Accurate identification of bovine deltapapillomavirus in equine sarcoids by ddPCR

Sarcoids are benign and locally aggressive skin lesions that commonly affect horses and other equid species. Sarcoids are generally considered to be caused by bovine delta-papillomaviruses (δPVs) types 1 and 2 (BPV1 and BPV2, respectively). Moreover, while bovine δPV types 13 and 14 (BPV13 and BPV14, respectively) are also suspected to induce sarcoids, information regarding this possibility and the occurrence of multiple bovine δPV infections in sarcoids is scarce. This study aimed, for the first time, to assess BPV1, BPV2, BPV13, and BPV14 infections and co-infections in equine sarcoid samples of Austrian provenance, and to determine the intralesional DNA loads of the detected bovine δPV types using highly sensitive droplet digital polymerase chain reaction (ddPCR). BPV DNA was detected in 93 sarcoid samples. The analyses revealed that BPV1 was the predominant bovine δPV type in sarcoids from Austria, with 83/93 lesions testing BPV1-positive. Importantly, 66 tumors also contained BPV2 DNA. In six cases, a triple infection including BPV13 or BPV14 was noted, and one lesion showed a quadruple infection. This is the first ddPCR-based study to show multiple infections by all four bovine δPVs in equine sarcoids. Clinical data suggest that BPV1/2 co-infection may be associated with more severe and therapy-resistant disease. In-depth studies are required to investigate this possibility in greater detail.