Elsevier

Veterinary Parasitology

Volume 238, 30 April 2017, Pages 30-34
Veterinary Parasitology

Short communication
Comparison of commercial ELISA tests for the detection of Toxoplasma antibodies in the meat juice of naturally infected pigs

https://doi.org/10.1016/j.vetpar.2017.03.012Get rights and content

Highlights

  • Commercial ELISA tests were compared using a commercial MAT as a reference test.

  • Various Toxoplasma antibody prevalences were obtained with the four tests.

  • Test accuracy was highest with a cut-off value of 0.30 for the three tests.

  • A very strong positive correlation of the S/P values was seen between the three of the four tests.

Abstract

Toxoplasmosis is a globally distributed protozoal zoonosis. Pigs are considered an important reservoir of Toxoplasma gondii and pork a major infection source of human toxoplasmosis. ELISA methods are commonly used diagnostic tools for detecting Toxoplasma infections. They are also used for slaughterhouse-based serological monitoring of toxoplasmosis in pigs to identify positive farms. The methods used are non-standardised with varying sensitivity and specificity. In our study, four commercial ELISA tests for the detection of Toxoplasma antibodies in the meat juice of slaughter pigs were compared with a modified agglutination test (MAT) as a reference. The cut-off values of the ELISA tests provided by the manufacturer varied between 0.20 and 0.50, and clearly influenced prevalence. The sensitivity of tests I, II and III varied between 96.4 and 78.6. Sensitivity was unacceptably low (3.6) for test IV (cut-off = 0.30). Tests I, II and III had the highest accuracy and the best agreement with the reference test when a cut-off of 0.30 was used. Test II and III showed very good agreement (K = 0.92 and 0.84, respectively) with the MAT. A very strong correlation (Pearson correlation >0.89) was observed between the S/P values of tests I, II and III. Our results demonstrate that the test and cut-off value used influence the results of the apparent seroprevalence studies.

Introduction

Toxoplasmosis is a globally distributed zoonosis and an estimated one-third of the world’s population is exposed to Toxoplasma (Robert-Gangneux and Darde, 2012). The infection can lead to abortions in pregnant women and neurological abnormalities in congenitally infected children. Toxoplasma infection in immunocompetent individuals is typically asymptomatic, but flu-like symptoms, such as swollen lymph glands, fever and/or muscle aches, may occur. The European Food Safety Authority (EFSA) has recognized toxoplasmosis as a parasitic zoonosis with a high incidence in humans and requiring representative data in Europe (EFSA, 2007).

Consumption of raw or undercooked meat is considered one of the most important sources of human Toxoplasma infections (Robert-Gangneux and Darde, 2012). Pigs, especially those with outdoor access, possess a high prevalence of cysts in meat (EFSA, 2011a, EFSA, 2011b). EFSA has identified Toxoplasma as one of the four most relevant biological hazards in the meat inspection of pigs (EFSA, 2011a). Toxoplasma infection in pigs at slaughter cannot be diagnosed because infected pigs are asymptomatic and the cysts are too small to be visually detectable during meat inspection. Other rapid, reliable and cost-effective control methods for large-scale monitoring and surveys are therefore required.

Serological testing of toxoplasmosis in slaughter pigs has been shown to be the most practical method for monitoring the exposure status of farms to implement control measures (Basso et al., 2013). The serological testing of fattening pigs raised especially in non-controlled housing conditions is recommended by EFSA (2011b). Various serological methods, such as the modified agglutination test (MAT), immunofluorescence antibody test (IFAT) and enzyme-linked immunosorbent assay (ELISA), have been used to detect Toxoplasma IgG antibodies in the blood and tissue samples of slaughter pigs (Gamble et al., 2005, Hill et al., 2006, Ranucci et al., 2012, Basso et al., 2013, Steinparzer et al., 2015, Papatsiros et al., 2016). ELISA tests, which can be semi-automated, are cost-effective and convenient diagnostic tools for large-scale screening. The levels of Toxoplasma gondii −specific IgG are lower in meat juice compared to serum and to compensate this, a lower dilution factor is used for the meat juice samples (Forbes et al., 2012, Wallander et al., 2015a). However, the specific antibody levels in meat juice depend on the muscle chosen for sampling (Forbes et al., 2012, Wallander et al., 2015a). Meat juice is a matrix easily available from pigs at slaughter, and can also be used for the detection of other public health hazards such as salmonellae, pathogenic yersiniae and trichinellae (Felin et al., 2015).

Previous studies have shown that serological methods are not standardised giving different results (Garcia et al., 2006, Steinparzer et al., 2015). The aim of our study was to compare commercially available ELISA kits for detecting Toxoplasma antibodies in the meat juice of naturally infected fattening pigs. The sensitivity and specificity of these tests were additionally evaluated with a commercial MAT as the reference method.

Section snippets

Material and methods

Ninety meat juice samples from the diaphragm tissue of 5–7-month-old pigs studied with PrioCHECK® Toxoplasma (Prionics AG, Zurich, Switzerland) were selected from the study of Felin et al. (2015). These 90 samples included all 43 positive samples with S/P ratios (= [sample optical density (OD) − mean OD of the negative control]/[mean OD of the positive control − mean OD of the negative control]) of at least 0.15, each of the 27 samples with a ratio between 0.10 and 0.14, and 20 random samples

Results and discussion

Four commercial ELISA tests for the detection of Toxoplasma antibodies in pigs at slaughter were compared with each other and with a modified agglutination test as a reference. Comparison is challenging as no reference method with 100% sensitivity and specificity is available for large-scale testing. A modified agglutination test (MAT) was used as the reference test in our study. It has also been used as the reference method in some earlier studies (Forbes et al., 2012, Steinparzer et al., 2015

Conclusions

Our results demonstrate that the test and cut-off value used have an influence on the results of apparent seroprevalence studies. A very strong positive correlation was observed between the S/P values of three of the ELISA tests when using meat juice. One of the tests did not correlate with the others. The cut-off S/P 0.30 was shown to give the highest accuracy and best Kappa agreement for the three tests. However, validation and comparison of the commercial ELISA tests with the meat juice of

Conflict of interest

The authors declare no potential conflict of interest with respect to the research, authorship and publication of this article.

Acknowledgements

We thank Maria Stark for excellent technical assistant. Funding from the Ministry of Agriculture and Forestry, Finland (1933/312/2011).

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