Rapid detection of equine infectious anaemia virus nucleic acid by insulated isothermal RT-PCR assay to aid diagnosis under field conditions.

Background: Control of equine infectious anaemia (EIA) currently depends on serological diagnosis of infected equids. However, recently infected equids may not produce detectable anti-EIAV antibodies up to 157 days post infection and so present a high transmission risk. Therefore, direct nucleic acid detection methods are urgently needed to improve EIAV surveillance and management programs in counties where the disease is endemic.

Objectives: To evaluate a field-deployable, reverse transcription-insulated isothermal PCR (RT-iiPCR) assay targeting the conserved 5' untranslated region (5' UTR)/exon 1 of the tat gene of EIAV.

Study Design: The analytical and clinical performance of the newly developed EIAV RT-iiPCR was evaluated by comparison with a EIAV real-time RT-PCR (RT-qPCR) along with the AGID test.

Methods: Analytical sensitivity was determined using in vitro transcribed RNA containing the target area of the 5' UTR/tat gene and samples from two EIAV-positive horses. Specificity was verified using nine common equine viruses. Clinical performance was evaluated by comparison with EIAV RT-qPCR and AGID using samples derived from 196 inapparent EIAV carrier horses.

Results: EIAV RT-iiPCR did not react with other commonly encountered equine viruses and had equivalent sensitivity (95% detection limit of eight genome equivalents), with a concordance of 95.41% to conventional EIAV RT-qPCR. However, the RT-qPCR and RT-iiPCR had sensitivities of 43.75 and 50.00%, respectively, when compared to the AGID test.

Main Limitations: Low viral loads commonly encountered in inapparent EIAV carriers may limit the diagnostic sensitivity of RT-PCR-based tests.

Conclusions: Although EIAV RT-iiPCR is not sufficiently sensitive to replace the current AGID test, it can augment control efforts by identifying recently exposed or "serologically silent" equids, particularly as the latter often represent a significant transmission risk because of high viral loads. Furthermore, the relatively low cost and field-deployable design enable utilisation of EIAV RT-iiPCR even in remote regions.