**8. Co-infections and heterologous infections**

In Brazil, the association between BPV infection and the occurrence of cutaneous papilloma‐ tosis, enzootic hematuria, and upper gastrointestinal neoplasias has been confirmed in cattle [46,48,57-59]. Previous studies have found BPV in tissues other than the skin epithelium. Thus, BPV1, 2, and 4 have been identified in the embryos and female reproductive tissues of infected cows [60-63]. Similarly, BPV DNA has been detected in samples of blood, milk, urine, seminal fluid, and spermatozoa from infected cattle. These findings point to the possible participation of these fluids and cell types in BPV transmission [64].

In addition, other studies have shown the occurrence of multiple infections in cattle exhibiting several cutaneous papillomas that are caused by different BPV types and the possibility of viral co-infections in single lesions [65-68]. Additionally, the presence of several BPV types in single lesions is similar to the situation in human skin, where co-infection by more than 10 viral types is frequently detected [69].

Each papillomavirus is known to exhibit specificity for a single animal host species in which it replicates productively. However, only a few viral types are also able to infect a second animal species. In such cases, non-productive infections, that is, infections without the production of infective virions, are the result [1]. This type of infection is the case for the equine sarcoid, which can be defined as a fibroblastic locally invasive skin tumor. Sarcoid is the most frequent neoplasia affecting equine species, and it represents the best-known example of heterologous PV infection because it is caused by BPV1 and 2 [70]. In addition to horses, donkeys, and mules, skin lesions caused by these viral types have also been described in zebras and buffaloes [71,72]. Another example of heterologous PV infection is provided by the detection of DNA from FeSarPV (feline sarcoid-associated papillomavirus), a putative new PV type that was initially identified in feline sarcoids with non-productive infections, in fibropa‐ pillomas and skin samples from cattle with dermatitis [73-75]. The recent detection of FeSarPV in biological samples from cattle strengthens the hypothesis that cattle might be the natural host of this virus [73].
