EcPV-2 is transcriptionally active in equine SCC but only rarely detectable in swabs and semen from healthy horses.

Squamous cell carcinomas (SCC) are malignant tumours arising from keratinocytes. In horses, there is increasing evidence for Equus caballus papillomavirus type 2 (EcPV-2) being causally involved in SCC development. However, only little is known regarding intralesional transcription of the virus, and sparse information on the incidence of EcPV-2 infection in healthy equids is available so far. Using RT-PCR, total mRNA from 8 EcPV-2 DNA-positive and 1 EcPV-2 negative SCC/SCC precursor lesions was screened for the presence of EcPV-2 E6 and E1 transcripts. Using PCR, we tested 193 sample specimens (30 ocular swabs, 94 genital swabs, 54 semen and 15 milk samples) from a total of 161 apparently healthy horses for the presence of EcPV-2 genes E7 and E6 or E2. Positive results were confirmed by repeating the PCR reactions, and by amplicon sequencing. E6 mRNA was detectable in 8/8 EcPV-2 DNA-positive lesions, whereas only 3/8 scored positive for E1 mRNA. EcPV-2 PCR scored positive for DNA from 1/30 ocular swabs, 4/94 genital swabs, 0/54 semen and 0/15 milk samples, thus resulting in an overall detection rate of 5/193, i.e. 2.6%. The demonstrated presence of viral mRNA in all EcPV-2 DNA-positive lesions is suggestive for an active pathogenic role of the virus in SCC development. This finding and the low incidence of EcPV-2 DNA in healthy equids further strengthen the concept of an aetiologic association of EcPV-2 with equine SCC disease.