kept sheep, two of whom had died of listeriosis. The cabbage had been
grown in fields fertilized by fresh and composted manure from the sheep
and the harvested cabbages had been stored in a large shed through the
winter – factors thought to account for the introduction of the organism
and its multiplication to dangerous levels.
Raw vegetables, in the form of a garnish containing celery, tomatoes
and lettuce, were also implicated on epidemiological grounds in an
outbreak that occurred in eight Boston hospitals in 1979.
Surveys in the UK, the United States, Australia and elsewhere have
reported a high frequency of isolation ofL. moncytogenesfrom meats and
meat products, where serotype 1 generally predominates. A number of
sporadic cases of listeriosis have been associated with products such as
pork sausage, turkey frankfurters, cook-chill chicken, and chicken nuggets.
L. monocytogenesis relatively resistant to curing ingredients and has
been found in a range of delicatessen meats such as salami, ham, corned
beef, brawn and pate ́. In an Australian survey 13.2% of samples were
found to be positive, largely as a result of cross-contamination in the
shop. In Britain in 1989/90, high levels on vacuum-packed ham and on
pate ́, from which serotype 4b was isolated, prompted the recall of both
products from the market. Pork tongues in aspic were identified as the
original source of a large outbreak in France caused by serotype 4b.
Between March and December 1992, 279 cases were reported with 63
deaths and 22 abortions.
Dairy products such as raw and pasteurized milk and soft cheeses have
been associated with a number of major outbreaks of listeriosis. The
overall incidence ofL. monocytogenesin raw milk derived from surveys
in Australasia, Europe and the United States averages at around 2.2%,
although one Spanish study reported an incidence in excess of 45%.
Pasteurized milk was responsible for an outbreak in Massachusetts in
1983 involving 42 adult and 7 perinatal cases with an overall mortality
rate of 29%. The milk had come from farms where bovine listeriosis is
known to have occurred at the time of the outbreak. It was the absence
of evidence of improper pasteurization at the dairy that gave rise to the
concern thatL. monocytogenesmight display marked heat resistance in
some instances (see Section 7.9.2 above).
Soft cheeses are also frequently contaminated withL. monocytogenes.
In 1985 there was an outbreak in California in which a Mexican-style
soft cheese which had been contaminated with raw milk was the vehicle.
One hundred and forty-two cases were recorded comprising 93 perinatal
and 49 adult cases with an overall mortality rate of 34%. This outbreak
served to focus attention on soft cheeses and there have since been other
incidents identified in which they have been implicated, including a major
outbreak covering the period 1983–87 with 122 cases and 31 deaths
associated with the Swiss cheese Vacherin Mont d’Or.
230 Bacterial Agents of Foodborne Illness