Challenges and proposed solutions for more accurate serological diagnosis of equine infectious anaemia

Abstract

Serological diagnosis of equine infectious anaemia virus (EIAV) infections has depended mainly on the agar gel immunodiffusion test (AGIDT). This study documents the presence of EIAV genetic sequences in a number of persistently infected horses and mules whose serums were interpreted as negative/equivocal on AGIDT, but positive on more than one ELISA test and in immunoblot tests. Strategies designed to take advantage of the combined strengths of the ELISA and AGIDT are shown effective in a national surveillance program for EIA in Italy where 17 per cent (25/149) of the equids considered to be infected with EIAV on combined/comparative serological data had reactions in the AGIDT that were interpreted as negative or equivocal. These data document the benefits of using a three-tiered laboratory system for the diagnosis of EIA. Although the ELISA-first strategy introduces some confusing results, the discovery of up to 20 per cent more cases of EIA makes it compelling. In our opinion, it is better and more defensible to find two samples in 1000 with resolvable but falsely positive ELISA tests for EIA than to release two to three horses in 10,000 with falsely negative test results for EIA (the rates seen in the Italian surveillance presented here).

FIG 2:

Immunoblot reactions of horses and mules to complement Table 1. (Please refer Table 1 for details)

FIG 1:

Standards used for reporting agar gel immunodiffusion test (AGIDT) reactions and challenges with accurate interpretations in the AGIDT: (a) graphical representation of standardised numerical interpretations by intensity and (b) photographs of reactions of weak positive reference serums from United States Department of Agriculture (USDA) (Panel 1) and University of Kentucky (Panel 2). All three test wells in each panel are the subject serum. Legend: Samples in (b) are the USDA (Panel 1) and the University of Kentucky (Panel 2) weak positive reference serums. Serum was washed from wells to facilitate photography. Both digital photographs receive identical editing

FIG 3:

Proposed decision tree for serodiagnosis of EIA using the three-tiered laboratory system