Many researchers have reported PBoV as a potential emerging pathogen (Blomström et al. 2009; Zhou et al. 2018) of the respiratory tract (Zhai et al. 2010) or the gastrointestinal tract (Amimo et al. 2017). Diversity of host species is assumed to be the specific feature of emerging viruses (Zhang C et al. 2018). To date, PBoV has not been reported as an enteric pathogen in the literature. However, the co-occurrence of this virus with circulating viruses has been reported since its discovery (Table 1) (Zhou et al. 2017; Zhang J et al. 2018). The most common circulating viruses are PEDV (Zhang et al. 2013; Zheng et al. 2020), GARV (Zhang et al. 2013), PRRSV (Blomström et al. 2009; Vlasakova et al. 2014), PCV-2 (Blomström et al. 2009; Blomstrom et al. 2010; Vlasakova et al. 2014; Zhang J et al. 2018), CSFV (Zhang et al. 2011; Zhou et al. 2018), porcine parvovirus (Zhang J et al. 2018), porcine pseudorabies virus (Luo et al. 2015) and porcine kobuvirus (Zhang et al. 2013). Clinically, the incidence of coinfection of PBoV with PCV-2 has been reported to be 83.8%, suggesting that PCV-2 enhances the infectivity of PBoV (Zhang J et al. 2018). Thus, PBoV may not be directly associated with disease and might function as a helper virus for triggering other infectious agents (McKillen et al. 2011).
Order Year Country Gene access number Co-infection References 1 2009 Sweden FJ872544 PCV-2, TTV Blomström et al. (2009) 2 2010 China GU556573 to GU556591 PCV-2, TTV-1, TTV-2, CSFV Zhai et al. (2010) 3 2010 China HM053693 and HM053694 PKV, PCV, Frog virus Cheng et al. (2010) 4 2010 USA GQ387500 and GQ387499 PCV-2 Cheung et al. (2010) 5 2010 Ireland JF512472 and JF512473 Porcine adenovirus, Enteroviruses, Reoviruses, Circovirus, Porcine parvovirus McKillen et al. (2011) 6 2011 Ireland JF512472 and JF512473 ND McNair et al. (2011) 7 2011 China JF429834, JF429836 ND Lau et al. (2011) 8 2011 China HQ223038 ND Zeng et al. (2011) 9 2011 China GU902971 ND Shan et al.(2011a, b) 10 2011 Romania JF721404–JF721421 PCV-2 (data not shown) Cadar et al. (2011) 11 2012 China NA PEDV, PKV, RVA, TGEV Zhang et al. (2013) 12 2012 Hungary JN400850 to JN400879 PPV2, PPV3, PPV4, PBoV1, PBoV2, 6 V and 7 V and PCV-2 Csagola et al. (2012) 13 2013 Croatia KC701291–KC701314, KC687097– KC687100, KC701315–KC701332, KC701333– KC701356, KC767891 ND Cadar et al. (2013) 14 2013 Cameroon JX869077 to JX869100 Mixed infection of different groups of porcine parvovirus Ndze et al. (2013) 15 2013 Uganda JX854557 TTSuV1 and TTSuV2 Blomstrom et al. (2013) 16 2013 Korea KF425330 to KF425337
KF728243 to KF728248
Co-infection between group1, 2 and 3 Choi et al. (2014) 17 2014 Slovakia and Czech Republic NA PCV-2, TTSuV1, TTSuV2, PBoV, PRRSV, PTV Vlasakova et al. (2014) 18 2015 North America KR709262 to KR709268 PRRSV Schirtzinger et al. (2015) 19 2015 Thailand AB973315-AB973334, AB973335-AB973354 PPV1, 2, 3, 4 Saekhow and Ikeda (2014) 20 2015 Japan LC090199 ND Zhang et al. (2016) 21 2016 Germany KU311698 Mycoplasma hyorhinis Pfankuche et al. (2016) 22 2017 Belgium KY426738 to KY426752 PAstV, PEDV, Porcine enterovirus, Porcine picobirnavirus Conceicao-Neto et al. (2017) 23 2018 Malaysia KX686996 to KX686700 PCV-2/PMWS Jacob et al. (2018)
Table 1. General information of documented porcine bocavirus cases.
Analysis of 12 fecal samples from diarrheic piglets and 24 fecal samples from healthy piglets by Shan et al. revealed that bocavirus (5/12 vs. 3/24) and coronavirus (11/12 vs. 13/24) are more prevalent in diarrheic than in healthy piglets (Shan et al. 2011b). The researchers concluded that the high rate of coinfection seen in high-density farms provides favorable conditions for recombination and accelerates viral evolution. In Uganda and Kenya, bocavirus was isolated from piglets with no signs of clinical diarrhea (Amimo et al. 2017). During a diarrhea outbreak in fattening pigs from Belgium, PEDV was detected along with PBoV, recombinant enterovirus-toroviruses, astroviruses, enteroviruses, and picobirnaviruses. The presence of PEVD (Cq > 30) in low loads along with high loads of PBoV and several enteroviruses suggests that the cause of viral diarrhea was a virus other than PEVD and that bocavirus was the most likely cause (Conceicao-Neto et al. 2017).
The pathogenesis of PBoV has not been determined to date. It is thought that, as PBoV is highly prevalent in the swine population and has high genetic diversity, its pathogenesis could be determined by direct evidence from clinical disease (Zhou et al. 2017). The detection of bocavirus in different tissues represents a wide range of tissue tropism (Lau et al. 2011). PBoV was detected for the first time in the lymph nodes of weaning pigs suffering from PMWS (Blomström et al. 2009). Soon after that, it was detected in fecal samples from piglets (Cheng et al. 2010) as well as the respiratory tract (Zhai et al. 2010). In diarrheal disease, histopathological changes, including microscopic lesions and villous atrophy, are mainly detected in the jejunum, ileum, and duodenum (Zhang et al. 2013). Bocavirus has been isolated from piglets with encephalomyelitis, demonstrating the pathological role of this virus (Pfankuche et al. 2016). In a study novel porcine parvovirus, PPV4, similar to porcine bocavirus was identified from the lung lavage of a diseased pig coinfected with PCV-2. Tissue homogenates consisting of lung, lymph node, spleen, and heart were prepared and inoculated into two colostrum-deprived pig oronasally and monitored clinically. Both pigs developed respiratory disease and were euthanized. However, due to the coinfecting virus PCV-2, it was not clear if it caused disease on its own or contributed to the clinical disease (Cheung et al. 2010). Information related to the isolation of PBoV from different tissues is presented in Table 2. Lymph nodes, spleen, and tonsil have the highest detection rate suggesting that these organs are the sites of active replication and the pathogenesis of PBoV infections may involve the lymphoid tissues and gastrointestinal tract (Jacob et al. 2018). Besides tissues, bocavirus has been detected from the saliva (Choi et al. 2014) and serum (Csagola et al. 2012; Choi et al. 2014; Meng et al. 2018). In brief, bocavirus has been isolated from different tissues of the pig, but it is still not clear which cells in pigs are responsible for its multiplication and dissemination throughout the body.
Tissue Key findings References Lymph nodes The infection rates of PBoV in the PMWS-affected pigs were twice higher than in the non-PMWS affected pigs. The co-infection of PBoV along with the TTSV and PCV-2 might have facilitate the development of PMWS Blomström et al. (2009) The co-existence of two bocavirus strain within the same fecal sample revealing inter and intra host genetic diversity Lau et al. (2011) The genetic diversity of the circulating bocavirus strains in Xinjiang belong to three subgroups of three different genetic groups Meng et al. (2018) Gastrointestinal tract The prevalence rate of PBoV was higher in stool samples and these viruses multiply in the intestinal tract of piglets Cheng et al. (2010) Respiratory tract The first evidence of infection of weaning piglets with respiratory tract symptoms representing an emerging virus for swine respiratory tract diseases Zhai et al. (2010) Nasopharyngeal sample PBoV were higher in nasopharyngeal samples in deceased pigs than in healthy pigs Lau et al. (2011) Mesenteric lymph nodes The mesenteric lymph node had the highest detection rate suggesting the pathogenesis of PBoV infection involves the lymphoid tissues Jacob et al. (2018) Inguinal lymph nodes 25% of the organ tested were positive for PBoV Jacob et al. (2018) Spleen 23.5% of the spleen tested were positive for PBoV Jacob et al. (2018) Tonsil Out of 80 tonsil samples, 23 samples were positive for the PBoV Saekhow and Ikeda (2014) The tonsil had the second highest detection rate suggesting the pathogenesis of PBoV infection involves the lymphoid tissues Jacob et al. (2018) Lung Porcine parvovirus 4 was similar to PBoV. After inoculation of tissue homogenate in the colostrum deprived piglets, clinical symptoms were observed. But due to coinfection with PCV-2. It was not clear whether PPV4 can cause disease on its own or contributed to the disease phenomenon Cheung et al. (2010) One lungs tissue sample was positive for the PBoV without coinfection of the PCV-2, suggesting PBoV as not the risk factor the Hungarian pigs Csagola et al. (2012) The first description of the prevalence of PBoV in Korean swine herds with the mean positive rate of 34.9% Choi et al. (2014) 33.3% of lungs tissues were positive for PBoV Jacob et al. (2018) Kidney PBoV was detected from kidney tissues of two pigs suggesting the ability of virus to replicate within kidney cells causing renal pathology Jacob et al. (2018) Cerebral tissue By using fluorescent in situ hybridization for histologic detection of encephalomyelitis assigns a potential role of PBoV in provoking CNS lesions Pfankuche et al. (2016) Liver 25% of liver tissue were positive for PBoV Jacob et al. (2018)
Table 2. Tissue tropism of porcine bocavirus.
However, in a study conducted in Thailand in dogs, it was reported that most of the viruses belonging to the Parvoviridae family replicate in mitotically active cells such as intestinal crypt epithelial cells. Analysis of tissue samples from the small intestine revealed nuclear signals for canine bocavirus 2 in enterocytes, mainly those located at the villus tips and crypts. Transmission electron microscopy showed numerous electron-dense icosahedral viral particles measuring approximately 20 nm in diameter that aggregated to form large intranuclear inclusion bodies within apical small intestinal enterocytes (Piewbang et al. 2018). This study provides novel insights into the pathogenicity of canine bocavirus infections, which could also apply to other bocaviruses isolated from different animals. Rats naturally infected with parvovirus are mostly healthy. However, in some cases, growth retardation and fetal loss is observed in pregnant rats experimentally inoculated with parvovirus (Lau et al. 2016).
Cells lines for the isolation of bocavirus have not yet been reported. Different cells such as porcine kidney cells (PK-15), swine testicular cells, porcine alveolar macrophages, monkey kidney cells (MARC-145), and human embryonic kidney epithelial cells (HEK293T) have been used for the propagation of PBoV. However, no success has been reported (Zeng et al. 2011). During the diarrheal disease outbreak in Belgium, an attempt was made to isolate bocavirus and recombinant enterovirus-torovirus from primary porcine kidney epithelial cells and swine testicular cells. Although a cytopathic effect was observed in porcine kidney epithelial cells after two days, no virus could be detected in passaged cells (Conceicao-Neto et al. 2017). In another study in Northern Ireland, fecal suspensions and homogenates of the small intestines of 6-week-old piglets exhibited cytopathic effect after four passages in cultured primary porcine kidney cells. PCV1, PCV-2, PPV, porcine enterovirus types 1, 2, and 3, porcine adenovirus, and porcine reovirus were not detected by PCR or RT-PCR; thus, this study was the first to describe the growth of bocavirus in a primary cell line (McKillen et al. 2011). Because this virus cannot be cultured in cell lines and since it commonly coexists with other circulating swine enteric viruses, the exact role of this virus in disease progression is unclear.
Due to the association of bocavirus with other circulating pathogenic enteric viruses, its nature has always been confusing. Due to the involvement of bocavirus in respiratory tract infection as well as gastrointestinal tract infection, it is still unclear whether bocaviruses are respiratory pathogens or enteric pathogens, especially in humans, pigs and other animals. Due to the lack of an experimental animal model, the pathogenic nature of PBoV has not been investigated.