8/8/2019 Field trial of the specificity of an ELISA test for Prrs (IDEXX HERDCHEK* PRRS 2XR) - High specificity confirmed, but

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Oral Presentations

Proceedings of the 21stIPVS Congress, Vancouver, Canada July 18-21, 2010

O.169

Field trial of the specificity of an ELISA test for PRRS (IDEXX HERDCHEK PRRS 2XR) highspecificity confirmed, but varying behaviour according to the age of the animals

Vincent Auvigne1 Jean-Louis Pinsard2 Herv Gourgues3

1. Ekipaj, Pozuelo de Alarcn, Spain; 2. Bio Chne Vert, Chateaubourg, France; 3. IDEXX Laboratories , Cergy Pontoise, France

Introduction

The laboratory diagnosis o the Porcine Respiratory and Repro-ductive Syndrome (PRRS) is mainly based on the use o sero-logical ELISA tests and PCR diagnostics. The aim o this study wasto assess the specifcity o a commercial ELISA test. In particular,the specifcity o the test is compared in two dierent subpopu-lations: adult breeding sows and young pigs.

Materials and methods

The study takes into account a set o results carried out onFrench pig arms that are considered ree o the PRRSV and witha low risk o contamination (areas with a low pig density or airfltration system). The arms are tested at least three times a year.

The IDEXX HerdChek PRRS 2XR is an indirect Elisa test or thedetection o antibodies against the European and Americanstrains o the PRRSV in the serum o pigs. Samples with an S/Pratio o 0.4 or more are positive. Those o less than 0.4 are nega-tive. Only the frst results were included in the study, beorecarrying out any confrmatory tests.

All arms with a positive result were checked to confrm thatthe arm was indeed considered as ree rom the disease, and tocheck i at least one subsequent negative result allowed exclud-ing the hypothesis o a recent contamination.

Results

Test results carried out between December 2007 and March

2009 were taken into account. 641 adult animals (breedingsows) and 1017 young stock (between 80 and 150 days o age)were included in the study.

Specifcity was signifcantly higher in young stock than in theadults (Chi2, p < 0.02, Table 1). Analysis o the S/P ratio distribu-tions o adult and young pigs showed a dierence between thetwo sub-populations (Figure 1). They diered in the average(Mann & Whitney test, p < 0.001) and in the standard deviation.

The percentage o borderline-negative results (S/P between 0.2and 0.4) was higher in adult pigs.

Table 1: Specifcity o the test in both populations

Figure 1:Distribution o S/P ratios according to the age

False positive results ranged rom values very close to the limit(5 out o 16 with S/P < 0.45) to clearly positive values (5 out o 16alse with S/P >1).

Discussion

The main di culty o specifcity studies in the absence o a reer-ence tests is to deal with a disease-ree study population. In thisstudy, the combination o a history o controls and a low risk ocontamination allowed a rming that the positive test resultswere alse positives.

The observed values o specifcity are high and confrm existingdata obtained with a smaller sample (1). These specifcity valuesare higher than those obtained with other tests in national con-

trol programmes in a disease-ree environment (2). The specif-city o a test should be put in balance with its sensitivity, as notest is 100% specifc.

The dierence in specifcity between the two sub-populations(young stock and adult animals) studied is an original result.

To our knowledge, this is the frst time it has been described inthe context o PRRS. One hypothesis to explain this dierencebetween the two age groups is the existence o immunologicalbackground noise, which increases with age due to the accumu-lation o non-specifc antigenic stimulation. It is surprising tonote that this lesser specifcity o adult animals is not due to anincrease o the S/P ratio but to their variability. It would be inter-esting to have equivalent data or other pathological agents opigs in order to determine whether this is a systematic phenom-

enon or only linked to PRRS.

References

1. Idexx Laboratories (1999).

2. Corbellini L.G. et al. Vet Microbiol (2006).118, 267-73

PRRS - Diagnostic

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