Challenges and proposed solutions for more accurate serological diagnosis of equine infectious anaemia.
Authors: Issel C J, Scicluna M T, Cook S J, Cook R F, Caprioli A, Ricci I, Rosone F, Craigo J K, Montelaro R C, Autorino G L
Journal: The Veterinary record
Summary
# Editorial Summary: Serological Diagnosis of Equine Infectious Anaemia Equine infectious anaemia (EIA) remains a significant threat to equine health, yet the agar gel immunodiffusion test (AGIDT)—long the gold standard for serological diagnosis—has substantial limitations that may allow infected horses to pass undetected. Issel and colleagues examined persistently infected horses and mules whose serum samples tested negative or equivocal on AGIDT but positive on ELISA and immunoblot tests, revealing the presence of confirmed EIAV genetic sequences in these animals. When the Italian national EIA surveillance programme implemented a three-tiered diagnostic approach using ELISA as a primary screening tool followed by confirmatory testing, they identified 17% of their positive cases (25/149 horses) that would have been missed or misclassified by AGIDT alone—potentially discovering up to 20% more EIA cases overall. The authors advocate for prioritising sensitivity over specificity in this context: whilst an ELISA-first strategy generates some false positives requiring resolution, the alternative risk of releasing infected horses into the population (estimated at 2–3 per 10,000 under AGIDT-only protocols) presents an unacceptable biosecurity hazard. For practitioners involved in EIA screening, this work underscores that relying solely on AGIDT is inadequate; employing combined serological approaches significantly improves case detection and helps prevent disease transmission.
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Practical Takeaways
- •Single AGIDT negative results should not be used to clear EIAV-infected horses from diagnosis; combined serological testing (ELISA plus AGIDT) is necessary for accurate diagnosis
- •Implement multi-tiered testing protocols in your surveillance and diagnostic programs to reduce the risk of releasing falsely negative EIA cases, which poses significant disease transmission risk
- •When AGIDT and ELISA results conflict, use immunoblot testing and/or molecular detection of viral sequences to resolve status before making final clinical or management decisions
Key Findings
- •17% (25/149) of EIAV-infected equids had negative or equivocal AGIDT results despite positive ELISA and immunoblot tests
- •EIAV genetic sequences were detected in persistently infected horses and mules with discordant serological test results
- •A three-tiered laboratory diagnostic system combining AGIDT and ELISA detected up to 20% more EIA cases compared to AGIDT alone
- •ELISA-first testing strategy identified more true positives despite some false positives, with a defensible false positive rate of 2 per 1000 samples versus a false negative rate of 2-3 per 10,000