A number of previous studies that were conducted in different countries have documented that nine EcPVs and three BPVs can infect the horses [3–5, 16–18]. However, before this study, no one has described about infections caused by these viruses in the Chinese horses. In the current study, we first investigated the existence of PVs in the different samples of Chinese horses. Interestingly, virome results revealed that of the 68,007,096 reads generated from the nasal swabs, 56,054 (0.08%) were annotated to mammalian viruses including papillomavirus, retrovirus, circovirus-like virus, herpesvirus, astrovirus, parvovirus, and paramyxovirus. Of these viral reads, 103 shared > 99.5% nt identity with the HPV124 E6 gene, suggesting that much like bovine papillomavirus 1 (BPV1), BPV2, and BPV13 [17, 18], HPV124 can also undergo trans-species transmission to infect horses. PCR analyses further confirmed that HPV124 was detectable in nasal swabs from two Thoroughbred horses in farm A in Yili city, 37 Thoroughbred horses in farm C in Changji city, and two Akhal-Teke horses in farm D located in Urumqi city (Table 1), with HPV124 case positivity rates of 4% (2/56), 100% (37/37) and 7% (2/30), respectively (Table 1). In addition, subsequent analysis of 50 samples of Yili horse aborted fetal lung tissue samples from farm B that were negative for EHVs revealed detectable HPV124 viral DNA in 17/50 of these samples via PCR (Table 1). Published reports have convincingly demonstrated HPVs only replication in human to date[1, 2], but our results indicated that HPV124 can readily infect heterologous hosts-horse, and this virus exist in mainland China. In order to avoid human viruses pollute samples of horses, workers wear gloves, masks, and protective clothing in sampling, thus HPV124 infection in horses is believable. These are also the first data to our knowledge demonstrating the presence of PV in aborted fetal tissue from horses, indicating that HPV124 is evidence for vertical transmission in horses as earlier reports of HPVs and BPVs having respectively demonstrated in humans and bovine species [19–27]. Similar to recently one study have demonstrated that EHV-1 cause abortions in Yili mares , our findings suggested that HPV124 may contribute to the incidence of Yili horse abortions, which highlight HPV124 as a candidate equine abortion virus worthy of further study.
To determine the potential for this HPV124 strain to transmit from humans to horses, we collected 62 nasal swabs from humans, including eight workers from farm A, 44 workers from farm B, and 10 workers from farm C (Table 1). PCR results confirmed that one worker in farm A, five workers in farm B, and one worker in farm C were positive for HPV124 (Table 1), which suggested that this HPV124 strain may be transmissible from workers to horses at an HPV124-positive farm. These results thus indicate that individuals working on a stud farm should take proper precautions to avoid HPV124 viral transmission. In humans, HPV124 has both been detected in healthy individuals and linked to diseases including viral warts, actinic keratosis, and squamous cell carcinoma [3, 7–11]. All seven HPV124-positive workers identified in this study were free of any clinical signs of disease, suggesting them to be healthy carriers. Future research should thus focus on the route of HPV124 viral transmission between humans and horses.
HPV124 partial E6 genes sequences were next amplified with appropriate primers (5'-GGAAGGAGCATTTAGTGTTTG-3'/5'-CTCCAAGCTCCTCTTACTTTATG-3'). Following sequence assembly, the E6 genes from nine nasal swabs (five workers, two Thoroughbred horses, two Akhal-Teke horses) and nine aborted Yili horse tissue samples were sequenced, with the resultant sequences being deposited in NCBI (GenBank nos. OK484563-OK484566 and OL474072-OK474085).
Multiple sequence alignment of these E6 gene sequences revealed the 18 sequences HPV124 strains from these human and equine samples to exhibit 96.9–100% nt identity, consistent with genetic diversity. Of these 18 HPV124 strains, sequences of seven (XJ-ZS5-52 from Thoroughbred horse nasal swab, XJ-ZS-A and XJ-ZS-G from human nasal swabs, as well as XJ-ZS2-1, XJ-ZS4-1, XJ-ZS20-1, and XJ-ZSi from aborted tissue samples of Yili mares) were compared with the HPV124 NJ3900 reference strain (GenBank no. GQ845446), revealing 100% DNA sequence similarity, which supported horse-origin HPV124 might be derived from human with HPV124 positive. Moreover, the remaining 11 HPV124 strains exhibited 97.4–99.5% nt similarity with this reference strain. In contrast, the E6 genes from these Chinese HPV124 strains exhibited just 51.5–62.9% and 29.8–45.6% nt similarity, respectively, with Beta2-Beta5 type HPV and EcPV.
A phylogenetic tree developed based upon E6 gene sequences from Beta type HPV and EcPV strains indicated that the Chinese HPV124 strain identified herein was closely related to the NJ3900 HPV124 reference strain, with HPV124, HPV8, HPV12, HPV14, and HPV19 all being clustered into the Beta 1 HPV lineage, which diverged from the Beta2-Beta5 HPV and EcPV lineages (Fig. 1).
In summary, this study is the first to have reported the detection of HPV124 among different breeds of horses or in any non-human species. As this virus was detected in aborted fetal lung tissue samples, HPV124 may be a causative agent responsible for abortions in Yili horses. Further studies revealed that this HPV124 isolate could be transmitted from humans working with HPV124 to horses. The results of this study will generate new awareness regarding the potential role of HPV124 in abortion incidence and zoonotic infections in horses and human throughout the world.