an epidemiological study of listeriosis in dairy cattle

Immunity reducing Septicaemia
factors Abortion
Infected feed Epithelial phase Bacteremia
Neuritis Encephalitis
Figure 1. 1. Pathogenesis of L. monocytogenes infection (Gronstol 1986)
b) Infectious dose: Most of the experimental studies in mice have shown that a variable
number of L. monocytogenes is required for infection to develop, varying between 1.7
x 10 3 and 2.5 x 10 8 organisms (Ferber and Paterkins 1991). In ruminants the results of
experimental studies have also varied, 1.5x10 3 cfu/ml injected into mammary gland
induced mastitis in cattle (Bourry and others 1995), 3x10 7 to 7.5x10 10 cfu/ml injected
into the pulp cavity of sheep produced encephalitis (Barlow and McGorum 1985) and
8x10 9 cfu/ml given orally induced abortion in ewes (Gitter and other 1986a) while 10 6
and 10 10 cfu/ml doses of the organism given orally to sheep and lambs produced no
clinical signs (Low and Donachie 1991, Lhopital and others 1993). The infectious dose
required to trigger disease in naturally occurring listeriosis in ruminants is not known.
c) Virulence: Several virulence determinants of L. monocytogenes have been identified
and all of them are associated with the entry, survival, multiplication and spread of
organism in the host’s cells (Portnoy and others 1992, Sheehan and others 1994).
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