strongly
discouraged because they are a potential source of VTEC infections as
well as other pathogens. It is possible that raw fresh vegetables and
fruit could be contaminated by manure. Other foods have been found to
be contaminated in isolated outbreaks, e.g. cheese, sandwiches, apple
juice. Waterborne outbreaks have occurred in the USA, Scotland and
Africa either because clean water was not available or perhaps because
of chlorination failure.  Recreational water has also caused
outbreaks.

The organism is heat-sensitive and should be destroyed by the same
temperature that is recommended to eliminate Salmonella and Listeria.
The advice in the UK is that minced beef and minced beef products
including beefburgers should be cooked to a minimum internal
temperature of 70 deg C for 2 minutes or equivalent.  Industry should
provide cooking instructions with burgers to ensure that they are
adequately cooked, that the meat juices run clear and there are no
pink bits inside cooked products. In North America the Food & Drug
Administration now recommend that ground beef products should be
cooked to heat all parts of the food to at least 68.3 deg C. (A
holding time at this temperature is not specified). Pasteurisation of
milk also effectively eliminates VTEC: at 72 deg C for 16.2 seconds
more than 104 cells/ml will be killed. Several of the recorded
outbreaks of VTEC illness are likely to have been caused by
undercooking of beefburgers and similar products.

The minimum pH for growth is thought to be pH 4.5, but some strains of
the organism can survive in low pH products such as mayonnaise and
yoghurt, particularly in refrigerated storage for several weeks. More
work is needed to assess the importance of these observations.
Refrigeration below 5 deg C is thought to prevent growth of VTEC and
is an important hygiene measure. However, any organisms present are
likely to survive at these temperatures perhaps for several weeks.

Outbreaks may also be caused by cross-contamination of ready-to-eat
foods from raw foods or dirty utensils and by post-pasteurisation
contamination of milk. Person-to-person spread also occurs and has
caused outbreaks in hospitals, day care centres, infant schools and
nursing homes.  It has not been possible to pinpoint the source of
infection in many sporadic cases and small outbreaks.  Normal good
manufacturing/catering practices should ensure that the chance of
cross-contamination occurring is minimised. The measures needed to
protect consumers from VTEC are the same as those needed to protect
against Salmonella, Campylobacter, Listeria and most other
non-spore-forming foodborne pathogens.

Food handlers suffering from O157 infection should be excluded from
work until two negative faecal specimens taken at intervals of not
less than 48 hours have been obtained.

QUALITY ASSURANCE

Quality assurance programmes in slaughterhouses should stress the need
to minimise faecal contamination of carcasses and to chill meat
rapidly. Elsewhere in the food industry, procedures to ensure that
incoming food materials and ingredients are of good quality should be
in place and adhered to.  Screening of raw meats for VTEC is not an
effective control mechanism because isolation rates from raw beef are
low and the organism has been found in the faeces of a small
proportion of healthy cattle, so currently it is unlikely that it can
be eliminated at source. Simple and reliable methods suitable for
routine VTEC detection in foods are not widely available. Similarly,
because of the low contamination rate of VTEC in other foods, routine
screening specifically for this organism is unlikely to be worthwhile
or successful.

The value of screening raw meat is being debated widely. Undoubtedly
screening will detect some contaminated material and this can then be
designated to a secure heat treatment process. But as screening can
never detect all contaminated lots it is a poor control procedure.
Adequate cooking of meat is the only sure way of eliminating the
danger of VTEC infection from this source.

A programme of microbiological surveillance undertaken by expert
laboratories is needed to increase awareness and understanding or the
epidemiology or VTEC in foods.  For most food manufacturers,
surveillance of raw and in-process materials, finished products arid
the manufacturing environment should be based on needs identified by a
Hazard Analysis Critical Control Point (HACCP) evaluation, and the end
product specification.   Monitoring trends of indicator organisms,
e.g. Enterobacteriaceae and Standard Plate Count should indicate
deviations from quality standards  A general method for E.coli
detection i.e. for non-pathogenic and potentially pathogenic strains,
is also useful as levels of all types of E.coli should be minimised in
food production.  Any increase in normal levels or indicators should
trigger an active investigation of the reasons for the increased
levels.   Effective process control of all cooking/pasteurisation
stages is  essential to ensure that the Correct heating temperatures
and times are achieved.

DETECTION

It is not easy to detect VTEC in raw meats and foods where low levels
of E.coli may be swamped by high numbers of other bacteria.  So far,
there are no widely recommended methods for routine food examination.
Methods used in medical laboratories to detect the organism from
stools are more successful, probably because the number VTEC cells
present in the stools of someone made ill by the organism is
relatively high in comparison to the background flora.

Currently traditional isolation methods for foods involve enrichment
in a selective broth followed by plating on to sorbitol MacConkey agar
with additives. This agar is only suitable for o157:H7 strains. (Most
but not all O157 VTEC strains do not ferment sorbitol). The
composition of the enrichment broth and plating agar is important if
VTEC is to be isolated from contaminated materials, and several groups
are working on determining the optimum combination of selective
agents.

For E.coli O157 (but not all VTEC serotypes) commercial kits are
available for isolation and identification (ELISA methods) and for
confirmation of suspect colonies (latex agglutination). An immunoassay
kit is available for Shiga-like toxins and it is reported to detect
several different VTEC strains. Recipes for several effective broths
and agars have been published but there is no consensus yet on which
is the best. Another technique used to enhance isolation of VTEC from
enrichment broths is the use of commercially-available immunomagnetic
beads coated with specific O157 antiserum. An immuno-blotting
technique is available for rapid identification of O157 colonies on
agar plates.  Biochemical and serological confirmation tests may also
be needed to exclude false positives. In clinical cases, confirmed
strains should be tested for the production of VT or the presence of
VT genes.  Confirmation can only be done at very specialised
laboratories. Most laboratories are only able to test for O157 VTEC.

It is important to note that many VTEC strains do not grow well at 44
deg C.  Many food and water E.coli methods rely on the ability of most
strains to grow and produce gas and indole at 44 deg C. Such methods
would probably fail to detect VTEC. Methods which rely on enrichment
followed by plating on to selective agar and biochemical confirmation
all at 37 deg C would be more likely to isolate VTEC. Incubation at
slightly elevated temperatures, e g 37-42 deg C, may be incorporated
in the future; method development using cultural and rapid techniques
is progressing fast.

CONCLUSION

VTEC can cause a very serious illness in man. Numbers of reported
cases are low but increasing.  The main source of the organism is
cattle and it is not likely that VTEC can be completely eliminated
>from  raw meats or the cattle population. Manufacturers, caterers and
consumers need to understand how they can each reduce the chance of
causing foodborne VTEC illness. Good slaughterhouse hygiene, thorough
cooking of raw meats, pasteurisation of milk and the avoidance of
cross-contamination from raw meats are the most effective ways of
preventing VTEC infections.

REFERENCES

Centers for Disease Control and Prevention (1993) Update: "Multistate
outbreak of Escherichia coli O157:H7 infections from hamburgers -
Western United States, 12992-1993", Morbidity and Mortality Weekly
Reports, 42, 258-263.

Chart H et al (1991) "Serological identification of Escherichia coli
O157 as cause of haemolytic uraemic syndrome in Netherlands", Lancet,
337, 437.

Paquet C et al (1993) "Aetiology of haemorrhagic colitis epidemic in
Lancet, 342, 175.

FURTHER READING:

    Advisory Committee on the Microbiological Safety of Food
    "Report on Verocytotoxin-Producing Escherichia coli"
    HMSO 1995 ISBN 0113219091

The Institute of Food Science & Technology (IFST) is the independent
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J Ralph Blanchfield
Food Science, Food Technology & Food Law Consultant
Chair, IFST Member Relations & Services Committee
Web Editor, Institute of Food Science & Technology
IFST Web address: http://www.easynet.co.uk/ifst/ 
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