Agriculture Reference
In-Depth Information
multiplication and toxin production. The foods usually
implicated are cooked meats, poultry and dairy produce.
This has been an uncommon cause of food poisoning
in the United Kingdom since the 1950s, with around
10 outbreaks reported each year.
Five types (A-E) of
Cl. perfringens
are known to exist,
but only type A has been implicated in food poisoning.
The clostridia form part of the normal intestinal flora of
animals and are widespread in the environment.
Infection in humans
The rapid production of
enterotoxin in the intestine results in a short incubation
period of 10-12 hours. The toxin causes diarrhoea and
abdominal pain but not usually vomiting. Illness nor-
mally lasts 24 hours and is self-limiting. Subsequent
complications and death are rare.
Clostridium botulinum
Botulism is one of the most feared causes of food poison-
ing because, although it is exceptionally rare, it is a severe
disease with a high mortality.
Clostridium botulinum
is a
Gram-positive spore-forming obligate anaerobe produc-
ing one or more of seven toxins (A-G). Toxins A, B and
E have been associated with human disease. Types A and
B are more commonly linked with meat and vegetables,
while type E is associated with fish. The toxin is not ther-
mostable and can be destroyed by cooking at 80°C for
30 minutes.
Source of human infection
The clostridial spores
survive normal cooking, and the heating process may
in fact stimulate them to germinate. As food cools, very
rapid multiplication can take place, since optimal
growth occurs at 43-47°C. The heating process of
cooking also drives off oxygen, creating the anaerobic
conditions necessary for growth. These conditions are
most likely to be found when large volumes of food are
cooked and cooled, particularly large joints of meat or
stews and casseroles. Thorough reheating (above 75°C)
will kill any vegetative cells present and prevent food
poisoning. Most cases occur as outbreaks, which may
be large and are often associated with hospital or com-
mercial catering. About 50 outbreaks are reported in
the United Kingdom each year.
Infection in humans
The clostridia produce
potent neurotoxins and the symptoms reflect muscular
paralysis. Symptoms can appear within 2 hours but
may take as long as 5 days. There can be an initial short
period of diarrhoea with vomiting and subsequent
constipation. Blurring or double vision is often the
first systemic sign, accompanied by dry mouth and
difficulty in swallowing. The patient is usually men-
tally alert and there is no loss of sensation. Paralysis
can extend to the limbs and eventually result in
respiratory failure. The effects of the toxin may persist
for several months.
Staphylococcus aureus
Unlike most bacterial food poisoning, illness caused by
Staph. aureus
is due to the consumption of pre-formed
toxin and not the bacteria, which may be absent. The
toxin is heat stable and may survive for 1½ hours at boil-
ing temperature, even though the staphylococci them-
selves are destroyed. Five major enterotoxins are known
to be produced (A-E) of which type A is the most com-
mon. The Gram-positive cocci can grow over a wide
range of temperatures (10-45°C) with the optimum at
35-40°C.
Source of human infection
Cl. botulinum
is
ubiquitous and can be found on a wide range of foods.
Toxin production only takes place when growth
occurs. This can happen during preservation pro-
cesses, such as smoking or fermentation of fish and
meat, which result in a suitable anaerobic environ-
ment. Improperly bottled or canned foods can also
allow growth to take place.
Botulism is very rare in the United Kingdom. The first
reported outbreak occurred in 1922 in Loch Maree in
Scotland when eight people became ill after eating pot-
ted duck paste and all died. The largest outbreak occurred
in 1989 when 27 cases (one death) were caused by hazel-
nut yoghurt. In total, only 10 outbreaks have been
described in the United Kingdom.
Infection in humans
The presence of pre-formed
toxin results in a short incubation period of 2-6 hours.
The symptoms are primarily nausea and vomiting, with
additionally diarrhoea. The illness may also be so acute
as to cause fainting or collapse. Most patients recover
within 12-48 hours.
Source of human infection
Staph. aureus
infection
usually follows contamination from a human source.
The organism can cause wound or skin infections and
is present in the nose of up to 40% of healthy people.
A failure of basic hygiene, including covering skin
lesions, results in contamination of food and subsequent
Vibrio parahaemolyticus
Vibrio parahaemolyticus
is a Gram-negative rod usually
found in seawater where the temperature is above 10°C.
It can multiply rapidly at ambient temperatures but is
easily killed by cooking.
