Herd-level factors associated with isolation of Salmonella in a multi-state study of conventional and organic dairy farms

Molecular Characterization of Multidrug-Resistant and Extended-Spectrum β-Lactamases-Producing Salmonella enterica Serovars Enteritidis and Typhimurium Isolated from Raw Meat in Retail Markets

In the present study, a total of 720 samples were collected from retail raw meat from 13 upazilas in Sylhet District, Bangladesh, of which 225 samples were from cattle meat, 210 samples were from goat meat, and 285 samples were from chicken meat. Salmonella enterica serovars Typhimurium and Enteritidis were screened for extended-spectrum β-lactamase (ESBL) genes using multiplex PCR. Among the 720 samples, Salmonella spp. was detected in 28.06% (202 out of 720) of the samples, with S. Enteritidis and S. Typhimurium were identified in 11.53% (83 out of 720) and 12.22% (88 out of 720) of the samples, respectively. It was found that all Salmonella enterica serovars isolated from cattle meat displayed multidrug resistance (MDR) based on antimicrobial susceptibility testing. Notably, a significant proportion of S. Enteritidis isolates and all S. Typhimurium isolates from goat meat demonstrated complete resistance to multiple drugs (ampicillin, cefuroxime, and ceftazidime). Regarding chicken meat, out of 89 isolates encompassing both S. Typhimurium and S. Enteritidis, 57 isolates (64.04%) exhibited MDR. Additionally, blaCTX-M-1 exhibited the highest occurrence at 15.69% for S. Typhimurium and 7.89% for S. Enteritidis in chicken meat. Moreover, blaCTX-M-9 was only detected at 3.92% for S. Enteritidis in chicken meat. Furthermore, blaOXA had the highest prevalence rate of 19.04% for S. Enteritidis and 25.80% for S. Typhimurium in cattle meat, followed by chicken meat. These findings highlight the urgency for monitoring ESBL-producing Salmonella in retail raw meat and the need for strict measure to manage antibiotic use to prevent the spread of multidrug-resistant and ESBL-producing Salmonella strains, thereby protecting humans and reducing public health risks.

Seasonal variation of Salmonella enterica prevalence in milk and cottage cheese along the dairy value chain in three regions of Ethiopia

Seasonal fluctuations influence foodborne illness transmission and affect patterns of microbial contamination of food. Previous investigations on the seasonality of Salmonella enterica prevalence in dairy products in Ethiopia have been minimal. However, such data are needed to inform strategic development of effective interventions to improve food safety, as seasonal differences may affect intervention strategies. This study was conducted to identify differences in the prevalence of Salmonella in milk and cheese samples between wet and dry seasons. A longitudinal study design was utilized with a random sampling occurring during both dry and wet seasons. A total of 448 milk and cottage cheese samples were collected from Oromia, Sidama, and Amhara regions. Samples were tested for Salmonella using the ISO 6579-1: 2008 method, followed by PCR confirmation. A chi-square test was conducted to assess the significance of differences in the prevalence of Salmonella in the samples between the two seasons. Results from this study showed a higher prevalence of Salmonella in all sample types during the dry season (P < 0.05). Moreover, when comparing raw milk, pasteurized milk, and cottage cheese samples, a significant difference was observed in Salmonella prevalence from raw milk samples (27.08%) collected in the Oromia region. Additionally, data showed a significantly higher prevalence of Salmonella in samples collected from raw milk producers (29.17%) during the wet season (P < 0.05). This study indicates that in order to enhance the safety of dairy products in Ethiopia, comprehensive, long-term awareness building on hygienic milk production and handling that consider seasonal influence is warranted.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40550-024-00108-4.

Occurrence of Salmonella spp. in animal patients and the hospital environment at a veterinary academic hospital in South Africa

Background and Aims

Nosocomial infections caused by Salmonella spp. are common in veterinary facilities. The early identification of high-risk patients and sources of infection is important for mitigating the spread of infections to animal patients and humans. This study investigated the occurrence of Salmonella spp. among patients at a veterinary academic hospital in South Africa. In addition, this study describes the environmental factors that contribute to the spread of Salmonella spp. in the veterinary facility.

Materials and Methods

This study used a dataset of Salmonella-positive animals and environmental samples submitted to the bacteriology laboratory between 2012 and 2019. The occurrence of Salmonella isolates at the veterinary hospital was described based on source, month, season, year, and location. Proportions and 95% confidence intervals were calculated for each variable.

Results

A total of 715 Salmonella isolates were recorded, of which 67.6% (483/715) came from animals and the remainder (32.4%, 232/715) came from environmental samples. The highest proportion (29.2%) of Salmonella isolates was recorded in 2016 and most isolates were reported in November (17.4%). The winter season had the lowest (14.6%) proportion of isolates reported compared to spring (31.3%), summer (27.8%), and autumn (26.4%). Salmonella Typhimurium (20.0%) was the most frequently reported serotype among the samples tested, followed by Salmonella Anatum (11.2%). Among the positive animal cases, most (86.3%) came from equine clinics. Most reported isolates differed based on animal species with S. Typhimurium being common in equines and S. Anatum in bovines.

Conclusion

In this study, S. Typhimurium emerged as the predominant strain in animal and environmental samples. Equines were the most affected animals; however, Salmonella serotypes were also detected in the production animals. Environmental contamination was also a major source of Salmonella species in this study. To reduce the risk of transmission, strict infection prevention and control measures (biosecurity) must be implemented.

Frequency of isolation and phenotypic antimicrobial resistance of fecal Salmonella enterica recovered from dairy cattle in Canada

Salmonellosis is one of the leading causes of gastrointestinal infections in humans. In Canada, it is estimated that approximately 87,500 cases of salmonellosis occur every year in humans, resulting in 17 deaths. In the United States, it is estimated that 26,500 hospitalizations and 420 deaths occur every year. In dairy cattle, infections caused by nontyphoidal Salmonella enterica can cause mild to severe disease, including enteritis, pneumonia, and septicemia. Our study objectives were to determine the proportion of fecal samples positive for Salmonella in dairy cattle in Canada and determine the resistance pattern of these isolates. We used data collected through the Canadian Dairy Network for Antimicrobial Stewardship and Resistance (CaDNetASR). Pooled fecal samples from preweaning calves, postweaning heifers, lactating cows, and manure storage were cultured for Salmonella, and the isolates were identified using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Antimicrobial susceptibilities were determined using the minimum inhibitory concentration test, and resistance interpretation was made according to the Clinical and Laboratory Standards Institute. A 2-level, multivariable logistic regression model was built to determine the probability of recovering Salmonella from a sample, accounting for province, year, and sample source. The proportion of farms with at least one positive sample were 12% (17/140), 19% (28/144), and 17% (24/144) for the sampling years 2019, 2020, and 2021, respectively. Out of the 113 Salmonella isolates, 23 different serovars were identified. The occurrence of Salmonella appeared to be clustered by farms and provinces. The most common serovars identified were Infantis (14%) and Typhimurium (14%). Overall, 21% (24/113) of the Salmonella isolates were resistant to at least one antimicrobial. Resistance to tetracycline was commonly observed (17%); however, very limited resistance to category I antimicrobials (categorization according to Health Canada that includes third-generation cephalosporins, fluoroquinolones, polymyxins, and carbapenems) was observed, with one isolate resistant to amoxicillin and clavulanic acid. The proportion of Salmonella isolates resistant to 2 and 3 antimicrobial classes was 3.5% and 8.8%, respectively. Our study provided valuable information on the proportion of fecal samples positive for Salmonella, the serovars identified, and the associated resistance patterns across CaDNetASR herds, at regional and national levels.

Review: Salmonella Dublin in dairy cattle

Salmonella enterica serovar Dublin (S. Dublin) is a bacterium host-adapted to cattle with increasing prevalence in dairy facilities. It can severely affect cattle health, producing high morbidity and mortality in young calves and reducing the performance of mature animals. Salmonella Dublin is difficult to control and eradicate from herds, as it can be shed from clinically normal animals. In addition, S. Dublin is a zoonotic bacterium that can be lethal for humans and pose a risk for human and animal health due to its multi-drug resistant characteristics. This review provides an overview of S. Dublin as a pathogen in dairy facilities, the risk factors associated with infection, and current strategies for preventing and controlling this disease. Furthermore, current gaps in knowledge are also discussed.

Genomic Diversity, Antimicrobial Resistance, Plasmidome, and Virulence Profiles of Salmonella Isolated from Small Specialty Crop Farms Revealed by Whole-Genome Sequencing

Salmonella is the leading cause of death associated with foodborne illnesses in the USA. Difficulty in treating human salmonellosis is attributed to the development of antimicrobial resistance and the pathogenicity of Salmonella strains. Therefore, it is important to study the genetic landscape of Salmonella, such as the diversity, plasmids, and presence antimicrobial resistance genes (AMRs) and virulence genes. To this end, we isolated Salmonella from environmental samples from small specialty crop farms (SSCFs) in Northeast Ohio from 2016 to 2021; 80 Salmonella isolates from 29 Salmonella-positive samples were subjected to whole-genome sequencing (WGS). In silico serotyping revealed the presence of 15 serotypes. AMR genes were detected in 15% of the samples, with 75% exhibiting phenotypic and genotypic multidrug resistance (MDR). Plasmid analysis demonstrated the presence of nine different types of plasmids, and 75% of AMR genes were located on plasmids. Interestingly, five Salmonella Newport isolates and one Salmonella Dublin isolate carried the ACSSuT gene cassette on a plasmid, which confers resistance to ampicillin, chloramphenicol, streptomycin, sulfonamide, and tetracycline. Overall, our results show that SSCFs are a potential reservoir of Salmonella with MDR genes. Thus, regular monitoring is needed to prevent the transmission of MDR Salmonella from SSCFs to humans.

Semi-Quantitative Biosecurity Assessment Framework Targeting Prevention of the Introduction and Establishment of Salmonella Dublin in Dairy Cattle Herds

An increasing average herd size and complexity in farm structures call for a higher level of biosecurity. It can reduce the risk of introducing and establishing pathogens with multiple-pathway and indirect spread mechanisms, such as Salmonella Dublin, a pathogen with an increasing occurrence in dairy cattle farms across different countries and continents. Therefore, this study aimed to use existing knowledge to develop a framework with a supporting tool allowing for a time-efficient, yet comprehensive, assessment of biosecurity measures that can help prevent the introduction and establishment of S. Dublin in dairy herds. Based on the literature review, a seven-step biosecurity assessment framework was developed and evaluated in collaboration with biosecurity experts. The resulting framework includes a weighted semi-quantitative assessment method with a scoring guide in an electronic supporting tool for 12 biosecurity sections assessed through on-farm observations and farmer interviews. The framework and tool provide a novel approach to comprehensively assess the overall (mainly external) on-farm biosecurity level by a trained biosecurity assessor. They can be used for systematic data collection in epidemiological studies on risk factors for the introduction and establishment of S. Dublin in dairy farms. Preliminary interrater reliability testing indicated moderate reliability between assessors with varying biosecurity skills.

Longitudinal Assessment of Prevalence and Incidence of Salmonella and Escherichia coli O157 Resistance to Antimicrobials in Feedlot Cattle Sourced and Finished in Two Different Regions of the United States

The objective was to investigate the influence of cattle origin and region of finishing on the prevalence of Salmonella, Escherichia coli O157:H7, and select antimicrobial resistance in E. coli populations. Yearling heifers (n = 190) were utilized in a 2 × 2 factorial arrangement. After determining fecal Salmonella prevalence, heifers were sorted into one of four treatments: heifers originating from South Dakota (SD) and finished in SD (SD-SD); heifers originating from SD and finished in Texas (SD-TX); heifers originating from TX and finished in SD (TX-SD); and heifers originating from TX and finished in TX (TX-TX). Fecal, pen, and water scum line samples were collected longitudinally throughout the study; hide swab and subiliac lymph node (SLN) samples were collected at study end. A treatment × time interaction was observed (p ≤ 0.01) for fecal Salmonella prevalence, with prevalence being greatest for TX-TX and TX-SD heifers before transport. From day (d) 14 through study end, prevalence was greatest for TX-TX and SD-TX heifers compared with SD-SD and TX-SD heifers. Salmonella prevalence on hides were greater (p ≤ 0.01) for heifers finished in TX compared with SD. Salmonella prevalence in SLN tended (p = 0.06) to be greater in TX-TX and SD-TX heifers compared with TX-SD and SD-SD. Fecal E. coli O157:H7 prevalence had a treatment × time interaction (p = 0.04), with SD-TX prevalence being greater than TX-SD on d 56 and SD-SD and TX-TX being intermediate. A treatment × time interaction was observed for fecal trimethoprim-sulfamethoxazole-resistant and cefotaxime-resistant E. coli O157:H7 prevalence (p ≤ 0.01). Overall, these data suggest that the region of finishing influences pathogenic bacterial shedding patterns, with the initial 14 d after feedlot arrival being critical for pathogen carriage.

Preparing Male Dairy Calves for the Veal and Dairy Beef Industry

Surplus male dairy calves experience significant health challenges after arrival at the veal and dairy beef facilities. To curb these challenges, the engagement of multiple stakeholders is needed starting with improved care on some dairy farms and better management of transportation. Differing management strategies are also needed if calves arrive at veal and dairy beef facilities under poor condition.

A risk-based scoring system to quantify biosecurity in cattle production

Farm biosecurity includes all measures preventing pathogens from entering (external) and spreading within a herd (internal) and is important in facilitating the shift from cure to prevention in veterinary medicine. To assess biosecurity on farm level quantitatively an objective measurement process is required. This study describes the development and implementation of risk-based weighted biosecurity evaluation tools for veal, beef and dairy cattle farms. Based on risk factors and biosecurity measures associated with priority cattle diseases and the results of a cross-sectional survey on Belgian farms, questions were selected for the Biocheck-tool. The scoring system consists of three separate questionnaires that contain 69 (veal), 104 (beef) and 124 (dairy) questions. Experts in various fields of veterinary medicine were asked to weigh the different biosecurity categories and questions according to the method of Gore. The system obtained provides biosecurity scores per category (external and internal biosecurity) and subcategory (e.g. purchase, transport, health management). The Biocheck tool was subsequently used in a survey to assess biosecurity in 20 veal, 50 beef and 50 dairy farms. For all production systems, both internal and external biosecurity were considered low, resulting in low mean total biosecurity scores of 39.7 points for veal (SD = 7.4), 44.3 for beef (SD = 8.4), and 48.6 points for dairy farms (SD = 8.1), out of a maximum of 100 points. For all farm types, the scores for internal biosecurity were lower compared to external biosecurity. Veal farms scored significantly lower for "purchase" than beef and dairy, while scoring higher for the other subcategories of external biosecurity. In dairy and beef, "purchase and reproduction" was the highest scoring subcategory. For internal biosecurity, "health management" was particularly low in the three farm types, while subcategories exceeding 50 points were rare. With this tool, implementation of biosecurity on cattle farms can be assessed in a standardized and reproducible manner. This evaluation allows for benchmarking of farms and herd-specific advice for improvements.

Biosecurity practices in Belgian veal calf farming: Level of implementation, attitudes, strengths, weaknesses and constraints

The shift from cure toward prevention in veterinary medicine involves the implementation of biosecurity. In cattle farming, the application of biosecurity measures has been described to a limited degree, yet no data on biosecurity on veal farms is available. A high degree of commingling of veal calves from multiple farms causes frequent disease outbreaks, and thereby high antimicrobial usage and increased risk of antimicrobial resistance. Therefore, this study aimed to describe the current implementation of biosecurity on veal farms in Belgium. To this extent, a list of the most important calf diseases (n = 34) was created, and risk factors and related biosecurity measures for these diseases were determined and included in a questionnaire. Herd visits and face-to-face interviews were conducted on 20 randomly selected veal farms, comprising 8.3% of the target population. A categorical principal component and clustering analysis were performed to determine the influence of the veal companies on the farms' biosecurity level. Awareness of biosecurity was very low among the farmers. All farms used an "all-in, all-out" production system with calves originating from multiple farms without quarantine. On average, farms were filled in 11.4 days (range 2-52). The degree of commingling for these farms was 1.24, meaning that, on average, 124 calves originated from 100 farms. Veterinarians wore farm-specific boots on eight farms (40%) and farm-specific clothes on six farms (30%), while technical advisors wore farm-specific boots on six farms (30%) and farm-specific clothes on four farms (20%). Disinfection footbaths were only used in five farms (25%) despite being present in all farms in the sample. Concerning internal biosecurity, none of the farmers isolated sick animals; only one farmer (5%) had a physically separated hospital pen, and only 11 farmers (55%) both cleaned and disinfected the stables after each production cycle. In most farms, animals were of comparable age. Healthy calves generally remained in the same compartment during the entire production cycle, limiting the risks associated with the movement of animals. No influence of the integrations on the biosecurity level could be determined. It can be concluded that a few biosecurity measures, such as "'all-in, all-out" and compartmentation, are implemented relatively well, while other measures, such as good cleaning and disinfection and proper entrance measures for visitors and personnel can easily be improved. The improvement of measures regarding the introduction of animals of different origins will require more fundamental changes in the veal industry.

Evaluation of herd-level sampling strategies for control of Salmonella in Swedish cattle

Based on Swedish legislation, all herds where Salmonella of any serotype is detected are put under restrictions, and measures aiming at eradication are required. Costs for sampling and control have increased in recent years and the aim of this study was to investigate the efficiency of different sampling strategies. We also compiled test results from recent surveillance activities and used these to complement and compare with calculated results. Sensitivities and specificities at group and herd level were calculated for different test strategies. A scenario-tree modeling approach was used to account for the hierarchy of animals within herds, and different relative risk of salmonella in different age groups. Negative and positive predictive values (NPV and PPV), and probability of freedom from Salmonella were calculated to compare the added value of different sampling strategies. Results showed that more fecal samples than serological samples per group are needed to reach a group sensitivity >0.50. This also means that serological testing leads to a higher NPV. For example, with 10 negative test-results from a group of 25 animals in a herd with a suspicion of Salmonella, the NPV based on serology was 0.75 and based on culture was 0.56. For the PPV, testing based on culture from fecal sampling was superior, as specificity of such testing was close to perfect. By changing the threshold for considering a group positive, from 1 test-positive animal to 2, the PPV of serological results could be increased without substantial loss in NPV. The herd sensitivity based on (1) bulk milk sampling, (2) fecal sampling of all animals, and (3) bulk milk sampling and individual sera from 20 animals within each age group was 0.53, 0.88, and 0.95, respectively. In low-prevalence regions, this sensitivity was enough to verify a high probability of freedom (>0.99), as the probability of infection in such Swedish regions has been shown to be 0.01. For herds with a higher prior probability of infection, repeated sampling (2-9 sampling occasions) was needed to reach the same level of confidence. Analysis of surveillance data indicated that boot swabs can be used to replace the standard fecal sampling presently used in Sweden. It was also confirmed that the individual specificity of the tests used for serological testing of Swedish calves is high (0.99). The results can form a basis for fit-for-purpose testing strategies (e.g., surveillance or prepurchase testing).

Development and application of the compression frictional treatment method for sterilizing and valorizing organic waste

Confined animal feeding operations are producing large amounts of organic waste with large fractions being mixtures of manure and sawdust. On one hand, the inhomogeneity of the material and the high water content restrict the optimal application of thermal treatment methods. On the other hand, the high concentration of bacteria in these mixed waste streams is an issue of concern. This study introduces a novel process for treating manure and woody biomass mixtures and upgrading them into valuable products. It is defined as compression frictional treatment (CFT) and takes place continuously in a rotary compression unit. Fresh poultry droppings and fresh anaerobic digester offal from a cattle feedlot were processed by means of CFT. Moisture was decreased by 52% and heating value was increased by 27% for treated poultry litter. The corresponding values were 63% moisture decrease and 25% increase for the heating value of cattle manure. On the aspect of bacteria reduction, the Escherichia coli and the Enterobacteriaceae family of bacterium were reduced by 94% and 91%, respectively.

A questionnaire study of associations between potential risk factors and salmonella status in Swedish dairy herds

In this study associations between potential risk factors and salmonella status in Swedish dairy herds were investigated. A case-control study design was used, including existing as well as new cases. Herds were assigned a salmonella status on the basis of antibody analysis of bulk milk samples. Information on potential risk factors was collected from registry data and from farmers via a questionnaire. Univariable and multivariable logistic regression analyses were used to investigate associations between salmonella status and potential risk factors. In addition, multivariate analysis with Additive Bayesian Network (ABN) modelling was performed to improve understanding of the complex relationship between all the variables. Because of the difficulty in identifying associations between potential risk factors and infections with low prevalence and a large regional variation, exposure of potential risk factors in the high-prevalence region (Öland) were compared to exposure in other regions in Sweden. In total 483 of 996 (48%) farmers responded to the questionnaire, 69 herds had test-positive bulk milk samples. The strongest association with salmonella status was 'presence of salmonella test-positive herds <5 km' (OR 4.3, 95% CI 2.0-9.4). Associations with salmonella status were also seen between 'feeding calves residue milk only' (OR 2.4, 95% CI 1.2-4.6), 'certified organic herds' (OR 2.5, 95% CI 1.2-4.9) and 'frequently seeing signs of rodents' (OR 0.4, 95% CI 0.13-0.97). The ABN model showed associations between Öland and four of the variables: salmonella status, presence of test-positive herds <5km, shared pastures and providing protective clothing for visitors. The latter is probably a reflection of increased disease awareness in Öland. The ABN model showed associations between herd size and housing as well as several management procedures. This provides an explanation why herd size frequently has been identified as a risk factor for salmonella by other studies. The study confirms the importance of local transmission routes for salmonella, but does not identify specific components in this local spread. Therefore, it supports the use of a broad biosecurity approach in the prevention of salmonella. In Öland, some potential risk factors are more common than in other parts of Sweden. Theoretically these could contribute to the spread of salmonella, but this was not confirmed in the present study. The study also highlights the difficulty in identifying associations between potential risk factors and infections with low prevalence and large regional variation.

Subsurface Transport of in Soils of Wisconsin's Carbonate Aquifer Region

is a waterborne pathogen known to have a significant reservoir in bovine manure. Land-dependent manure disposal may not result in significant or reliable pathogen attenuation and, therefore, presents a risk for transport of pathogenic spp. to groundwater. One factor missing in the existing literature is the role soil characteristics play in affecting oocyst transport. Of specific concern in regions with carbonate geology are macropores and other soil structures that contribute to preferential flow. Therefore, research is needed to understand soil type effects and important transport pathways for pathogens such as oocysts to drinking water wells. This study investigated transport potential in several soils overlying Wisconsin's vulnerable carbonate aquifer and related the soil transport to soil series and textural class. Experimental work involved monitoring the transport of oocysts through intact soil columns of different soil series under simulated rain conditions. Results demonstrate that a significant portion of oocysts will sorb or be physically entrapped in the soil, especially in soil with high clay content. However, silt loam soils with comparatively lower clay content demonstrated an ability to transport oocysts through the soil profile primarily via the first flush of water infiltrating through soil macropores. The rate of oocyst migration in silt loam soils paralleled the bromide tracer front, thus bypassing the soils' ability to strain or adsorb oocysts out of infiltrating water. Nevertheless, proper manure treatment and management are necessary to minimize public health risks.

Epidemiology of Salmonella sp. in California cull dairy cattle: prevalence of fecal shedding and diagnostic accuracy of pooled enriched broth culture of fecal samples

BACKGROUND

The primary objective of this cross-sectional study was to estimate the crude, seasonal and cull-reason stratified prevalence of Salmonella fecal shedding in cull dairy cattle on seven California dairies. A secondary objective was to estimate and compare the relative sensitivity (Se) and specificity (Sp) for pools of 5 and 10 enriched broth cultures of fecal samples for Salmonella sp. detection.

METHODS

Seven dairy farms located in the San Joaquin Valley of California were identified and enrolled in the study as a convenience sample. Cull cows were identified for fecal sampling once during each season between 2014 and 2015, specifically during spring, summer, fall, and winter, and 10 cows were randomly selected for fecal sampling at the day of their sale. In addition, study personnel completed a survey based on responses of the herd manager to questions related to the previous four month's herd management. Fecal samples were frozen until testing for Salmonella. After overnight enrichment in liquid broth, pools of enrichment broth (EBP) were created for 5 and 10 samples. All individual and pooled broths were cultured on selective media with putative Salmonella colonies confirmed by biochemical testing before being serogrouped and serotyped.

RESULTS

A total of 249 cull cows were enrolled into the study and their fecal samples tested for Salmonella. The survey-weighted period prevalence of fecal shedding of all Salmonella sp. in the cull cow samples across all study herds and the entire study period was 3.42% (N = 249; SE 1.07). The within herd prevalence of Salmonella shed in feces did not differ over the four study seasons (P = 0.074). The Se of culture of EBP of five samples was 62.5% (SE = 17.12), which was not statistically different from the Se of culture of EBP of 10 (37.5%, SE = 17.12, P = 0.48). The Sp of culture of EBP of five samples was 95.24% (SE = 3.29) and for pools of 10 samples was 100.00% (SE = 0). There was no statistical difference between the culture relative specificities of EBP of 5 and 10 (P > 0.99).

DISCUSSION

Our study showed a numerically higher prevalence of Salmonella shedding in the summer, although the results were not significant, most likely due to a lack of power from the small sample size. A higher prevalence in summer months may be related to heat stress. To detect Salmonella, investigators may expect a 62.5% sensitivity for culture of EBP of five, relative to individual fecal sample enrichment and culture. In contrast, culture of EBP of 10 samples resulted in a numerically lower Se. Culture of EBP of size 5 or 10 samples, given similar prevalence and limit of detection, can be expected to yield specificities of 95 and 100%, respectively.

Fecal prevalence, serotype distribution and antimicrobial resistance of Salmonellae in dairy cattle in central Ethiopia

Background

Salmonellae are major worldwide zoonotic pathogens infecting a wide range of vertebrate species including humans. Consumption of contaminated dairy products and contact with dairy cattle represent a common source of non-typhoidal Salmonella infection in humans. Despite a large number of small-scale dairy farms in Addis Ababa and its surrounding districts, little is known about the status of Salmonella in these farms.

Results

Salmonella was recovered from the feces of at least one animal in 7.6 % (10/132) of the dairy farms. Out of 1203 fecal samples examined, 30 were positive for Salmonella resulting in a weighted animal level prevalence of 2.3 %. Detection of diarrhea in an animal and in a farm was significantly associated with animal level (p = 0.012) and herd level (p < 0.001) prevalence of Salmonella. Animal level prevalence of Salmonella was significantly associated with age (p = 0.023) and study location; it was highest among those under 6 months of age and in farms from Adaa district and Addis Ababa (p < 0.001). Nine different serotypes were identified using standard serological agglutination tests. The most frequently recovered serotypes were Salmonella Typhimurium (23.3 %), S. Saintpaul (20 %), S. Kentucky (16.7 %) and S. Virchow (16.7 %). All isolates were resistant or intermediately resistant to at least one of the 18 drugs tested. Twenty-six (86.7 %), 19 (63.3 %), 18 (60 %), 16 (53.3 %) of the isolates were resistant to streptomycin, nitrofurantoin, sulfisoxazole and tetracycline , respectively. Resistance to 2 drugs was detected in 27 (90 %) of the isolates. Resistance to 3 or more drugs was detected in 21 (70 %) of the isolates, while resistance to 7 or more drugs was detected in 11 (36.7 %) of the isolates. The rate of occurrence of multi-drug resistance (MDR) in Salmonella strains isolated from dairy farms in Addis Ababa was significantly higher than those isolated from farms outside of Addis Ababa (p = 0.009). MDR was more common in S. Kentucky, S. Virchow and S. Saintpaul.

Conclusion

Isolation of Salmonella serotypes commonly known for causing human salmonellosis that are associated with an MDR phenotype in dairy farms in close proximity with human population is a major public health concern. These findings imply the need for a strict pathogen reduction strategy.

Within-Farm Changes in Dairy Farm-Associated Salmonella Subtypes and Comparison to Human Clinical Isolates in Michigan, 2000-2001 and 2009

Temporal changes in the distribution of Salmonella subtypes in livestock populations may have important impacts on human health. The first objective of this research was to determine the within-farm changes in the population of subtypes of Salmonella on Michigan dairy farms that were sampled longitudinally in 2000-2001 and again in 2009. The second objective was to determine the yearly frequency (2001 through 2012) of reported human illnesses in Michigan associated with the same subtypes. Comparable sampling techniques were used to collect fecal and environmental samples from the same 18 Michigan dairy farms in 2000-2001 and 2009. Serotypes, multilocus sequence types (STs), and pulsed-field gel electrophoresis (PFGE) banding patterns were identified for isolates from 6 farms where >1 Salmonella isolate was recovered in both 2000-2001 and 2009. The distribution of STs was significantly different between time frames (P < 0.05); only two of 31 PFGE patterns were identified in both time frames, and each was recovered from the same farm in each time frame. Previously reported within-farm decreases in the frequency of multidrug-resistant (MDR) Salmonella were due to recovery of MDR subtypes of S. enterica serotypes Senftenberg and Typhimurium in 2000-2001 and genetically distinct, pansusceptible subtypes of the same serotypes in 2009. The annual frequency of human illnesses between 2001 and 2012 with a PFGE pattern matching a bovine strain decreased for patterns recovered from dairy farms in 2000-2001 and increased for patterns recovered in 2009. These data suggest important changes in the population of Salmonella on dairy farms and in the frequency of human illnesses associated with cattle-derived subtypes.

Survival of Salmonella enterica in aerated and nonaerated wastewaters from dairy lagoons

Salmonella is the most commonly identified foodborne pathogen in produce, meat and poultry. Cattle are known reservoirs of Salmonella and the pathogen excreted in feces ends up in manure flush lagoons. Salmonella enterica survival was monitored in wastewater from on-site holding lagoons equipped or not with circulating aerators at two dairies. All strains had poor survival rates and none proliferated in waters from aerated or settling lagoons. Populations of all three Salmonella serovars declined rapidly with decimal reduction times (D) of <2 days in aerated microcosms prepared from lagoon equipped with circulators. Populations of Salmonella decreased significantly in aerated microcosms (D = 4.2 d) compared to nonaerated waters (D = 7.4 d) and in summer (D = 3.4 d) compared to winter (D = 9.0 d). We propose holding the wastewater for sufficient decimal reduction cycles in lagoons to yield pathogen-free nutrient-rich water for crop irrigations and fertilization.

Short communication: Survey of animal-borne pathogens in the farm environment of 13 dairy operations

A survey was conducted on 13 dairies to determine the occurrence of 5 animal-borne pathogens (Salmonella enterica, Escherichia coli O157:H7, Campylobacter jejuni, Mycobacterium avium ssp. paratuberculosis, and Cryptosporidium parvum) and their distributions across farm elements (feces, bedding, milk filters, stored manure, field soil, and stream water). Presence of C. parvum was measured only in feces and stored manure. All but one farm were positive for at least one pathogen species, and 5 farms were positive for 3 species. Escherichia coli O157:H7 was detected on 6 farms and in all farm elements, including milk filters. Mycobacterium avium ssp. paratuberculosis was detected on 10 of 13 farms and in all farm elements except for milk filters. Salmonella enterica and C. jejuni were detected at lower frequencies and were not identified in soil, stream water, or milk filters on any of the 13 farms. Cryptosporidium parvum was detected in feces but not in stored manure. Stored manure had the highest occurrence of pathogens (73%), followed by feces (50%), milk filters, bedding, soil, and water (range from 23 to 31%). Association of pathogen presence with farm management factors was examined by t-test; however, the small number of study farms and samples may limit the scope of inference of the associations. Pathogens had a higher prevalence in maternity pen bedding than in calf bedding, but total pathogen occurrence did not differ in calf compared with lactating cow feces or in soils with or without manure incorporation. Herd size and animal density did not appear to have a consistent effect on pathogen occurrence. The extent of pathogen prevalence and distribution on the farms indicates considerable public health risks associated with not only milk and meat consumption and direct animal contact, but also potential dissemination of the pathogens into the agroecosystem.

An evaluation of microbial health risks to livestock fed with wastewater-irrigated forage crops

This paper presents the results of five experiments in which animal health risks associated with the consumption of crops irrigated with domestic wastewater were evaluated. Forage maize and Tanner grass were irrigated with treated wastewater and used in goats and calves feeding trials. The irrigated crops presented high levels of surface contamination with E. coli (10(4) -10(7) 25 g(-1) ) and salmonellae (up to 1.6 × 10(4) 25 g(-1)), but none of the animals showed signs of infection or of disease. Further, the microbiological quality of animal products always complied with the Brazilian and European Union standards for food safety. It is suggested that the WHO guideline values for restricted irrigation (≤ 10(4) E. coli 100 ml(-1) and ≤ 1 helminth egg l(-1)), which were developed to protect the health of agricultural field workers, would be equally protective of the health of both animals fed with wastewater-irrigated crops and humans consuming products from such animals.

Time-to-event analysis of predictors for recovery from Salmonella Dublin infection in Danish dairy herds between 2002 and 2012

Salmonella Dublin infections reduce gross margins and compromise animal health and welfare in dairy cattle herds. Despite on-going control efforts in several countries the duration and risk factors of a persistent infection have been difficult to study due to a lack of suitable data. This study utilised the unique opportunity to extract systematically collected repeated bulk-tank milk antibody measurements from all the Danish dairy herds during a 10-year period to perform a time-to-event analysis of the factors that affect the duration of test-positivity and the hazards of recovery from S. Dublin at herd level. Recovery was defined as a shift from test-positive to test-negative between two year-quarters followed by at least three more test-negative year-quarters. The average duration of infection was approximately 2 years. Predictors of recovery were tested in a multivariable Cox proportional hazard model allowing herds to recover from infection multiple times over the 10-year surveillance period. The model results were based on 36,429 observations with data on all the predictors, representing 3563 herds with a total of 3246 recoveries. Sixty-seven herds (2.4%) remained test-positive throughout the study period. The rest of the 317 herds that did not have any recoveries were censored, mainly due to a cessation of milk production. Prior recovery from test-positivity turned out not to be a significant predictor of recovery in the model. The effect of the duration of infection on the conditional probability of recovery (i.e. the hazard) was time-dependent: early in the study period, long durations of infection were predictive of a low hazard of recovery. Later in the control programme the effect of duration of infection was reduced indicating a desired effect of an intensified control programme. There was an increasing tendency towards longer durations and lower hazard of recovery with: (i) increasing herd sizes, (ii) increasing bulk-tank milk somatic cell counts, (iii) increasing local prevalence within a 5 km radius, (iv) organic farming and (v) recent purchase of cattle from test-positive herds. Participation in a voluntary paratuberculosis control programme reduced the duration of infection, and there were indications that recovery from S. Dublin infection was stimulated by a centrally organised and targeted control campaign. This is the first large-scale study that investigated duration of infection and predictors of recovery from S. Dublin in cattle herds over an extended period of time. The results provide useful knowledge for the design of control programmes for S. Dublin.

Microfluidic chip-based detection and intraspecies strain discrimination of Salmonella serovars derived from whole blood of septic mice

Salmonella is a zoonotic pathogen that poses a considerable public health and economic burden in the United States and worldwide. Resultant human diseases range from enterocolitis to bacteremia to sepsis and are acutely dependent on the particular serovar of Salmonella enterica subsp. enterica, which comprises over 99% of human-pathogenic S. enterica isolates. Point-of-care methods for detection and strain discrimination of Salmonella serovars would thus have considerable benefit to medical, veterinary, and field applications that safeguard public health and reduce industry-associated losses. Here we describe a single, disposable microfluidic chip that supports isothermal amplification and sequence-specific detection and discrimination of Salmonella serovars derived from whole blood of septic mice. The integrated microfluidic electrochemical DNA (IMED) chip consists of an amplification chamber that supports loop-mediated isothermal amplification (LAMP), a rapid, single-temperature amplification method as an alternative to PCR that offers advantages in terms of sensitivity, reaction speed, and amplicon yield. The amplification chamber is connected via a microchannel to a detection chamber containing a reagentless, multiplexed (here biplex) sensing array for sequence-specific electrochemical DNA (E-DNA) detection of the LAMP products. Validation of the IMED device was assessed by the detection and discrimination of S. enterica subsp. enterica serovars Typhimurium and Choleraesuis, the causative agents of enterocolitis and sepsis in humans, respectively. IMED chips conferred rapid (under 2 h) detection and discrimination of these strains at clinically relevant levels (<1,000 CFU/ml) from whole, unprocessed blood collected from septic animals. The IMED-based chip assay shows considerable promise as a rapid, inexpensive, and portable point-of-care diagnostic platform for the detection and strain-specific discrimination of microbial pathogens.

Intraspecies variation in the emergence of hyperinfectious bacterial strains in nature

Salmonella is a principal health concern because of its endemic prevalence in food and water supplies, the rise in incidence of multi-drug resistant strains, and the emergence of new strains associated with increased disease severity. Insights into pathogen emergence have come from animal-passage studies wherein virulence is often increased during infection. However, these studies did not address the prospect that a select subset of strains undergo a pronounced increase in virulence during the infective process- a prospect that has significant implications for human and animal health. Our findings indicate that the capacity to become hypervirulent (100-fold decreased LD₅₀) was much more evident in certain S. enterica strains than others. Hyperinfectious salmonellae were among the most virulent of this species; restricted to certain serotypes; and more capable of killing vaccinated animals. Such strains exhibited rapid (and rapidly reversible) switching to a less-virulent state accompanied by more competitive growth ex vivo that may contribute to maintenance in nature. The hypervirulent phenotype was associated with increased microbial pathogenicity (colonization; cytotoxin production; cytocidal activity), coupled with an altered innate immune cytokine response within infected cells (IFN-β; IL-1β; IL-6; IL-10). Gene expression analysis revealed that hyperinfectious strains display altered transcription of genes within the PhoP/PhoQ, PhoR/PhoB and ArgR regulons, conferring changes in the expression of classical virulence functions (e.g., SPI-1; SPI-2 effectors) and those involved in cellular physiology/metabolism (nutrient/acid stress). As hyperinfectious strains pose a potential risk to human and animal health, efforts toward mitigation of these potential food-borne contaminants may avert negative public health impacts and industry-associated losses.

Salmonellosis continues to compromise human health, animal welfare, and modern agriculture. Developing a comprehensive control plan requires an understanding of how pathogens emerge and express traits that confer increased incidence and severity of disease. It is well-established that animal passage often results in increased virulence; however, our findings indicate that the capacity to undergo a pronounced increase in virulence after passage was much more prevalent in certain Salmonella isolates than in others. The resultant hyperinfectious strains are among the most virulent salmonellae reported; were restricted to certain serotypes; and were able to override the immunity conferred in vaccinated animals. The induction of hypervirulence was responsive to subtle changes in environmental conditions and, potentially, may occur in other salmonellae serotypes after passage through certain hosts and/or exposure to certain environmental variables; a response that may be common across the microbial realm. Thus, management practices and environmental conditions inherent to livestock production have the potential to inadvertently trigger hypervirulence (e.g., diet; herd size; exposure to livestock waste and/or antimicrobials). From a farm management perspective, careful consideration must be given to risk-management strategies that reduce emergence/persistence of these potential food-borne contaminants to safeguard public health and reduce industry-associated losses.

Influence of aerobic and anaerobic conditions on survival of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium in Luria-Bertani broth, farm-yard manure and slurry

The influence of aerobic and anaerobic conditions on the survival of the enteropathogens Escherichia coli O157:H7 and Salmonella serovar Typhimurium was investigated in microcosms with broth, cattle manure or slurry. These substrates were inoculated with a green fluorescent protein transformed strain of the enteropathogens at 10(7) cells g(-1) dry weight. Survival data was fitted to the Weibull model. The survival curves in aerobic conditions generally showed a concave curvature, while the curvature was convex in anaerobic conditions. The estimated survival times showed that E. coli O157:H7 survived significantly longer under anaerobic than under aerobic conditions. Survival ranged from approximately. 2 weeks for aerobic manure and slurry to more than six months for anaerobic manure at 16 °C. On average, in 56.3% of the samplings, the number of recovered E. coli O157:H7 cells by anaerobic incubation of Petri plates was significantly (p < 0.05) higher in comparison with aerobic incubation. Survival of Salmonella serovar Typhimurium was not different between aerobic and anaerobic storage of LB broth or manure as well as between aerobic and anaerobic incubation of Petri dishes. The importance of changes in microbial community and chemical composition of manure and slurry was distinguished for the survival of E. coli O157:H7 in different oxygen conditions.

Efficacy of European starling control to reduce Salmonella enterica contamination in a concentrated animal feeding operation in the Texas panhandle

BACKGROUND

European starlings (Sturnus vulgaris) are an invasive bird species known to cause damage to plant and animal agriculture. New evidence suggests starlings may also contribute to the maintenance and spread of diseases within livestock facilities. Identifying and mitigating the risk pathways that contribute to disease in livestock is necessary to reduce production losses and contamination of human food products. To better understand the impact starlings have on disease transmission to cattle we assessed the efficacy of starling control as a tool to reduce Salmonella enterica within a concentrated animal feeding operation. We matched a large facility, slated for operational control using DRC-1339 (3-chloro-4-methylaniline hydrochloride, also 3-chloro p-toluidine hydrochloride, 3-chloro-4-methylaniline), with a comparable reference facility that was not controlling birds. In both facilities, we sampled cattle feed, cattle water and cattle feces for S. enterica before and after starling control operations.

RESULTS

Within the starling-controlled CAFO, detections of S. enterica contamination disappeared from feed bunks and substantially declined within water troughs following starling control operations. Within the reference facility, detections of S. enterica contamination increased substantially within feed bunks and water troughs. Starling control was not observed to reduce prevalence of S. enterica in the cattle herd. Following starling control operations, herd prevalence of S. enterica increased on the reference facility but herd prevalence of S. enterica on the starling-controlled CAFO stayed at pretreatment levels.

CONCLUSIONS

Within the starling-controlled facility detections of S. enterica disappeared from feed bunks and substantially declined within water troughs following control operations. Since cattle feed and water are obvious routes for the ingestion of S. enterica, starling control shows promise as a tool to help livestock producers manage disease. Yet, we do not believe starling control should be used as a stand alone tool to reduce S. enterica infections. Rather starling control could be used as part of a comprehensive disease management plan for concentrated animal feeding operations.

Impact of antibiotic use in adult dairy cows on antimicrobial resistance of veterinary and human pathogens: a comprehensive review

Antibiotics have saved millions of human lives, and their use has contributed significantly to improving human and animal health and well-being. Use of antibiotics in food-producing animals has resulted in healthier, more productive animals; lower disease incidence and reduced morbidity and mortality in humans and animals; and production of abundant quantities of nutritious, high-quality, and low-cost food for human consumption. In spite of these benefits, there is considerable concern from public health, food safety, and regulatory perspectives about the use of antimicrobials in food-producing animals. Over the last two decades, development of antimicrobial resistance resulting from agricultural use of antibiotics that could impact treatment of diseases affecting the human population that require antibiotic intervention has become a significant global public health concern. In the present review, we focus on antibiotic use in lactating and nonlactating cows in U.S. dairy herds, and address four key questions: (1) Are science-based data available to demonstrate antimicrobial resistance in veterinary pathogens that cause disease in dairy cows associated with use of antibiotics in adult dairy cows? (2) Are science-based data available to demonstrate that antimicrobial resistance in veterinary pathogens that cause disease in adult dairy cows impacts pathogens that cause disease in humans? (3) Does antimicrobial resistance impact the outcome of therapy? (4) Are antibiotics used prudently in the dairy industry? On the basis of this review, we conclude that scientific evidence does not support widespread, emerging resistance among pathogens isolated from dairy cows to antibacterial drugs even though many of these antibiotics have been used in the dairy industry for treatment and prevention of disease for several decades. However, it is clear that use of antibiotics in adult dairy cows and other food-producing animals does contribute to increased antimicrobial resistance. Although antimicrobial resistance does occur, we are of the opinion that the advantages of using antibiotics in adult dairy cows far outweigh the disadvantages. Last, as this debate continues, we need to consider the consequences of "what would happen if antibiotics are banned for use in the dairy industry and in other food-producing animals?" The implications of this question are far reaching and include such aspects as animal welfare, health, and well-being, and impacts on food quantity, quality, and food costs, among others. This question should be an important aspect in this ongoing and controversial debate.

Mathematical modeling of the transmission and control of foodborne pathogens and antimicrobial resistance at preharvest

Foodborne diseases are a significant health-care and economic burden. Most foodborne pathogens are enteric pathogens harbored in the gastrointestinal tract of farm animals. Understanding the transmission of foodborne pathogens and the dissemination of antimicrobial resistance at the farm level is necessary to design effective control strategies at preharvest. Mathematical models improve our understanding of pathogen dynamics by providing a theoretical framework in which factors affecting transmission and control of the pathogens can be explicitly considered. In this review, we aim to present the principles underlying the mathematical modeling of foodborne pathogens and antimicrobial resistance at the farm level to a broader audience.

Transmission dynamics of a multidrug-resistant Salmonella typhimurium outbreak in a dairy farm

Cattle are recognized as an important source of foodborne Salmonella causing human illness, particularly for antimicrobial-resistant strains. The transmission dynamics of multidrug-resistant (MDR) Salmonella after the onset of a clinical outbreak in a dairy farm has been rarely monitored. The early transmission of a pathogen influences the outbreak size and persistence of the pathogen at the farm level and, therefore, how long the herd represents a risk for Salmonella zoonotic transmission. The objective of this study was to describe the transmission dynamics of MDR Salmonella Typhimurium after the onset of a clinical outbreak in a dairy herd. For that purpose, fecal shedding and serological response to MDR Salmonella were monitored in a longitudinal study conducted in a dairy herd after a few cases of salmonellosis, and a stochastic transmission model was developed to predict Salmonella persistence at the pen level. The outbreak was limited to five clinical cases, and only 18 animals out of 500 cows shed Salmonella in feces. The longest shedder was culture-positive for Salmonella for at least 68 days. The isolates (n = 27) were represented by four pulsed-field gel electrophoresis patterns; three patterns were similar. With one exception, isolates were resistant to nine or more antimicrobial drugs. Simulations of the transmission model indicated that approximately 50% of the outbreaks were likely to die out within 20 days after the first animal was infected. The simulation studies indicated that salmonellosis outbreaks with few clinical cases were likely due to the extinction of the pathogen in the premises in the early phase of the outbreaks. Small population size and group structure within the farm decrease the on-farm persistence of the pathogen.

The incidence of salmonellosis among dairy herds in the northeastern United States

The objectives of this study were to estimate the incidence of salmonellosis among a large sample of dairy herds in the northeastern United States (both at the animal level and the herd level), to describe the serotypes and antimicrobial resistance profiles of the positive samples, and to determine whether various herd-level factors were important predictors of incidence. Participating veterinarians enrolled 831 dairy herds and submitted fecal samples from 2,565 female dairy cattle for Salmonella culture because of suspicion of clinical disease. Estimates of animal-level incidence rates were calculated for each age group as the number of cases per animal time at risk, and an estimate of herd-level incidence rate was calculated as the number of positive herds per herd time at risk. Descriptive analysis of serotype data and level of antimicrobial resistance was performed, and Poisson regression analysis was used to study associations between the within-herd incidence of salmonellosis and certain predictor variables (herd size, housing type, vaccination status, and prior history of Salmonella infection). Salmonella was isolated from 576 (22.5%) samples representing 93 herds. The animal-level incidence rates for preweaned female calves, heifers, and adult cows were 8.1, 0.04, and 1.8 cases per 1,000 animal-years, respectively. The herd-level incidence rate was 8.6 positive herds per 100 herd-years. Salmonella Newport was the predominant serotype, accounting for 41% of the cases, followed by Salmonella Typhimurium. Over 68% of all isolates were resistant to 5 or more antimicrobial agents. Herd size was the only significant predictor of the incidence of salmonellosis in a multivariable model; herds with at least 400 female dairy cattle had a higher incidence rate than smaller herds. Our results shed light on the impact of salmonellosis on the dairy industry in the northeastern United States, and they help clarify the role of dairy cattle as a source of Salmonella serotypes that are also important human pathogens.

Role of ceftiofur in selection and dissemination of blaCMY-2-mediated cephalosporin resistance in Salmonella enterica and commensal Escherichia coli isolates from cattle

Third-generation cephalosporin resistance of Salmonella and commensal Escherichia coli isolates from cattle in the United States is predominantly conferred by the cephamycinase CMY-2, which inactivates beta-lactam antimicrobial drugs used to treat a wide variety of infections, including pediatric salmonellosis. The emergence and dissemination of bla(CMY-2)(-)-bearing plasmids followed and may in part be the result of selection pressure imposed by the widespread utilization of ceftiofur, a third-generation veterinary cephalosporin. This study assessed the potential effects of ceftiofur on bla(CMY-2) transfer and dissemination by (i) an in vivo experimental study in which calves were inoculated with competent bla(CMY-2)-bearing plasmid donors and susceptible recipients and then subjected to ceftiofur selection and (ii) an observational study to determine whether ceftiofur use in dairy herds is associated with the occurrence and frequency of cephalosporin resistance in Salmonella and commensal E. coli. The first study revealed bla(CMY-2) plasmid transfer in both ceftiofur-treated and untreated calves but detected no enhancement of plasmid transfer associated with ceftiofur treatment. The second study detected no association (P = 0.22) between ceftiofur use and either the occurrence of ceftiofur-resistant salmonellosis or the frequency of cephalosporin resistance in commensal E. coli. However, herds with a history of salmonellosis (including both ceftiofur-resistant and ceftiofur-susceptible Salmonella isolates) used more ceftiofur than herds with no history of salmonellosis (P = 0.03) These findings fail to support a major role for ceftiofur use in the maintenance and dissemination of bla(CMY-2)-bearing plasmid mediated cephalosporin resistance in commensal E. coli and in pathogenic Salmonella in these dairy cattle populations.