The role of animal movement, including off-farm rearing of heifers, in the interherd transmission of multidrug-resistant Salmonella

Abstract

Fifty-nine commercial dairy farms were sampled 7 times over 15 to 21 mo to determine the role of animal movement, including off-farm rearing of heifers, in the interherd transmission of multidrug-resistant (MDR) Salmonella spp. Farm management data were collected by on-site inspections and questionnaires on herd management practices before and after the study. Forty-four percent (26/59) of herds did not acquire any new MDR Salmonella strains. The number of newly introduced MDR Salmonella strains acquired by the remaining 56% (33/59) of herds ranged from 1 to 8. Logistic regression models indicated that off-farm heifer raising, including contract heifer raising where heifers commingle with cattle from other farms [commingled heifers, odds ratio (OR) = 8.9, 95% confidence interval (CI): 2.4, 32.80], and herd size per 100-animal increment (herd size, OR = 1.04, 95% CI, 1.01, 1.05) were significantly associated with the introduction of new MDR Salmonella strains. The negative binomial regression similarly revealed that commingled heifers [relative risk (RR) = 2.3, 95% CI: 1.1, 4.7], herd size per 100 animals (RR = 1.02, 95% CI, 1.01, 1.03), and a history of clinical salmonellosis diagnosed before the study (RR = 2.5, 95% CI, 1.3, 5.0) were significantly associated with the number of new MDR Salmonella strains that were introduced. Factors not associated with the introduction of new MDR Salmonella strains were housing of heifers and cows in the same close-up pen, a common hospital-maternity pen, and the number of purchased cattle. This study highlights the role of animal movement in the interherd transmission of MDR Salmonella spp.

Antimicrobial Resistance Profiles and Diversity in Salmonella from Humans and Cattle, 2004–2011

“…All human isolates in this study were derived from clinical cases, whereas most cattle isolates (82.7%) were non‐clinical and of dairy cattle origin with the remainder from clinical cases. The non‐clinical cattle isolates from 2005 to 2007 were collected by a previous study that recruited herds from the three main dairy cattle areas in Washington State and was active surveillance (Adhikari et al., ). A follow‐up to that study in 2008 used the same sampling scheme and collected a similar number of isolates.…”

“…A partial explanation for this observation is that Salmonella mainly circulates within each population as a consequence of movement and persistence. A study utilizing bovine non‐clinical isolates demonstrates resistant Salmonella can persist as unobserved infections in cattle, and cattle movement is a risk factor for herd infection (Adhikari et al., ). There is some evidence for Salmonella circulation within the human population.…”

Bioexclusion of diseases from dairy and beef farms: Risks of introducing infectious agents and risk reduction strategies

“…Co-grazing with cattle from other herds is a significant risk factor for introduction to the resident herd of BHV-1, BVDv (OR 3.4 compared to not sharing pasture), Leptospira hardjo (OR 1.63 compared to not sharing pasture) and Salmonella spp. (OR 8.9 compared to no co-mingling during contract heifer rearing) (Valle et al, 1999;van Schaik et al, 2002;Davison et al, 2006;Adhikari et al, 2009;Oliveira et al, 2010). Co-grazing with sheep and goats constitutes a risk for BVD, Johne's disease and leptospirosis (Caldow, 2004).…”

Introduction of new multidrug-resistant Salmonella enterica strains into commercial dairy herds

“…Herds sampled represented a wide range of production practices and herd management types, which in turn are representative of most large-scale and intensive dairying production practices outside the Pacific Northwest. The management factors associated with the rate of Salmonella introduction are described elsewhere (Adhikari et al, 2009). Further studies are needed to determine the epidemiology of MDR Salmonella introduction, maintenance, and dissemination at the farm level and to examine whether the patterns of new strain introduction reported in the current study are similar to that in other US dairy herds.…”