Molecular Epidemiology of Ovine Papillomavirus Infections Among Sheep in Southern Italy.

Ovine papillomaviruses (OaPVs) were detected and quantified, for the first time, using droplet digital polymerase chain reaction (ddPCR) and real-time quantitative PCR (qPCR) blood samples of 165 clinically healthy sheep. OaPV DNA was detected in 126 blood samples (76.4%). DdPCR detected OaPV DNA in 124 samples; in only two additional samples positive for real-time qPCR, ddPCR failed to detect the presence of any OaPVs. In 70 of the positive samples (55.6%), a single OaPV infection was observed, 12 of which were caused by OaPV1 (17.1%) and 14 by OaPV2 (20%). OaPV3 was responsible for 19 single infections (27.1%), and OaPV4 for 25 single infections (35.7%). Multiple OaPV coinfections were observed in 56 (44.4%) positive samples. OaPV coinfections caused by two genotypes were observed in 31 positive samples (55.4%), with dual OaPV3/OaPV4 infection being the most prevalent as seen in 11 blood samples. In addition, five OaPV1/OaPV4, four OaPV1/OaPV2, four OaPV2/OaPV3, four OaPV1/OaPV3, and three OaPV2/OaPV4 dual coinfections were also detected. OaPV coinfections by triple and quadruple genotypes were detected in 24 (42.8%) and only one (~1.8%) of coinfected blood samples, respectively. Multiple infections caused by OaPV1/OaPV3/OaPV4 genotypes were the most prevalent, as observed in 12 (50%) blood samples harboring triple OaPV infections. This study showed that ddPCR is the most sensitive and accurate assay for OaPV detection and quantification thus outperforming real-time qPCR in terms of sensitivity and specificity. Therefore, ddPCR may represent the molecular diagnostic tool of choice, ultimately providing useful insights into OaPV molecular epidemiology and field surveillance.