Generalized papillomatosis in three horses associated with a novel equine papillomavirus (EcPV8)

Background Currently, seven equine papillomaviruses (PV) are known and are associated with one of three different and distinct clinical presentations. Hypothesis/Objectives To report the clinical, histopathological and immunohistochemical findings in horses with generalized papillomatosis associated with a novel equine PV, Equus caballus papillomavirus 8 (EcPV8). Animals Three client-owned quarter horses. Methods Case report, retrospective. Results Dozens to thousands of papillomas involved the axilla, inguinal area and proximal limbs as well as the ventral and lateral neck, thorax and abdomen. Lesions were sometimes confluent in ventral areas. Fewer lesions were on the face, ears, distal limbs and genitalia. Plaque-type papillomas were small, 0.5 to 1.5 cm in diameter and hyperkeratotic. Histologically, plaque-type papillomas prominently involved follicular infundibula. Immunohistochemical findings demonstrated PV antigen in superficial keratinocyte nuclei. PCR using degenerate primers for the PV L1 gene and sequencing of amplicons revealed PV DNA sequences that were 98% identical for all three cases, but shared <70% identity to other PVs. Horses were otherwise healthy; serum immunoglobulin levels and peripheral blood lymphocyte phenotyping did not identify a known immunodeficiency syndrome. Lesions nearly completely resolved after 1.5 years in one horse and persisted for two years in another, despite intralesional human IFN-alpha treatment. The oldest horse was lost to follow-up. Conclusion and clinical importance A novel equine papillomavirus (EcPV8) is associated with a distinct, plaque-type, generalized papillomatosis. Papillomas persisted for months to years, with or without treatment.