Evaluation of the antibacterial residue surveillance programme in Danish pigs using Bayesian methods.

Residues of pharmacological active substances or their metabolites might be found in food products from food-producing animals. Maximum Residue Limits for pharmacological active substances in foodstuffs of animal origin are established to assure high food safety standards. Each year, more than 20,000 samples are analysed for the presence of antibacterial residues in Danish pigs. This corresponds to 0.1% of the size of the slaughter pig population and more than 1% of the sows slaughtered. In this study, a Bayesian model was used to evaluate the Danish surveillance system accuracy and to investigate the impact of a potential risk-based sampling approach to the residue surveillance programme in Danish slaughter pigs. Danish surveillance data from 2005 to 2009 and limited knowledge about true prevalence and test sensitivity and specificity were included in the model. According to the model, the true antibacterial residue prevalence in Danish pigs is very low in both sows (∼0.20%) and slaughter pigs (∼0.01%). Despite data constraints, the results suggest that the current screening test used in Denmark presents high sensitivity (85-99%) and very high specificity (>99%) for the most relevant antibacterial classes used in Danish pigs. If high-risk slaughter pigs could be identified by taking into account antibacterial use or meat inspection risk factors, a potential risk-based sampling approach to antibacterial residue surveillance in slaughter pigs would allow reducing the sample size substantially, while increasing or maintaining the probability of detection. Hence, the antibacterial residue surveillance programme in Danish pigs would be more cost-effective than today.

Modelling food safety and economic consequences of surveillance and control strategies for Salmonella in pigs and pork.

Targets for maximum acceptable levels of Salmonella in pigs and pork are to be decided. A stochastic simulation model accounting for herd and abattoir information was used to evaluate food safety and economic consequences of different surveillance and control strategies, based among others on Danish surveillance data. An epidemiological module simulated the Salmonella carcass prevalence for different scenarios. Cost-effectiveness analysis was used to compare the costs of the different scenarios with their expected effectiveness. Herd interventions were not found sufficient to attain Salmonella carcass prevalence <1%. The cost-effectiveness of abattoir interventions changed with abattoir size. The most cost-effective strategy included the use of steam vacuum and steam ultrasound. Given uncertainty of the effect of steam vacuum and steam ultrasound, model results should be updated as more information becomes available. This framework contributes to informed decision-making for a more cost-effective surveillance and control of Salmonella in pigs and pork.

Use of herd information for predicting Salmonella status in pig herds.

Salmonella surveillance-and-control programs in pigs are highly resource demanding, so alternative cost-effective approaches are desirable. The aim of this study was to develop and evaluate a tool for predicting the Salmonella test status in pig herds based on herd information collected from 108 industrial farrow-to-finish pig herds in Portugal. A questionnaire including known risk factors for Salmonella was used. A factor analysis model was developed to identify relevant factors that were then tested for association with Salmonella status. Three factors were identified and labelled: general biosecurity (factor 1), herd size (factor 2) and sanitary gap implementation (factor 3). Based on the loadings in factor 1 and factor 3, herds were classified according to their biosecurity practices. In total, 59% of the herds had a good level of biosecurity (interpreted as a loading below zero in factor 1) and 37% of the farms had good biosecurity and implemented sanitary gap (loading below zero in factor 1 and loading above zero in factor 3). This implied that they, among other things, implemented preventive measures for visitors and workers entering the herd, controlled biological vectors, had hygiene procedures in place, water quality assessment, and sanitary gap in the fattening and growing sections. In total, 50 herds were tested for Salmonella. Logistic regression analysis showed that factor 1 was significantly associated with Salmonella test status (P = 0.04). Herds with poor biosecurity had a higher probability of testing Salmonella positive compared with herds with good biosecurity. This study shows the potential for using herd information to classify herds according to their Salmonella status in the absence of good testing options. The method might be used as a potentially cost-effective tool for future development of risk-based approaches to surveillance, targeting interventions to high-risk herds or differentiating sampling strategies in herds with different levels of infection.

Factors influencing Salmonella carcass prevalence in Danish pig abattoirs.

The Danish Salmonella Surveillance-and-Control Programme in finisher pigs includes both herd and carcass surveillance. Herd surveillance consists of serological testing of meat-juice samples and classification of herds into three Salmonella seroprevalence levels. At the abattoirs, carcass swabs from five pigs are collected daily and analysed as a pooled sample to evaluate the Salmonella carcass prevalence. This study aimed to investigate factors associated with Salmonella carcass prevalence in Denmark. A total of 20,196 pooled carcass swabs collected in 23 Danish abattoirs were included in the analysis. A multilevel logistic regression model was used taking into account the two-level data structure (abattoir, carcass pool) and adjusting the parameter estimates to the random variation at the abattoir level. Study results indicated that carcass contamination was mainly influenced by the probability that at least one pig contributing to the pool was seropositive, the log-transformed number of seropositive pigs delivered to the abattoir on the same day and weekday. No other factors were found to be significant (P>0.05). Large reductions in the number of seropositive pigs delivered to slaughter are unlikely to result in large reductions of the Salmonella carcass prevalence, unless the number of seropositive pigs can be kept below approximately 200. On average, individual Salmonella carcass prevalence can be kept below 1% by keeping a Salmonella input to the abattoir below approximately 50 seropositive pigs. Variation between abattoirs suggested that improved hygiene practices in some of the abattoirs would reduce the Salmonella carcass prevalence further.

Factors affecting persistence of high Salmonella serology in Danish pig herds.

In 1995, Denmark implemented a National Surveillance-and-Control Programme for Salmonella in finisher pigs. It was based on serological surveillance that enabled assignment of pig herds into three prevalence levels. Herds in levels 2 and 3 (corresponding to moderate and high Salmonella seroprevalence) were subjected to control measures, compulsory pen faecal sampling for bacteriological examination and serotyping. The initial dataset contained all herds that had mandatory bacteriological samples taken (N=3691 samples), out of which 44.4% were negative. Data from January 2005 to November 2007 obtained from the Danish programme were used to investigate the persistence of high serology (Salmonella index > or =40, corresponding to levels 2 or 3) for herds infected with one of the four most common Salmonella serotypes found in pig herds, S. Typhimurium, S. Derby, S. Infantis or S. Livingstone, using survival analysis. The effects of other potential factors, such as season, year, herd size and herd health status were also evaluated. An event was defined as return to low serology measured by a Salmonella index <40, for at least 6 consecutive months. A total of 1242 pig herds positive for one of the four serotypes of interest and 1433 events were included in the preliminary analysis using the Kaplan-Meyer method. The median high serology time (survival time) was 8 months and all explanatory variables were found to be significant (p<0.05), except the herd health status (p=0.49). Cox proportional hazards regression was used for herds infected with S. Typhimurium (N=1066) or S. Derby (N=314). Herds infected with S. Typhimurium presented a lower probability for returning to low serology (HR=0.69, p<0.001), compared to S. Derby, indicating longer high serology periods. Herds that started having high seroprevalence in Summer and Fall had a higher probability of returning to low serology compared to those starting in Winter and Spring (p<0.001). Compared to 2005, periods with high Salmonella serology were fewer and shorter in 2006 and 2007 (p<0.001). A positive impact of herd size was found, with larger herds being associated with shorter high serology periods, compared to smaller herds (p=0.012). This might indicate effective implementation of biosecurity measures in large herds. The health status of the herds was not found to be a significant factor (p=0.87), which might reflect that biosecurity measures in place in SPF herds are not effective for clearing Salmonella infections.

Association between the presence of antibodies to Mycobacterium avium subspecies paratuberculosis and somatic cell count.

Somatic cell counts (SCC) in bulk tank milk delivered for human consumption are one of the indicators of milk quality and are used for milk pricing. Consequently, milk from cows with high SCC is frequently used by farmers for feeding of calves to lower the SCC in bulk tank milk. Young calves are more susceptible to Mycobacterium avium ssp. paratuberculosis (MAP) and may acquire the infection early in life through ingestion of MAP-contaminated milk. The occurrence of MAP antibodies can be an indicator of MAP shedding. Because MAP can be shed in milk from infected cows, and antibodies to MAP can be an indicator of the infectious status, an association between antibodies to MAP and high SCC can result in high-SCC milk being at risk of containing MAP. Feeding milk containing high SCC to susceptible calves may result in MAP infections. Somatic cell counts and MAP antibodies in milk were measured repeatedly in 7,251 cows from 26 Danish dairy herds to investigate the association between the occurrence of MAP antibodies and high SCC. The results of robust regression showed a log-linear relationship between the age at first positive ELISA and the age at first high SCC sample (R2 = 0.51). Of the 1,733 cows positive for MAP antibodies and with high SCC, high SCC was detected prior to MAP antibodies in 46% of the cows. Still, in 40% of the cows, MAP antibodies were detected before a high SCC. Therefore, the findings do not point to a causal relationship between high SCC and antibodies to MAP, but suggest a strong association and highlight a potentially increased risk of MAP transmission when milk with high SCC is fed to calves.