Agriculture Reference
In-Depth Information
important than O157. As with campylobacter, a small
infective dose - perhaps as few as 10 cells - is required
to cause human illness, but unlike campylobacter, they
can multiply on food.
Infection in humans
The incubation period in
people is variable but is usually between 12 and 36 hours.
The typical presenting symptom is diarrhoea, but this
may be accompanied by nausea and abdominal pain,
although vomiting is not usual. There may also be
headache and fever. While the infection is normally
self-limiting and does not require antibiotic treatment,
occasionally, with more invasive salmonellae such as
S. virchow
, bacteraemia can occur. The infection is rarely
fatal in people.
Infection in humans
The low infective dose plays
a major part in the spread of these bacteria. Incubation
is normally 1-10 days and a considerable spectrum of
symptoms can be seen. These range from asympto-
matic infection, abdominal pain and diarrhoea, bloody
diarrhoea and haemorrhagic colitis to haemolytic
uraemia resulting in renal failure. Although the total
number of cases, with about 1000 reported each year
in the United Kingdom, is low, the consequences are
serious. In one series of cases in Scotland, 59% of cases
required hospitalisation, 15% required renal dialysis,
and 3% died. Serious systemic disease is a particular
feature in the old and the young. Following a large
milk-borne outbreak in Scotland in 1994, several chil-
dren required renal transplants. Conversely, many
people appear to carry and shed the bacteria yet show
no signs of infection.
Source of human infection
People can become
infected following a failure of personal hygiene after con-
tact with infected animals (or other infected people).
Environmental contamination, especially
untreated water
,
is also important. Most cases are thought to be the result of
food-borne infection and highlight the importance of
controlling hygiene in the food chain. Meat can become
contaminated during the slaughter process either from
intestinal contents or from faecal contamination on
the hide. As with any faecally spread organism,
it is
essential that clean animals are presented for slaughter
to help minimise the latter and that the abattoir operates
hygienically to prevent both
.
Source of human infection
Various studies have
identified three main routes of transmission: consump-
tion of contaminated food or water, direct or indirect
contact with animals and person-to-person spread.
Beefburgers were recognised as a significant vehicle of
infection following a large outbreak across several states
in the United States in 1993 in which 732 people were
affected and 195 hospitalised with four deaths. However,
a wide range of foods have been implicated, including
apple juice, vegetables, potatoes, bean sprouts and water.
Contamination of the food with animal faeces is thought
to cause most of the problem.
Cattle are considered to be the primary reservoir of
VTEC with an estimated 0-17% of cattle carrying
VTEC O157. As with all organisms found in animal
faeces, control is based on minimising carcase contami-
nation. This begins on the farm by reducing the num-
ber of animal carriers, but at present, not much is
known about the natural transmission of
E. coli
O157 to
devise a control strategy.
The most important measure is
to ensure that only clean animals
, with the minimum of
faecal contamination,
are slaughtered
. This was a major
recommendation of the Pennington group which
reported after a large outbreak in Central Scotland (at
Wishaw) in 1996 in which 496 people were affected and
20 elderly people died. Products from a single butcher's
premises, including cooked meat, were implicated as
the cause of the outbreak.
Slaughter of clean animals must be accompanied by
the hygienic operation of the whole slaughterhouse and
Escherichia coli
O157
Verotoxigenic
E. coli
infections, particularly serogroup
O157, were first reported in the United Kingdom in
1983. Since then, despite a relatively low notification rate
of 0.45 cases per 100,000 population across the EU, they
have become recognised as a major source of human
morbidity and mortality due to the potential serious
consequences of infection.
E. coli
are ubiquitous inhabitants of the intestinal
tracts of animals and man. A variety of serogroups cause
infections, usually in a single animal species, for exam-
ple, bowel oedema in pigs. In humans, a number of dif-
ferent disease syndromes are recognised, including:
Enteropathogenic
E. coli
(EPEC)
Enteroinvasive
E. coli
(EIEC)
Enterotoxigenic
E. coli
(ETEC)
Enterohaemorrhagic
E. coli
(EHEC)
Some of these EHEC have the ability to produce one
or more toxins (verotoxins), which can be detected
using tissue culture (Vero cells), and are often referred
to as verotoxin-producing
E. coli
(VTEC). Together
with other virulence factors, they have the ability to
cause human illness but apparently no animal illness.
While several serogroups such as O111 and O26 have
been involved in the United Kingdom, the majority of
infections are caused by serogroup O157. In other
countries, other serogroups have been reported as more
