DETECTION OF EQUINE INFECTIOUS ANEMIA VIRAL-RNA IN PLASMA SAMPLES FROM RECENTLY INFECTED AND LONG-TERM INAPPARENT CARRIER ANIMALS BY PCR

Control of equine infectious anemia (EIA) is currently based on detect ion of anti-EIA virus (EIAV) antibodies. However, serologic diagnostic methods may give false-negative results in infected horses that fail to respond adequately or are in the early stages of infection. We deve loped a reverse transcriptase nested PCR (RT-nPCR) assay for the detec tion of viral gag gene sequences in plasma from EIAV-infected horses. The ability of RT-nPCR to detect field strains of EIAV was investigate d by assaying plasma samples from 71 horses stabled on EIA quarantine ranches. Positive PCR signals were detected in 63 of 63 horses with EI AV antibody test-positive histories on approved serologic tests, demon strating that RT-nPCR was probably directed against highly conserved s equences in the viral genome. The RT-nPCR assay, agar gel immunodiffus ion test, and conventional virus isolation were compared for detection of early infection in 12 experimentally infected ponies. Viral gag se quences were detected in all 12 animals by 3 days postinfection (p.i.) by RT-nPCR, whereas virus could not be routinely isolated on cell cul ture until 9 to 13 days p.i. and EIAV antibodies could not be detected by agar gel immunodiffusion until 20 to 23 days p.i. Finally, specifi city of the RT-nPCR assay was examined by testing plasma from 43 horse s with serologic test-negative histories and no known contact with EIA V-infected animals. Viral gag sequences were not detectable in this co ntrol group. These data suggest that the EIAV RT-nPCR assay effectivel y detects EIAV and is more sensitive than current standard methods for detection of early stages of infection.