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an epidemiological study of listeriosis in dairy cattle

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a variable number <strong>of</strong> cases <strong>of</strong> encephalitis <strong>in</strong> a survey <strong>of</strong> 60 flocks with the me<strong>an</strong> attack<br />

rate be<strong>in</strong>g 2.5%. This c<strong>an</strong> be as high as 35% <strong>in</strong> serious outbreaks. The attack rate for<br />

abortion is reported <strong>in</strong> the region <strong>of</strong> 10% (reviewed by Radostits <strong>an</strong>d others 1994).<br />

b) Distribution: L. monocytogenes is ubiquitous <strong>in</strong> the environment <strong>an</strong>d has been<br />

isolated from a variety <strong>of</strong> <strong>an</strong>imals <strong>an</strong>d people (Gray <strong>an</strong>d Kill<strong>in</strong>ger 1966). It has also<br />

been isolated from the faeces <strong>of</strong> healthy <strong>an</strong>imals <strong>an</strong>d people which <strong>in</strong>dicates that it c<strong>an</strong><br />

live <strong>in</strong> the host’s <strong>in</strong>test<strong>in</strong>e without caus<strong>in</strong>g <strong>an</strong>y cl<strong>in</strong>ical illness.<br />

Several studies <strong>in</strong>dicate that a variable proportion <strong>of</strong> <strong>an</strong>imals excrete L.<br />

monocytogenes <strong>in</strong> their faeces. Kampelmacher <strong>an</strong>d v<strong>an</strong> Noorle-J<strong>an</strong>sen (1969) isolated<br />

L. monocytogenes from 15.3% <strong>an</strong>d 6% <strong>of</strong> <strong>cattle</strong> kept <strong>in</strong> two different regions, similarly<br />

v<strong>an</strong> Renterghem <strong>an</strong>d colleagues (1991) isolated it from 20% <strong>of</strong> <strong>cattle</strong> faeces samples<br />

exam<strong>in</strong>ed. In a survey carried out by Husu (1990) between 9. 2 % (<strong>in</strong>door period) <strong>an</strong>d<br />

3.1% (outdoor period) <strong>of</strong> <strong>dairy</strong> <strong>cattle</strong> excreted L. monocytogenes <strong>in</strong> their faeces <strong>an</strong>d the<br />

figures were higher when all Listeria spp. were considered. However Skovgaard <strong>an</strong>d<br />

Morgen (1988) reported the highest excretion rates <strong>in</strong> <strong>cattle</strong> faeces 67% Listeria spp.<br />

<strong>an</strong>d 51% L. monocytogenes. It has also been noted that the seasonal distribution <strong>of</strong><br />

faecal carriage is the same as that <strong>of</strong> cl<strong>in</strong>ical Listeriosis <strong>in</strong> northern hemisphere (Husu<br />

1990).<br />

The presence <strong>an</strong>d concentration <strong>of</strong> L. monocytogenes <strong>in</strong> nature is believed to be<br />

<strong>in</strong>fluenced by environment <strong>an</strong>d climate (Picar-Bonnaud <strong>an</strong>d others 1989). L.<br />

monocytogenes has been isolated from soil, vegetation (soybe<strong>an</strong>, corn, grass, forest,<br />

etc.), cultivated <strong>an</strong>d uncultivated fields (Welshimer 1968, Welshimer <strong>an</strong>d Donker-Voet<br />

1971, Welshimer 1975, Weis 1975). This character <strong>of</strong> L. monocytogenes led some<br />

researchers to conclude that there was a saprophytic existence <strong>of</strong> the org<strong>an</strong>ism <strong>in</strong> the<br />

pl<strong>an</strong>t-soil environment where<strong>in</strong> this environment served as a reservoir (Welshimer <strong>an</strong>d<br />

22

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