Latency-associated transcripts of equine herpesvirus type 4 in trigeminal ganglia of naturally infected horses.

Equine herpesvirus type 4 (EHV-4) is a major respiratory pathogen of horses. Unlike most other members of the Alphaherpesvirinae, EHV-4 was regarded as non-neurotropic. Here, neural and lymphoid tissues of 17 horses have been analysed post-mortem. EHV-4 DNA was detected in 11 cases (65%) by PCR, exclusively in the trigeminal ganglia. In order to define the transcriptional activity, RNA preparations of 10 EHV-4 DNA-positive ganglia were investigated by nested RT–PCR. EHV-4-specific transcripts derived from genes 63 [herpes simplex virus type 1 (HSV-1) ICP0 gene homologue] and 64 (HSV-1 ICP4 gene homologue) were detected in six trigeminal ganglia. In one other case, only gene 64-specific transcripts were present. All of the transcripts proved to be antisense orientated when a strand-specific RT–PCR was applied. Type-specific primers for gene 33 (encoding glycoprotein B) served to detect transcripts of an acute EHV-4-infection, which were found in only one of the six ganglia positive for gene 63- and gene 64-specific transcripts. Overall, these studies clearly demonstrate that EHV-4 is latent in trigeminal ganglia.