Prevalence of Listeria species in raw milk from farm bulk tanks in Nagano prefecture.

Raw milk samples from bulk tanks of a total of 943 farms, which corresponded to approximately 60% of all dairy farms in Nagano Prefecture were examined for Listeria species between December 1990 and April 1991. Listeria spp. were isolated from 29 (3.1%) of 943 milk specimens. In the southern, central, eastern and northern areas of the prefecture, Listeria spp. were isolated from 6.1% (22/362), 1.5% (4/272), 1.4% (2/143) and 0.6% (1/166) of samples, respectively. Listeria monocytogenes was isolated from three (0.3%) bulk tanks in the southern area: two of the strains isolated from two different farm bulk tanks were serovar 4b, and the other one was 1/2a. Besides, between February 1991 and January 1992, 504 samples of raw milk from farm bulk tanks were collected nine times from 56 farms in the southern area, where the prevalence of Listeria spp. was the highest, and examined for the seasonal variation in the presence of Listeria spp. The prevalence of Listeria spp. was higher in spring (14.3%) than in autumn (4.8%). The 56 farms were divided into three groups according to the prevalence of Listeria spp., namely, three farms in Group 1 gave a high contamination rate (50% < or =), 14 farms in Group 2 a low contamination rate and the remaining 39 farms in Group 3 no recovering of Listeria spp. Sixteen strains of L. monocytogenes serovar 4b were isolated from four farms.