3.5 The pathogenicity of YNG strain in vivo
In order to evaluate the pathogenicity of YNG strain in vivo, Kunming mice (six-week-old) were chosen as animal models and inoculated with 0.1 mL of 105 TCID50/mL of YNG, YNP, or HNLY strain via intramuscular injection at the hind leg, respectively. The clinical signs and death time of the mice in different groups were carefully observed. The results revealed that all of the mice challenged with YNG or YNP strain displayed severe itching before 42 hpi, and died before 72 hpi, while the mice inoculated with HNLY strain only exhibited slightly clinical signs before 72 hpi (Fig. 5A ). Additionally, the brains of mice infected with YNG and YNP strains showed similar pathological changes, such as severe hemorrhage, while those challenged with HNLY strain only presented slight hemorrhage lesion when compared with the control group (Fig. 5B ). Accordingly, higher viral copies in the cerebrum, kidney, and spleen of YNG or YNP strain-infected mice were found, relative to those in HNLY strain-infected mice (Fig. 5C ), indicating a similar pathogenicity of YNG strain as PRV variant to mice.
DiscussionThough extensive efforts have been devoted to the eradication of the disease caused by this pathogen, PRV remains pandemic in some countries/regions, mainly in Asia and Europe (Q. Liu et al., 2020). For instance, the latest researches revealed that the average positive rate of PRV nucleic acid among 16256 tissue specimens collected from the mainland China between 2012 and 2017 was 8.3% (Sun et al., 2018), while the sero and molecular prevalence rates of wild PRV in pigs in Shandong province of China from 2015 to 2018 even reached to 52.7% (8667/16457), 15.7% (257/1638), respectively (Ma et al., 2020), indicating the still severe status of wild PRV in Chinese pig populations. The current study reported the first case of natural PRV infection in goats, thereby further expanding the list of non-natural hosts challenged by this pathogen. Thus, our new finding, together with previously confirmed species including humans and its lethality to these susceptible animals (Ai et al., 2018; Cheng et al., 2020; Jin et al., 2016; H. Liu et al., 2017) and multiple infection ways (Q. Liu et al., 2020), collectively suggest that PRV may be a zoonotic pathogen (Ai et al., 2018; Yang et al., 2019) and more attention should be paid to it in the future.
Large numbers of ruminants raised in China, severe prevalence of PRV in Chinese pig population and frequent transportation of various animals (pigs, goats/sheep, cattle, etc.,) among different regions greatly enhance the possibility of the contact between pigs and these PRV susceptible animals, thereby leading to the transmission of PRV from host pigs to non-host animals (Tan et al., 2021). Consistent with this, our study for the first time confirmed the PRV transmission from pigs to goats, mainly due to the close contact of raising places for those two species. This conclusion was stemmed from a series of evidences: a) serum or fecal specimens from pigs raised nearby presented PRV antibody or nucleic acid positive; b) PRV viruses were confirmed and isolated from infected or died goats; c) isolated YNG strain exhibited highly genetic sequence homology with PRV variant strain YNP, moreover, its pathogenicity was successfully reproduced in mice as its counterpart YNP. These highlight the really threat of PRV to goats or even ruminants, as partially supported by a previous report (Kong et al., 2013). As a precaution measure, far physical isolation of raising places for different animal species is therefore suggested.
Genetically, PRV strains prevalent in the world are divided into two genotypes, most of PRV strains in China belong to the genotype II (He et al., 2019), while the latter one can be further classified into two sub-genotypes (PRV classical and variant strains). Furthermore, PRV variant strains emerged since 2011 (e.g. JS-2012, HN1201, hSD-1/2019) in China showed higher pathogenicity to animal models than classical strains (i.e. Ea and Fa) (Luo et al., 2014; Tong et al., 2015). More direct evidences substantiating the conclusion were that vaccination with PRV Bartha-K61 strain could prevent the occurrence of classical PRV infection but fail to provide complete protection against the newly emerging PRV variants (8, 9). Importantly, the attenuated live vaccine (Bartha-K61 strain) instead led to high pathogenicity to sheep (Kong et al., 2013). Likewise, immunization with Bartha-K61 strain or HB98 strain (another PRV attenuated live vaccine designed based on Ea strain) caused histopathological lesions in dogs (Lin et al., 2019; Willemse, Rondhuis, Goedegebuure, & Maas, 1977). It is worth noting that the recombinant PRV variant strain with US7&US8&UL23 gene deletion is highly lethal to dogs, while the US7&US8&UL23&US3 gene-deleted recombinant PRV strain is safe to this species. Of course, whether this genetically modified PRV strain can be developed for vaccine candidates against variant PRV infection in dogs needs further investigations (Yin et al., 2020). Taken together, these facts described above suggest that currently commercial vaccines might be not suitable for the eradication of PR in ruminants (eg. sheep/goats and cattle), accordingly, safer vaccines with efficient protection should be developed for this purpose in the future.
In summary, we reported the first case of the occurrence of PR in goats and confirmed that the goat-originated PRV strain was a Chinese variant strain by genetic sequence analysis. These suggest that variant PRV strains should receive sufficient attention in terms of the healthy development of goat industry in China. Considering the confirmed PRV infection to humans (Q. Liu et al., 2020), the potential transmission of PRV from goats to humans cannot be ruled out.