Foodborne Listeriosis
607
number of cases/million persons in several countries were as follows:
Australia (1992)
2
Canada
2–4
Denmark
4–5
United Kingdom
2–3
United States
∼ 4
The estimated number of cases in the United States for 1993 was 1092 with 248 deaths. Not all
cases are of direct food origin, as other sources have been documented.
From a risk assessment study, a person on average is exposed 3.8 times via food to 5.0 log 10
organisms and 0.8 times to >10
6 log 10 organisms/year with about five to seven cases of listeriosis per
year.
101 After considering other factors such as mouse infective doses, the investigators concluded that
listeriosis is a rare disease in humans despite frequent exposure to the causative organism.
Source of Pathogens
With the incidence of human foodborne listeriosis being so low and sporadic, the source of the
causative strains of L. monocytogenes is of great interest. Although the outbreaks traced to dairy
products may be presumed to result from the shedding of virulent strains into milk, this is not always
confirmed. In a study of 1,123 raw milk samples from the 27 farms that supplied milk to the incriminated
cheese plant in California in 1985, Donnelly et al.
28 were unable to recover the responsible 4b serovar.
A serotype 1 was isolated from 16 string samples from one control farm. In a review of the human cases
through most of 1986, Hird
57 concluded that whereas the evidence was not conclusive in all cases, it
nevertheless supported zoonotic transmission to some degree (zoonosis: disease transmissible under
natural conditions from vertebrate animals to humans). Hird believes the healthy animal carrier is an
important source of the organism, along with clinical listeriosis in livestock, but the relative degree to
which each contributes to foodborne cases is uncertain.
L. monocytogenes was found to be shed in milk from the left forequarter of a mastitic cow, but milk
from the other quarters was uninfected.
48 About 10% of healthy cattle tested in The Netherlands were
positive for L. monocytogenes, and about 5% of human fecal samples from slaughterhouse workers
in Denmark contained the organism.
68 The carriage rate for healthy humans seemed to be about the
same regardless of their work position within food processing plants.
68 Over an 18-month period
in the United Kingdom, 32 of 5,000 (0.6%) fecal samples were positive.
75 Cross-infection with L.
monocytogenes from congenitally infected newborn infants to apparently healthy neonates in hospitals
has been shown to occur.
94 Thus, although the organism is known to be fairly common in environmental
specimens, it also exists in healthy humans at rates from less than 1% to around 15%. The relative
importance of environmental, animal, and human sources to foodborne episodes awaits further study.
Among ready-to-eat (RTE) foods, meat and poultry products are the leading vehicles for human
listeriosis. In its annual survey of RTE meat and poultry products, the Food Safety and Inspection
Service (FSIS) of the U.S. Department of Agriculture found the following percent of these products
to contain L. monocytogenes during the 9 years noted:
1995—3.02
2000—1.45
1996—2.91
2001—1.32
1997—2.25
2002—1.03
1998—2.54
2003—0.75 (January–September)
1999—1.91
607
number of cases/million persons in several countries were as follows:
Australia (1992)
2
Canada
2–4
Denmark
4–5
United Kingdom
2–3
United States
∼ 4
The estimated number of cases in the United States for 1993 was 1092 with 248 deaths. Not all
cases are of direct food origin, as other sources have been documented.
From a risk assessment study, a person on average is exposed 3.8 times via food to 5.0 log 10
organisms and 0.8 times to >10
6 log 10 organisms/year with about five to seven cases of listeriosis per
year.
101 After considering other factors such as mouse infective doses, the investigators concluded that
listeriosis is a rare disease in humans despite frequent exposure to the causative organism.
Source of Pathogens
With the incidence of human foodborne listeriosis being so low and sporadic, the source of the
causative strains of L. monocytogenes is of great interest. Although the outbreaks traced to dairy
products may be presumed to result from the shedding of virulent strains into milk, this is not always
confirmed. In a study of 1,123 raw milk samples from the 27 farms that supplied milk to the incriminated
cheese plant in California in 1985, Donnelly et al.
28 were unable to recover the responsible 4b serovar.
A serotype 1 was isolated from 16 string samples from one control farm. In a review of the human cases
through most of 1986, Hird
57 concluded that whereas the evidence was not conclusive in all cases, it
nevertheless supported zoonotic transmission to some degree (zoonosis: disease transmissible under
natural conditions from vertebrate animals to humans). Hird believes the healthy animal carrier is an
important source of the organism, along with clinical listeriosis in livestock, but the relative degree to
which each contributes to foodborne cases is uncertain.
L. monocytogenes was found to be shed in milk from the left forequarter of a mastitic cow, but milk
from the other quarters was uninfected.
48 About 10% of healthy cattle tested in The Netherlands were
positive for L. monocytogenes, and about 5% of human fecal samples from slaughterhouse workers
in Denmark contained the organism.
68 The carriage rate for healthy humans seemed to be about the
same regardless of their work position within food processing plants.
68 Over an 18-month period
in the United Kingdom, 32 of 5,000 (0.6%) fecal samples were positive.
75 Cross-infection with L.
monocytogenes from congenitally infected newborn infants to apparently healthy neonates in hospitals
has been shown to occur.
94 Thus, although the organism is known to be fairly common in environmental
specimens, it also exists in healthy humans at rates from less than 1% to around 15%. The relative
importance of environmental, animal, and human sources to foodborne episodes awaits further study.
Among ready-to-eat (RTE) foods, meat and poultry products are the leading vehicles for human
listeriosis. In its annual survey of RTE meat and poultry products, the Food Safety and Inspection
Service (FSIS) of the U.S. Department of Agriculture found the following percent of these products
to contain L. monocytogenes during the 9 years noted:
1995—3.02
2000—1.45
1996—2.91
2001—1.32
1997—2.25
2002—1.03
1998—2.54
2003—0.75 (January–September)
1999—1.91