Comparative assessment of the surveillance systems for equine infectious anaemia, equine viral arteritis and contagious equine metritis in France

Equine infectious anemia (EIA) is caused by a persistent lentivirus infection. Infected horses are lifelong reservoirs for vector mediated or iatrogenic transmission of EIA virus (EIAV). No vaccine is currently available or likely to become so. Therefore control of new cases depends upon the time honored strategy of serological identification of carriers and their disposition by quarantine and or slaughter. In the United States EIA prevalence has declined from 4% in 1972 when the control program began to its current prevalence of approximately 0.001%. The agar gel immunodiffusion (AGID) test has been the regulatory test since EIAV control program began 43 years ago. This 4,000 fold reduction in prevalence underscores the effectiveness of the AGID test but it has a couple of drawbacks: it requires 48 hours to read and is not as sensitive as newer serological tests. Alert clinicians, regulators and horse owners seek to implement and facilitate better management of outbreaks, pre purchase evaluations and import/ export requirements. To address the need for a faster and more sensitive assay we carried out this study to compare a USDAlicensed AGID test kit with a USDA-licensed modified indirect ELISA (MI-ELISA) kit. The serum sample from an EIAV carrier horse was serially diluted and used to determine the analytical sensitivity of each assay: the MI-ELISA was 4-fold more sensitive. A Center for Veterinary Biologics proficiency panel (validated by AGID test) of 20 samples was tested: there was 100% concordance between the two assays. Analytical time point samples from 6 experimentally infected horses were collect over a period 4 months (n¼100): analytical sensitivity of experimentally infected horses was comparable for the two assays. The two assays were used to test known weak positive samples collected from nine carrier horses: the AGID test detected only two but the MI-ELISA detected all nine (Table). Due to the speed (45 minutes) and significantly greater sensitivity of the MI-ELISA reflected in the results of this study, screening of routine samples with it and confirmation of positives by the regulatory assay, AGID test, would serve the needs of the equine industry for better and more rapid risk management. Comparison between the modified indirect enzyme-linked immunosorbent assay (MI-ELISA) and agar gel immunodiffusion (AGID) test in the sensitivity against the sera from weak EIA positive carriers