Food Safety Magazine - June/July 2013

SANITATION
By Ewen C. D. Todd, Ph.D.
The Burden of Listeriosis
Taking a closer look at the
food safety risks associated
with Listeria monocytogenes
Listeria monocytogenes is estimated to cause nearly
1,600 illnesses each year in the United States
and is responsible for more than 1,400 hospitalizations
and 250 deaths. 1 Nearly all cases of
Listeria infections (listeriosis) result from persons
eating food contaminated with L. monocytogenes or
affect newborn infants whose mothers eat contaminated
food during pregnancy. Rates of listeriosis per 100,000
individuals are relatively consistent across nations that
report; estimates for cases of the febrile gastrointestinal
form of the disease are rarely considered, even though
a few outbreaks have been reported. 2 Outbreaks from
L. monocytogenes are not frequent compared with those
caused by pathogens like Salmonella and Clostridium
perfringens, but they often generate national or even
international notoriety. This is because: 1) the resultant
infections are serious, with hospitalizations and deaths;
2) they are often caused by lack of management oversight
in manufacturing plants, lack of understanding of
the nature of the pathogen or where errors by workers
have been allowed to repeatedly occur; 3) these plants
are then found to be typically out of compliance with
existing regulations; and 4) the outbreaks have major
economic consequences, especially if the products affect
international trade. Outbreaks have involved a whole
range of facilities, from small artisanal operations to
well-established major processors. The implicated foods
are typically deli meats and artisanal cheeses. However,
case-control studies in the U.S. have implicated melons
and hummus, and in 2011, a large outbreak associated
with contaminated cantaloupes causing 147 cases and
33 deaths reinforced a risk of listeriosis from consuming
produce items. Most listeriosis
outbreaks have been reported from
Europe, the U.S., Canada, Australia and
New Zealand, but the pathogen is likely
causing illnesses in many other nations
that do not investigate cases, which
are mainly sporadic. Even in the U.S.,
where listeriosis is nationally notifiable,
it is believed that many cases are not
recognized and reported. The fact that
contaminated ready-to-eat (RTE) food is
responsible for almost all illnesses—and
the organism surprises us in the range
of food it can contaminate in numbers
large enough to cause outbreaks—has
forced both government and the food
industry to make a major effort to
reduce the public’s exposure to the
pathogen.
Characteristics of Listeria
monocytogenes
Among pathogens, L. monocytogenes
has distinctive characteristics that must
be recognized before implementing prevention
and control strategies applied to
the food processing and food preparation
environments. These include the
extensive occurrence of the pathogen in
the natural environment. Listeria spp.,
including L. monocytogenes, are present
in soil, water, sewage, vegetation and
wild animal feces, as well as on the farm
and in food processing facilities. 3 Farm
animals can be asymptomatic or cause
encephalitis, septicemia and abortions,
and may be a source of L. monocytogenes
on the farm. The pathogen can survive
and grow at a wide range of temperatures
(-1 to 45 °C); for instance, it can
be a long-term resident in chilled food
processing premises under high humidity,
but it can also survive conditions of
low water activity, such as under high
salt concentrations. 4 Some strains can
survive for long times under adverse
environmental conditions and persist
in niches in food processing equipment
and associated drains, walls and
28 F o o d S a f e t y M a g a z i n e