Estimation of the Diagnostic Accuracy of theinvA‐gene‐based PCR Technique and a Bacteriological Culture for the Detection ofSalmonellaspp. in Caecal Content from Slaughtered Pigs using Bayesian Analysis

Abstract

The goal of this study was to estimate the accuracy of the invA-gene-based polymerase chain reaction (PCR) and a culture technique based on pre-enrichment with buffered peptone water, three selective enrichment media (selenite, tetrathionate and Rappaport-Vassiliadis broths) and four selective, solid media (Xylose-Lysine-Tergitol-4, Salmonella/Shigella, Hekton-Enteric and MacConkey), for the detection of Salmonella organisms from caecal samples from slaughter pigs. For this purpose a latent-class (Bayesian) approach was used. Two hundred and three slaughtered pigs were used after grouping them into two groups of 96 and 107 animals. Sensitivity (Se) was estimated to be 56% (95% probability interval 40, 76) for culture and 91% (81, 97) for PCR. The specificity (Sp) of the PCR was 88% (80, 95) while the Sp of the culture had been considered 100% in the statistical analysis as all culture-positive samples were confirmed by serotyping. PCR Se was not affected by the Salmonella serotypes present in the samples analysed. Accordingly, a minimum of 25.5% of the pigs was estimated to harbour Salmonella organisms in their faeces. It was concluded that bacteriology on caecal samples alone was a poor diagnostic method, and that the PCR method could be considered a cost-effective alternative to culture in Salmonella monitoring programmes. However, given the moderate Sp of this molecular technique, PCR-positive samples should be further confirmed through bacteriology.

Sensitivity of the ISO 6579:2002/Amd 1:2007 Standard Method for Detection of Salmonella spp. on Mesenteric Lymph Nodes from Slaughter Pigs

“…Overall, these results confirmed that the Se ISO was somewhat below 87.6%, which is rather high for a culture protocol and higher than most of the results reported from other studies using different culture protocols and matrices (9,10,14,(24)(25)(26). Both the large amount of sample (25 g) and the repeated enrichment and subculturing in different media used by ISO would have had an effect on the recovery of Salmonella organisms (7,10,11) and would explain this high overall Se ISO .…”

“…(32). Regarding the Sp PCR , it has been suggested that it may be affected by the proportion of potential cross-reacting bacteria in the guts of the animals sampled in each population (14). Hence, it was decided to check the assumption of constant test accuracy across populations by running separate Bayesian analyses of the two populations (39).…”

“…The reported Se's of different culture protocols on fecal samples vary widely, ranging from as low as 56% to as high as 90% (9,11,12,14,(24)(25)(26). The authors are also aware of two studies assessing the Se ISO on fecal samples.…”

“…In fact, most studies estimate the Se of a given bacteriological technique relative to a combination of different but imperfect bacteriological methods (7,9,11). To overcome this problem, latent-class methods with Bayesian approaches have been used to obtain unbiased estimates of the Se's of different culture protocols for the diagnosis of pig salmonellosis on fecal samples under field conditions (12,14).…”

Serological survey of Salmonella spp. infection in finishing pigs from northeastern Spain and associated risk factors

“…Bacteriology, the method of reference, is expensive, tedious, time-consuming and less sensitivity, particularly when performed on feces (Hurd et al, 2004;Mainar-Jaime et al, 2008a), making this approach unfeasible for large-scale control programs. Serology is thus foreseen as one of the best alternatives.…”