Ecology and transmission of Listeria monocytogenes infecting ruminants and in the farm environment

Abstract

A case-control study involving 24 case farms with at least one recent case of listeriosis and 28 matched control farms with no listeriosis cases was conducted to probe the transmission and ecology of Listeria monocytogenes on farms. A total of 528 fecal, 516 feed, and 1,012 environmental soil and water samples were cultured for L. monocytogenes. While the overall prevalence of L. monocytogenes in cattle case farms (24.4%) was similar to that in control farms (20.2%), small-ruminant (goat and sheep) farms showed a significantly (P < 0.0001) higher prevalence in case farms (32.9%) than in control farms (5.9%). EcoRI ribotyping of clinical (n = 17) and farm (n = 414) isolates differentiated 51 ribotypes. L. monocytogenes ribotypes isolated from clinical cases and fecal samples were more frequent in environmental than in feed samples, indicating that infected animals may contribute to L. monocytogenes dispersal into the farm environment. Ribotype DUP-1038B was significantly (P < 0.05) associated with fecal samples compared with farm environment and animal feedstuff samples. Ribotype DUP-1045A was significantly (P < 0.05) associated with soil compared to feces and with control farms compared to case farms. Our data indicate that (i) the epidemiology and transmission of L. monocytogenes differ between small-ruminant and cattle farms; (ii) cattle contribute to amplification and dispersal of L. monocytogenes into the farm environment, (iii) the bovine farm ecosystem maintains a high prevalence of L. monocytogenes, including subtypes linked to human listeriosis cases and outbreaks, and (iv) L. monocytogenes subtypes may differ in their abilities to infect animals and to survive in farm environments.

FIG. 1.

Prevalence of L. monocytogenes (LM) in fecal, soil, feedstuff, and water samples collected on all farms (A), cattle farms (B), and small-ruminant farms (goat and sheep farm results were pooled) (C). Different letters indicate statistically different levels of L. monocytogenes in comparisons of prevalence of L. monocytogenes in each sample category of case and control farms at the P < 0.05 level. The total number (n) of samples collected in each sample category is indicated above each bar.

FIG. 2.

Scatter plots of percentages of L. monocytogenes (LM)-positive fecal samples versus L. monocytogenes-positive soil samples (A), L. monocytogenes-positive fecal samples versus L. monocytogenes-positive feed samples (B), and L. monocytogenes-positive soil samples versus feed samples (C). Triangles represent bovine farms, while squares indicate small-ruminant farms (goat and sheep farm results were pooled). Control farms are depicted by open symbols, while case farm symbols are shaded. Regression equations and R² values determined by regression lines for each farm system in each scatter plot comparison are indicated.

FIG. 3.

Scatter plot of SID for individual farms with more than four L. monocytogenes-positive samples. Triangles represent cattle farms, while squares indicate small-ruminant farms (goat and sheep farm results were pooled). Control farms are depicted by open symbols, while case farm symbols are shaded. CUFARM 11, results from a bovine case farm that showed an extremely low SID (0.22); eight of the nine isolates from this farm were identical with respect to ribotype (DUP-1039E).