an epidemiological study of listeriosis in dairy cattle

values (Dean and others 1994). A probability of P< 0.05 was accepted as statistically
significant.
5. 3. Results:
5. 3. 1. Results of bacteriology:
The effect of extending the incubation period: There was no change in the frequency of
isolating Listeria with the extended incubation period but it enhanced the quality of the
Listeria growth by reducing contamination on the plates.
The effect of cold storage: The samples collected in September (30) and October (70)
were cultured without cold enrichment to detect the presence of the organism. Only 3 of
them were presumably positive for L. monocytogenes. The samples collected in
subsequent months were cold enriched.
Cold enrichment increased the frequency of isolation of Listeria spp especially
after the 3rd week (Table 5. 2.). As the length of cold storage was extended the number
of Listeria isolates increased.
After cold enrichment and weekly culture for 15 weeks, Listeria spp. had been
identified in 18 of the 20 samples. This was taken as a “gold standard” to evaluate the
sensitivity of culture after cold enrichment for different lengths of time. The sensitivity
of culture was 0.72 (13/18) after 3 weeks of cold enrichment and was highest after 7
weeks of cold enrichment (0.94, 17/18). Similar results were found for L.
monocytogenes. The sensitivity of the test was 0.56 (10/18) after 3 weeks of cold
enrichment and 0.61 (11/18) after 7 weeks. When these results of culture after 3 and 7
weeks of cold enrichment were combined the sensitivity was 1 for Listeria spp. (18/18)
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