Evaluation of enzyme-linked immunosorbent assay (ELISA) tests and culture for determining Salmonella status of a pig herd

Bayesian evaluation of meat juice ELISA for detecting Salmonella in slaughtered pigs without specifying a cut-off

BACKGROUND

Consumption of pork and pork products is a major source of human infection with Salmonella. Salmonella is typically subclinical in pigs, making it difficult to identify infected pigs. Therefore, effective surveillance of Salmonella in pigs critically relies on good knowledge on how well the diagnostic tests used perform. A test that has been used in several countries for Salmonella monitoring is serological testing of meat juice using an ELISA (MJ ELISA) to detect antibodies against Salmonella. This MJ ELISA data could be used to estimate infection prevalence and trends. However, as the MJ ELISA output is a sample-to-positive (S/P) ratio, which is a continuous outcome rather than a binary (positive/negative) result, the interpretation of this data depends upon a chosen cut-off.

AIM

To apply Bayesian latent class models (BLCMs) to estimate diagnostic accuracy of the MJ ELISA test values in the absence of a gold standard without needing to apply a cut-off.

METHODS AND RESULTS

BLCMs were fitted to data from a UK abattoir survey carried out in 2006 in order to estimate the diagnostic accuracy of MJ ELISA with respect to the prevalence of active Salmonella infection. This survey consisted of a MJ ELISA applied in parallel with the bacteriological testing of caecal contents, carcass swabs and lymph nodes (n = 625). A BLCM was also fitted to the same data but with dichotomisation of the MJ ELISA results, in order to compare with the model using continuous outcomes. Estimates were obtained for sensitivity and specificity of the ELISA over a range of S/P values and for the bacteriological tests and were found to be similar between the models using continuous and dichotomous ELISA outcomes.

CONCLUSION

The Bayesian method without specifying a cut-off does allow prevalence to be inferred without specifying a cut-off for the ELISA. The study results will be useful for estimating infection prevalence from serological surveillance data.

Building a predictive model for assessing the risk of Salmonella shedding at slaughter in fattening pigs

Salmonellosis continues to be a major cause of foodborne outbreaks worldwide, and pigs are one of the main sources of human infection. Salmonella pork contamination is a major concern for abattoirs and is related to the presence of Salmonella in pigs' feces at slaughter. Being able to predict the risk of Salmonella shedding in pigs arriving at the slaughterhouse could help mitigate abattoir and carcass contamination. For this purpose, 30 batches of 50 pigs each were selected from 30 different fattening units. The pigs were tagged and bled for the detection of antibodies against Salmonella approximately one month before slaughter. Pooled floor fecal samples were also collected from 10 pens per unit for Salmonella detection, and a questionnaire on biosecurity was administered to each farm. At the abattoir, colon content was collected from each tagged pig for the Salmonella shedding assessment. A predictive model for Salmonella shedding at slaughter was built with two-third of the pigs by employing random-effects logistic regression analysis, with Salmonella shedding as the dependent variable and pig serology and other farm/environmental characteristics as the independent variables. The model included farm as the grouping factor. Data from the remaining one-third of the pigs were used for model validation. Out of 1,500 pigs initially selected, 1,341 were identified at the abattoir and analyzed. Salmonella was detected in 13 (43.3%; 95%CI = 27.4-60.8) of the fattening units. The mean batch seroprevalence (cut-off OD% ≥40) among the fattening units was 31.7% (95%CI = 21.8-41.0), and a total of 316 pigs (23.6%; 95%CI = 21.4-25.9) shed Salmonella at slaughter. The model predicted reasonably well (Area under the curve = 0.76; P < 0.05) whether a pig would shed Salmonella at slaughter, with estimates of sensitivity and specificity at 71.6% and 73.6%, respectively. Serology, the percentage of Salmonella-positive pens on the farm, and the internal biosecurity score were significantly associated (P < 0.05) with Salmonella shedding at the abattoir, and several scenarios were observed by the model. The study highlighted that although serology may be helpful for identifying batches of pigs at risk of shedding Salmonella upon their arrival at the abattoir, it may not be necessary in some scenarios.

Immunochromatographic assay for the point-of-care diagnosis of food borne Salmonella strains using smartphone application

Salmonella strain is a prevalent pathogen, affecting poultry industry and hence human population around the world. Host-specific pathogen infections including fowl typhoid, pullorum disease and typhoid fever affects poultry birds, causing huge economic loss worldwide. This study explored the fabrication of immunochromatographic (ICG) strip by colorimetric method integrated with smartphone ColorGrab application for the detection of Salmonella using in-house generated antibodies (Abs) conjugated with gold nanoparticles. The developed point-of-care diagnostic platform was fabricated in-house and tested to detect the presence of Salmonella in a linear range of 10⁷-10⁰ CFU/mL with the limit of detection (LOD) of 10³, 10² and 10⁴ CFU/mL respectively, for Salmonella gallinarum (S.gal), Salmonella pullorum (S.pul) and Salmonella enteritidis (S.ent), which was further confirmed by smartphone-based ColorGrab application. The fabricated ICG strips were further validated using spiked fecal, meat, and milk samples which provided results in 10 mins with stability at 4 °C and 37 °C up to 28 days. Hence, the fabricated in-house ICG strip can be used as a portable, cost-effective diagnostic device for rapid detection of Salmonella strains in food samples.

Pilot Investigation of Anti-Salmonella Antibodies in Oral Fluids from Salmonella Typhimurium Vaccinated and Unvaccinated Swine Herds

Simple Summary

The consumption of pork meat is responsible for a significant number of outbreaks of salmonellosis in people. Surveillance in pig herds is constrained by the cost-effectiveness and efficiency of sampling methods. The last decade has seen significant advances in the routine use of pool samples, including oral fluids (OFs). This study aimed to investigate the OF collected passively via chewed sampling ropes as a potential sample type for assessing anti-Salmonella antibodies in two Salmonella-vaccinated (V) and two non-vaccinated (NV) farrow-to-finish pig farms, comparing the results with the Salmonella shedding of tested animals. Sows in the V farms were vaccinated prior to farrowing. Pooled faecal and OF samples were collected from sows and their offspring. Salmonella was isolated with direct bacteriological methods. A commercial ELISA assay was adapted to detect IgG and IgA antibodies in OF. Overall, a higher Salmonella prevalence was observed in the NV farm and in the offspring (76.3%) compared to sows (36.4%). The protocol used to test anti-Salmonella IgA in pig OF samples was found to lack sensitivity and specificity. At herd level, IgG is the most reliable isotype for monitoring Salmonella specific antibody via OF.

Abstract

Oral fluid (OF) can be a simple, cheap and non-invasive alternative to serum or meat juice for the diagnosis and surveillance of important pathogens in pigs. This study was conducted on four Salmonella Typhimurium-positive farrow-to-finish pig farms: two Salmonella-vaccinated (V) and two non-vaccinated (NV). Gilts and sows in the V farms were vaccinated with a live, attenuated vaccine prior to farrowing. Pooled faecal and OF samples were collected from the sows and their offspring. Salmonella was isolated according to ISO6579–1:2017. In parallel, IgG and IgA levels were assessed in OF samples using a commercial ELISA assay. Salmonella was detected in 90.9% of the pooled faecal samples from the NV farms and in 35.1% of the pooled faecal samples from the V farms. Overall, a higher prevalence was observed in the pooled faecal samples from the offspring (76.3%) compared to the sows (36.4%). IgG antibodies measured in V farms are likely to be related to vaccination, as well as exposure to Salmonella field strains. The detection of IgA antibodies in OF was unreliable with the method used. The results of this study show that IgG is the most reliable isotype for monitoring Salmonella-specific antibody immunity in vaccinated/infected animals via OF.

Modernization of Control of Pathogenic Micro-Organisms in the Food-Chain Requires a Durable Role for Immunoaffinity-Based Detection Methodology-A Review

Food microbiology is deluged by a vastly growing plethora of analytical methods. This review endeavors to color the context into which methodology has to fit and underlines the importance of sampling and sample treatment. The context is that the highest risk of food contamination is through the animal and human fecal route with a majority of foodborne infections originating from sources in mass and domestic kitchens at the end of the food-chain. Containment requires easy-to-use, failsafe, single-use tests giving an overall risk score in situ. Conversely, progressive food-safety systems are relying increasingly on early assessment of batches and groups involving risk-based sampling, monitoring environment and herd/flock health status, and (historic) food-chain information. Accordingly, responsible field laboratories prefer specificity, multi-analyte, and high-throughput procedures. Under certain etiological and epidemiological circumstances, indirect antigen immunoaffinity assays outperform the diagnostic sensitivity and diagnostic specificity of e.g., nucleic acid sequence-based assays. The current bulk of testing involves therefore ante- and post-mortem probing of humoral response to several pathogens. In this review, the inclusion of immunoglobulins against additional invasive micro-organisms indicating the level of hygiene and ergo public health risks in tests is advocated. Immunomagnetic separation, immunochromatography, immunosensor, microsphere array, lab-on-a-chip/disc platforms increasingly in combination with nanotechnologies, are discussed. The heuristic development of portable and ambulant microfluidic devices is intriguing and promising. Tant pis, many new platforms seem unattainable as the industry standard. Comparability of results with those of reference methods hinders the implementation of new technologies. Whatever the scientific and technological excellence and incentives, the decision-maker determines this implementation after weighing mainly costs and business risks.

Influence of colostrum supply on Salmonella spp. seroprevalence in piglet rearing and possibilities to increase colostrum production by optimised feeding

A commercial farm study investigated whether colostrum production and antibody transfer can be improved by extra feeding in late pregnancy sows, and whether such improvements have an influence on disease status (measured as Salmonella spp. seroprevalence) in piglets, using a rapid and cost effective, ELISA-based estimation method (IDEXX Swine Salmonella spp. Ab Test). Four farms with established high Salmonella spp. seroprevalence were selected, and 16 sows in each farrowing group were selected over six farrowing cycles for the feeding experiment (n=368). One half (n=184) of the sows were fed a conventional feed following official nutrient recommendations concerning energy, amino acids and minerals when they entered the farrowing pen. The other half (n=184) received 1.25 kg of a test feed daily containing fibre rich cereals (barley, oats), a fibre component (soy husks) and potassium diformate in addition to the conventional feed until two days post-partum. Blood samples were taken from two light and two heavy piglets aged 2 d in each litter (n=1,469) and at the end of rearing (approximately nine weeks old; average body weight of 25 kg; n=588). In the test-fed group, piglet immunocrit value (as a measure of immunoglobulin transfer) was significantly improved (P<0.0219; K:0.1226±0.0437; V: 0.1278±0.0406). A moderate correlation (r=0.40935; P<0.0001) was found between immunocrit value and Salmonella spp. antibodies at 2 d old. There was no correlation between immunocrit value and Salmonella spp. antibodies at the end of the rearing period (r=0.09914), when the pigs were around nine weeks of age. Despite better colostrum supply, the animals in the test group did not show a significantly lower prevalence of Salmonella spp. seroprevalence (test group optical density (OD) 9.8000±17.4954%; control group OD 8.9486±14.2426%; P<0.5344) at nine weeks of age. It could be shown that the colostrum supply can be optimised by providing sows with extra feed. The moderate correlation between immunocrit and Salmonella spp. antibodies on the second day of life suggests that measuring antibodies by rapid, cost effective ELISA could be a practical tool to for the estimation of colostrum supply and the corresponding health of piglets. A suspected effect of reduced colostrum supply on the Salmonella spp. seroprevalence at the end of piglet rearing was not detectable. Other effects (e.g. hygiene) seem to be more significant.

Imaging Inflammation and Infection in the Gastrointestinal Tract

A variety of seemingly non-specific symptoms manifest within the gastrointestinal (GI) tract, particularly in the colon, in response to inflammation, infection, or a combination thereof. Differentiation between symptom sources can often be achieved using various radiologic studies. Although it is not possible to provide a comprehensive survey of imaging gastrointestinal GI tract infections in a single article, the purpose of this review is to survey several topics on imaging of GI tract inflammation and infections. The review discusses such modalities as computed tomography, positron emission tomography, ultrasound, endoscopy, and magnetic resonance imaging while looking at up-an-coming technologies that could improve diagnoses and patient comfort. The discussion is accomplished through examining a combination of organ-based and organism-based approaches, with accompanying selected case examples. Specific focus is placed on the bacterial infections caused by Shigella spp., Escherichia coli, Clostridium difficile, Salmonella, and inflammatory conditions of diverticulitis and irritable bowel disease. These infectious and inflammatory diseases and their detection via molecular imaging will be compared including the appropriate differential diagnostic considerations.

Longitudinal study describing time to Salmonella seroconversion in piglets on three farrow-to-finish farms

BACKGROUND

Pigs are frequently colonised with Salmonella enterica, and this constitutes a major risk for human salmonellosis. The infection can be assessed by the serological response of pigs to S enterica. A longitudinal study was undertaken on-farm to correctly describe this serological response and investigate factors associated with age at Salmonella seroconversion.

METHODS

Three pig farms and in each farm three successive batches were considered. Per batch, 40 piglets were selected at random from 10 sows (four piglets per sow). Blood was sampled from sows one week after farrowing and from piglets at weeks 1, 6, 10, 14, 18 and 22 and at the slaughterhouse. Salmonella antibodies were detected in serum using a commercial ELISA test. Factors related to farm characteristics, batch management system, porcine reproductive and respiratory syndrome infection, and sows' Salmonella serological status were recorded to assess their effect on age at seroconversion.

RESULTS

At week 1 after farrowing, 96.5 per cent of the sows had antibodies against Salmonella. The serological results of piglets at weeks 1 and 6 only were positively correlated with those of the sows. The average age at Salmonella seroconversion was 137±2.2 days (confidence interval at 95 per cent). The first seroconversions occurred from weeks 10 to 14, but most of the pigs (54.6 per cent) were seropositive at the end of the fattening period, with variations between farms and batches (28.9-75.7 per cent). Herd/farm was significantly associated with age at seroconversion.

CONCLUSION

This longitudinal study allowed the authors to follow precisely the evolution of Salmonella seroconversion from maternity to slaughterhouse and confirm the relationship between the seroconversion of sows and serology of their piglets. Moreover, factors related to farm practices and management as a whole are more influential than individual factors (at the pig level) on age at Salmonella seroconversion.

Looking for new approaches for the use of serology in the context of control programmes against pig salmonellosis

Most swine Salmonella national control programmes in Europe have been based on the categorization of herds according to risk levels based on serological results. However, none of the non-Scandinavian countries have reported of any significant success on Salmonella infection reduction in fattening pigs or the number of human cases attributable to pigs or pork. The limited accuracy of the tests used, the small number of animals sampled and the likely lack of herd representativeness of the samples used could be major factors affecting the suitability of these programmes. Focusing on minimizing Salmonella shedding at slaughter appears more important to prevent human infections than focusing on detection of seropositive pigs/herds at this stage. This study assessed whether performing on-farm serology may help to predict shedding at slaughter. Between 2010 and 2016, pigs from six cohorts from a Salmonella-positive herd were bled at 30, 60 and 90 days on fattening and before slaughter, and faecal samples collected at slaughter. Serology on days 60, 90 and before slaughter predicted somewhat shedding at slaughter with no significant differences among them. Pigs with higher OD% values at these point times would have higher risk of shedding when arriving to slaughter. The probability of shedding for a pig sampled on day 90 and showing an OD% value of 10 was 43%, and the risk increased up to 65% if the OD% was 40. Concluding, on-farm serology may help to determine to some extent the risk of Salmonella shedding at slaughter from seropositive fattening units, which would allow for prompt on-farm and slaughter interventions to reduce the likelihood of slaughter contamination with Salmonella.

Transmission parameters estimated for Salmonella typhimurium in swine using susceptible-infectious-resistant models and a Bayesian approach

Background

Transmission models can aid understanding of disease dynamics and are useful in testing the efficiency of control measures. The aim of this study was to formulate an appropriate stochastic Susceptible-Infectious-Resistant/Carrier (SIR) model for Salmonella Typhimurium in pigs and thus estimate the transmission parameters between states.

Results

The transmission parameters were estimated using data from a longitudinal study of three Danish farrow-to-finish pig herds known to be infected. A Bayesian model framework was proposed, which comprised Binomial components for the transition from susceptible to infectious and from infectious to carrier; and a Poisson component for carrier to infectious. Cohort random effects were incorporated into these models to allow for unobserved cohort-specific variables as well as unobserved sources of transmission, thus enabling a more realistic estimation of the transmission parameters. In the case of the transition from susceptible to infectious, the cohort random effects were also time varying. The number of infectious pigs not detected by the parallel testing was treated as unknown, and the probability of non-detection was estimated using information about the sensitivity and specificity of the bacteriological and serological tests. The estimate of the transmission rate from susceptible to infectious was 0.33 [0.06, 1.52], from infectious to carrier was 0.18 [0.14, 0.23] and from carrier to infectious was 0.01 [0.0001, 0.04]. The estimate for the basic reproduction ration (R₀) was 1.91 [0.78, 5.24]. The probability of non-detection was estimated to be 0.18 [0.12, 0.25].

Conclusions

The proposed framework for stochastic SIR models was successfully implemented to estimate transmission rate parameters for Salmonella Typhimurium in swine field data. R₀ was 1.91, implying that there was dissemination of the infection within pigs of the same cohort. There was significant temporal-cohort variability, especially at the susceptible to infectious stage. The model adequately fitted the data, allowing for both observed and unobserved sources of uncertainty (cohort effects, diagnostic test sensitivity), so leading to more reliable estimates of transmission parameters.

The use of meat juice or blood serum for the diagnosis of Salmonella infection in pigs and its possible implications on Salmonella control programs

Serology is the method of choice for country-scale Salmonella control programs in pigs and can be carried out both on blood serum or meat juice. However, the diagnostic performance of enzyme-linked immunosorbent assays (ELISAs) on these sample matrices has not been sufficiently compared. The agreement between the serum ELISA and meat juice ELISA on samples taken from commercial farms was assessed in 2 pig populations (adult sows and finishers). Results of optical density percentage (OD%) from the serum ELISA were consistently higher than those from the meat juice ELISA (38.5 vs. 27.9; P<0.001), and the mean difference between them was significantly different from zero (P<0.0001). The overall correlation coefficient between serum ELISA and meat juice ELISA results was low (r=0.53). These results indicated an important disagreement between ELISA performed on serum and meat juice matrices and suggested that before implementing a control program to reduce the prevalence of Salmonella in swine the choice of matrix on which to perform the ELISA should be carefully considered.

Comparison of bacterial culture, polymerase chain reaction, and a mix-enzyme-linked immunosorbent assay for the detection of Salmonella status in grow-to-finish pigs in western Canada with a Bayesian approach

Among grow-to-finish pigs from 10 herds in Alberta and Saskatchewan, 23 (16%) of 144 fecal samples were culture-positive and 40 (28%) of 144 pigs were seropositive for Salmonella. With a Bayesian model specifying dependence between the 2 tests, the sensitivity (Se) of culture and real-time polymerase chain reaction (RT-PCR) was 79% to 86%, depending on the cut-off value for the enzyme-linked immunosorbent assay (ELISA). Culture specificity (Sp) was assumed to be 100%; RT-PCR Sp was found to be 94%. The ELISA Se was 76% and 51% at optical density cut-off values ≥ 20% and ≥ 40%, respectively; the Sp was 94% at each cut-off value. The model showed some sensitivity to ELISA prior information, the ELISA Se being approximately 8% lower when informative prior information was specified in the model. When there was no adjustment for dependence between culture and RT-PCR, the posterior estimates for both culture and RT-PCR Se were 11% higher than with the conditional-dependence model and had considerably narrower probability intervals, which suggests that correlation between culture and PCR is important and should be adjusted for in future studies.

Salmonella Typhimurium LPS mutations for use in vaccines allowing differentiation of infected and vaccinated pigs

Contaminated pork is a major source of human salmonellosis and the serovar most frequently isolated from pigs is Salmonella Typhimurium. Vaccination could contribute greatly to controlling Salmonella infections in pigs. However, pigs vaccinated with the current vaccines cannot be discriminated from infected pigs with the LPS-based serological tests used in European Salmonella serosurveillance programmes. We therefore examined which LPS encoding genes of Salmonella Typhimurium can be deleted to allow differentiation of infected and vaccinated pigs (DIVA), without affecting the vaccine strain's protective capacity. For this purpose, deletion mutants in Salmonella strain 112910a, used as vaccine strain, were constructed in the LPS encoding genes: ΔrfbA, ΔrfaL, ΔrfaJ, ΔrfaI, ΔrfaG and ΔrfaF. Primary inoculation of BALB/c mice with the parent strain, ΔrfaL, ΔrfbA or ΔrfaJ strain but not the ΔrfaG, ΔrfaF or ΔrfaI strain protected significantly against subsequent infection with the virulent Salmonella Typhimurium strain NCTC12023. Immunization of piglets with the ΔrfaJ or ΔrfaL mutants resulted in the induction of a serological response lacking detectable antibodies against LPS. This allowed a clear differentiation between sera from pigs immunized with the ΔrfaJ or ΔrfaL strains and sera from pigs infected with their isogenic wild type strain. In conclusion, applying deletions in the rfaJ or the rfaL gene in Salmonella Typhimurium strain 112910a allows differentiation of infected and vaccinated pigs in an LPS based ELISA without reducing the strain's protective capacities in mice.

Evaluation of three commercial enzyme-linked immunosorbent assays for the detection of antibodies against Salmonella spp. in meat juice from finishing pigs in Spain

The control of animal salmonellosis is considered as a major objective in Europe and indirect ELISAs will be important tools for the implementation of control programs for this infection in pigs. We analyse the results yielded by three commercial ELISAs (Herdcheck Swine Salmonella, SALMOTYPE Pig Screen, and PrioCHECK Salmonella) on meat juice samples from a population of slaughter pigs of Aragon, NW Spain, to assess their efficacy using traditional and latent-class approaches. Overall, the Herdcheck Swine Salmonella detected more Salmonella-infected pigs than the other two tests, but its relative sensitivity was low (65.9%). A similar result was observed when only serotypes detectable by this test were considered (69.1%). When a Bayesian approach was used the Herdcheck Swine Salmonella showed also the highest overall accuracy (sensitivity = 88% and specificity = 74%). Our results suggest that a relatively small proportion of the observed prevalence in herds would be explained by using these ELISAs. Also, this study points out that when different ELISA tests are used within the same herd, results may differ substantially. Thus, caution is advised if it is decided to use these assays for herd health classification in Spanish Salmonella control programs.

Tracking the Salmonella status of pigs and pork from lairage through the slaughter process in the Republic of Ireland

Salmonella Typhimurium is the predominant serotype isolated from humans in Europe. Pork and pork products are recognized vehicles of Salmonella and are responsible for outbreaks of human salmonellosis. Pigs can become infected with Salmonella on the breeding or fattening farm and during transport, lairage, and slaughter. The aim of this study was to investigate selected points of Salmonella contamination from the time pigs entered the lairage to the time the carcass was processed in the boning hall and to determine the importance of different sources of Salmonella along the Irish pork production chain. A second objective was to evaluate whether the serological status or category of a herd influenced the levels of bacteriological contamination detected on individual carcasses and pork cuts during slaughter and dressing operations. All samples were tested for the presence and numbers of Salmonella. Enterobacteriaceae numbers were also determined. Serotype, phage type, and pulsed-field gel electrophoresis were utilized to determine similarity among Salmonella isolates. Lairage was a major source of cross-contamination with Salmonella as were the hands of evisceration operatives, conveyor belts, and equipment in the boning hall. Cross-contamination within the slaughter plant environment accounted for up to 69 % of Salmonella carcass contamination. In general, herd category reflected the bacteriological status of carcasses and pork cuts. Major findings were a strong association (P < 0.01) between Enterobacteriaceae counts and Salmonella occurrence on prechill carcasses and a significant association (P < 0.05) between Enterobacteriaceae counts and Salmonella occurrence on pork cut samples.

An appraisal of the use of meat-juice serology monitoring data for estimating prevalence of cecal Salmonella carriage of pigs at slaughter by means of herd-level and animal-level simulation

Some attempts have been made to elucidate the association between positive serology and Salmonella detection by bacterial culture in individual pigs and pig herds. This study aimed to appraise whether the existing knowledge on such association provides grounds for the utilization of serology monitoring data for predicting Salmonella subclinical infection of pigs entering the abattoir. Serology test results of pig carcasses (taken at abattoirs) originating from 436 representative active herds in Ireland were utilized to estimate the overall cecal Salmonella carriage of Irish slaughter pigs. To this effect, two separate simulations were conducted using (i) herd-level regression data and (ii) animal-level sensitivity (0.2890) and specificity (0.8895) data, which were extracted from published articles. The herd-level approach estimated a moderate prevalence of cecal Salmonella carriage of 0.222 (sigma = 0.094; 95% confidence interval [CI]: 0.069 to 0.431), which matched closely the mean prevalence value from the surveys' validation data of Salmonella-positive cecal samples (n = 1,098) obtained at Irish abattoirs (0.215; 95% CI: 0.192 to 0.240). The animal-level simulation generated an output distribution with slightly more uncertainty (sigma = 0.102 and 95% CI: 0.146 to 0.537) and a higher estimate of cecal carriage (0.312), which was an effect of the low relative sensitivity of serology, common under field conditions. While the herd-level simulation appeared to be technically more appropriate, since its correlation is only moderate, further elucidation of other factors related to subclinical infection should be attained for their incorporation in prospective dynamic on-farm models, which would be useful in the ultimate goal of estimating the risk of carcass contamination during slaughter.

Longitudinal study of Salmonella infection in Italian farrow-to-finish swine herds

A longitudinal study of Salmonella enterica infection was carried out in five Italian farrow-to-finish swine herds previously known to be infected by Salmonella. Five litters were randomly selected from each herd and in each litter six piglets were randomly selected and individually identified. Thus, the study included 30 pigs from each farm. At weaning, individual blood samples were collected for serological examination from all selected piglets and on the same day from all sows in the farrowing unit. Piglets were bled again at approximately 60, 90, 150, 210 and 270 days of life whereas the last blood sample was collected at slaughtering. In one of the herds, in which the duration of productive cycle was about 12 months, the last blood samples were collected at 350 days of life. With the same time scheduling, five pen pooled faecal samples were collected from each herd for bacteriological examination. At slaughtering, mesenteric lymph nodes were collected from each ear-tagged pig. Sero-prevalence (cut off S/P ratio 0.25) in sows varied from 93.8% to 100%. In four herds, sero-prevalence in piglets showed a similar profile with complete decline of maternal antibodies at day 60 and clear sero-conversion between day 90 and day 150. In one herd, sero-conversion was observed earlier and 56% of piglets were positive at day 90. The peak of sero-prevalence was observed between day 210 and day 270. Sero-prevalence at slaughtering varied from 66% to 100%. Salmonella was isolated from faecal samples in four of five herds. No Salmonella was isolated from mesenteric lymph nodes at slaughter in two of the herds. Culture prevalence from mesenteric lymph nodes in the other three herds ranged from 3.3% to 30%. This longitudinal study provides original information about epidemiological dynamics of Salmonella enterica infection in Italian swine herds in consideration of the unique extended fattening period typical of the Italian production.

Distribution of Salmonella serovars and phage types on 80 Ontario swine farms in 2004

The objective of this study was to describe the distribution of Salmonella spp. on Ontario grower-finisher pig farms. Eighty swine farms were visited from January through July 2004. On each farm, fecal samples were collected from 5 pens, 2 rectal samples and 1 pooled sample from fresh manure on the floor per pen. Salmonella was isolated from 91 (11%) of the 800 rectal samples and 73 (18%) of the 397 pooled samples. Overall, Salmonella was recovered from 37 (46%) of the 80 farms. On each positive farm, Salmonella was cultured from 1 to 7 pigs or 1 to 5 pens. Of the 37 farms, 18, 13, 5, and 1 yielded 1, 2, 3, and 4 serovars, respectively. The most common serovars were S. Typhimurium var. Copenhagen, S. Infantis, S. Typhimurium, S. Derby, S. Agona, S. Havana, and S. enterica subsp. I:Rough-O. The 3 most frequent phage types were PT 104, PT 104a, and PT 104b. There was a statistically fair agreement between samples collected directly from pigs and pooled pen samples in determining the Salmonella status at the pen and farm level (kappa = 0.6, P < 0.0001). However, in 62 pens, Salmonella status, serovars, or phage types differed between the pig and pooled pen samples. The distribution of Salmonella on the swine farms in this study indicates that, in developing an intervention strategy, priority should be given to farms positive for S. Typhimurium var. Copenhagen. Also, the variation in Salmonella status between pig and pooled pen samples deserves consideration in a sampling strategy.