606 Modern Food Microbiology
that caused a 50% infectious dose (ID 50 ) were 3.24–4.55 log cfu for normal mice, 1.91–2.74 for the
cortisone-treated, and 2.48 for pregnant mice.
50 The ID 50 for those administered cimetidine was similar
to the normals. These investigators found no significant difference between IP and IG administration
relative to ID 50 . Employing neonatal mice (within 24 hours of birth), the LD 50 by IP injection of
L. monocytogenes was 6.3 × 10/cfu, but for 6- to 8-week-old mice, the LD 50 was 3.2 × 10
6
by the same route of administration.
17 The neonatal mice were protected against a lethal dose of
L. monocytogenes when γ -interferon was injected (see below). With 15- to 20-g Swiss mice treated
with carrageenan, LD 50 was found to range from about 6 to 3,100 cfu.
25
When nude mice are challenged with virulent strains of L. monocytogenes, chronic infections
follow, and for baby mice and macrophage-depleted adult mice, virulent strains are lethal. With the
adult mouse model, rough strains of L. monocytogenes multiplied only weakly, and a weak immunity
was induced; baby mice were killed, but nude mice survived.
59 In nonfatal infections by virulent
strains, the organisms multiply in the spleen, and protection against reinfections results regardless of
the serovar used for subsequent challenge.
59
Overall, studies with the mouse model confirm the greater susceptibility to L. monocytogenes
of animals with impaired immune systems than normal animals, as is the case with humans. The
correspondence of minimal infectious doses for normal adult mice to humans is more difficult. It
has been suggested that levels of L. monocytogenes less than 10
2 /cfu appear to be inconsequential
to healthy hosts.
50 From the nine cheeses reported by Gilbert and Pini
47 that contained 10
4 –10
5 /g of
L. monocytogenes, no known human illness resulted.
INCIDENCE AND NATURE OF THE LISTERIOSIS SYNDROMES
Incidence
Although L. monocytogenes may have been described first in 1911 by H¨ ulphers,
67 its unambiguous
description was made in 1923 by Murray et al.
98 Since that time it has been shown to be a pathogen in
over 50 mammals, including humans, in addition to fowls, ticks, fish, and crustaceans. The first human
case of listeriosis was reported in 1929, and the disease has since been shown to occur sporadically
throughout the world. L. monocytogenes is the etiological agent of about 98% of human and 85% of
animal cases.
92 At least three human cases have been caused by L. ivanovii and one by L. seeligeri.
There were around 60 human cases in the United Kingdom in 1981 but around 140 in 1985, along with
a similar increase in animal cases.
93 Between 1986 and 1988, human listeriosis increased in England
and Wales by 150%, along with a 100% increase in human salmonellosis. The overall mortality rate
for 558 human cases in the United Kingdom was 46%, with 51 and 44%, respectively, for perinatal and
adult cases.
93 For the period 1983–1987, 775 cases were reported in Britain, with 219 (28%) deaths,
not including abortions. When the 44 abortions are added to the deaths, the fatality rate is 34%.
53
Prior to 1974, 15 documented cases were seen yearly in western France, but in 1975 and 1976 there
were 115 and 54, respectively.
16 All but 3 of 145 strains that were serotyped were serotype 4. There
were 687 cases in France in 1987.
22 In a 9-year period prior to early 1984, Lausanne, Switzerland,
experienced a mean of 3 cases of human listeriosis per year, but in a 15-month period in 1983–1984,
25 cases were seen.
90 Thirty-eight of 40 strains examined were serovar 4b, and 92% had the same
phage type.
By and large, foodborne outbreaks of human listeriosis seemed to have waned over the past several
years with a few exceptions, as may be noted from Table 25–5. In the early to mid-1990s, the estimated
that caused a 50% infectious dose (ID 50 ) were 3.24–4.55 log cfu for normal mice, 1.91–2.74 for the
cortisone-treated, and 2.48 for pregnant mice.
50 The ID 50 for those administered cimetidine was similar
to the normals. These investigators found no significant difference between IP and IG administration
relative to ID 50 . Employing neonatal mice (within 24 hours of birth), the LD 50 by IP injection of
L. monocytogenes was 6.3 × 10/cfu, but for 6- to 8-week-old mice, the LD 50 was 3.2 × 10
6
by the same route of administration.
17 The neonatal mice were protected against a lethal dose of
L. monocytogenes when γ -interferon was injected (see below). With 15- to 20-g Swiss mice treated
with carrageenan, LD 50 was found to range from about 6 to 3,100 cfu.
25
When nude mice are challenged with virulent strains of L. monocytogenes, chronic infections
follow, and for baby mice and macrophage-depleted adult mice, virulent strains are lethal. With the
adult mouse model, rough strains of L. monocytogenes multiplied only weakly, and a weak immunity
was induced; baby mice were killed, but nude mice survived.
59 In nonfatal infections by virulent
strains, the organisms multiply in the spleen, and protection against reinfections results regardless of
the serovar used for subsequent challenge.
59
Overall, studies with the mouse model confirm the greater susceptibility to L. monocytogenes
of animals with impaired immune systems than normal animals, as is the case with humans. The
correspondence of minimal infectious doses for normal adult mice to humans is more difficult. It
has been suggested that levels of L. monocytogenes less than 10
2 /cfu appear to be inconsequential
to healthy hosts.
50 From the nine cheeses reported by Gilbert and Pini
47 that contained 10
4 –10
5 /g of
L. monocytogenes, no known human illness resulted.
INCIDENCE AND NATURE OF THE LISTERIOSIS SYNDROMES
Incidence
Although L. monocytogenes may have been described first in 1911 by H¨ ulphers,
67 its unambiguous
description was made in 1923 by Murray et al.
98 Since that time it has been shown to be a pathogen in
over 50 mammals, including humans, in addition to fowls, ticks, fish, and crustaceans. The first human
case of listeriosis was reported in 1929, and the disease has since been shown to occur sporadically
throughout the world. L. monocytogenes is the etiological agent of about 98% of human and 85% of
animal cases.
92 At least three human cases have been caused by L. ivanovii and one by L. seeligeri.
There were around 60 human cases in the United Kingdom in 1981 but around 140 in 1985, along with
a similar increase in animal cases.
93 Between 1986 and 1988, human listeriosis increased in England
and Wales by 150%, along with a 100% increase in human salmonellosis. The overall mortality rate
for 558 human cases in the United Kingdom was 46%, with 51 and 44%, respectively, for perinatal and
adult cases.
93 For the period 1983–1987, 775 cases were reported in Britain, with 219 (28%) deaths,
not including abortions. When the 44 abortions are added to the deaths, the fatality rate is 34%.
53
Prior to 1974, 15 documented cases were seen yearly in western France, but in 1975 and 1976 there
were 115 and 54, respectively.
16 All but 3 of 145 strains that were serotyped were serotype 4. There
were 687 cases in France in 1987.
22 In a 9-year period prior to early 1984, Lausanne, Switzerland,
experienced a mean of 3 cases of human listeriosis per year, but in a 15-month period in 1983–1984,
25 cases were seen.
90 Thirty-eight of 40 strains examined were serovar 4b, and 92% had the same
phage type.
By and large, foodborne outbreaks of human listeriosis seemed to have waned over the past several
years with a few exceptions, as may be noted from Table 25–5. In the early to mid-1990s, the estimated