an epidemiological study of listeriosis in dairy cattle

is seldom clinical illness in the dam. In sheep as well as in cattle the incidence of
abortion in a group is low but may reach as high as 15%. Liveborn offspring are usually
too weak to survive for long (reviewed by Radostits and others 1994). Abortions due to
L. ivanovii have also been reported and are similar to those due to L. monocytogenes
(Alexander and others 1992).
In abortion the pathological picture depends on the stage of pregnancy. If it
occurs in the early stages of the last trimester the placenta is quickly invaded by the
bacteria and the foetus dies as a result of septicaemia. The dead foetus is expelled
within 5 days and by this time autolytic changes cover the minor gross lesions produced
by the organism. Metritis usually occurs and results in retention of the foetal
membranes. If it occurs at a late stage the offspring may be born in the normal way but
is usually unable to survive. In the aborted foetus the lesions are less severe. Gross
lesions are tiny pin-point yellow foci in the liver. Similar foci but visible only
microscopically are seen in the lung, myocardium, kidney, spleen and brain. The
bacteria can be demonstrated in the centre of these focal areas (Ladds and others 1974,
Thomson 1988).
c) Septicaemia: Although it is believed to be a syndrome of young ruminants and
monogastric species outbreaks of septicaemia have been reported both in cattle (Price
1975) and in sheep (Low and Renton 1985). This syndrome comprises depression,
weakness, emaciation, pyrexia and diarrhoea. At necropsy some cases show hepatic
necrosis, gastroenteritis, serofibrinous meningitis and ophthalmitis.
d) Mastitis: Since listeric mastitis goes unnoticed due to lack of clinical illness, mastitis
associated with L. monocytogenes is not well documented (Gitter 1989). In some cases
skin discolorization on the udder or teats and firm and nodular tissue development may
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