<FIGURE 2>
The HEV infection in pigs normally occurs during early life, and the virus shedding peaks at about at age of 3 months. The people occupationally exposed to pigs increase the probability of the past HEV infection by almost threefold.51 A cross-sectional study was conducted in Lao People’s Democratic Republic, the prevalence of anti-HEV IgG antibody in slaughter pigs was 54.0% (136/252), in professionals exposed to pigs was 41.0% (57/139), and 18.1% (38/210) was in non-risk controls. The results showed that the individuals occupationally exposed to pigs are at higher risk of HEV infection (p < 0.001). Furthermore, contact with young piglets was a major risk factor as compared to contact with the older slaughter pigs (p = 0.011).52 Another recent study published from 10 US states, reported that the prevalence of HEV-IgG antibody was 40% of 5,033 serum samples from market-weight pigs at 25 slaughterhouses, of which the HEV RNA was detected in 6.3%. The detected sequences belonged to HEV genotype 3, group 2 (HEV-3abchij).53
Studies from the European countries reported a high prevalence of HEV in pigs. In Germany, the reported anti-HEV IgG seroprevalence in pigs (at farms) was 70.7%.54 Another study from Germany, reported the overall seroprevalence of anti-HEV IgG in serum to be 68.6% and in meat juice samples from pigs (at slaughter) as 67.6%.55 In the Netherlands, HEV seroprevalence in pigs (at slaughter) was 70%, and 50% RNA +ve on the farm.56 In Scotland, the overall seroprevalence of anti-HEV antibodies in slaughter-age pigs was 61·4%.57 In Switzerland, the overall HEV seroprevalence of 12.5% was documented in wild boar, and 58.1% in domestic pigs.58
In Bulgaria overall, HEV seroprevalence has been noted to be 60% (95% CI 42.7-77.1) in domestic pigs and 12.5% (4/32) in wild boar with seroprevalence of the slaughter-aged pigs being 73.65% (95% Cl 58.7-87.3), higher prevalence in domestic pigs indicating the probable reason for cross-species transmission.21 The involvement of other animal species including both domestic and wild ones risks human life also. Around 10.12% samples (95% CI: 5.44-14.8) from wild boar and 16.05% samples (95% CI: 8.06-24.04) from red deer have been positive for HEV RNA detected by real-time RT-PCR.59 Though none of the 48 samples from Iberian pigs, was positive for HEV RNA detection but 43.75% (95% CI: 29.75-57.75) serological samples from Iberian pig were positive for anti-HEV antibodies when the 57.40% (95% CI: 48.10-66.70) from wild boar and 12.85% (95% CI: 5.01-20.69) samples from red deer were positive.59
Constant contact and consumption of pig and pig products increase risk and hence warrants proper inspection and surveillance.59-60 Raw viscera more likely harbors HEV than pork, and ground pork and pig liver could act as a potential source of this virus.60-61 In one study HEV RNA has been detected in liver, kidney, and blood samples with positivity of 6.1% (7/114), 3.1% (4/129), and 1.2% (2/170) respectively, and viral loads ranged from 102.4to104.4 (2.4Log-4.4Log) genome equivalents per gram whereas none has been detected in pork indicating a higher transmission risk related to consuming pig organs.61 Similarly, HEV has been detected in 99 pork and 63 wild boar sausages and salami sold in Southern Italy few belonging to the HEV-3 genotype.62 However the mere presence of HEV in samples may rarely matter than the infectivity of HEV and there should be a robust system for measurement of HEV infectivity in food samples with cell culture-based systems having promising prospects.63
Though detection of HEV RNA in pigs and retail pork livers have been found comparable to that noted in the USA and Europe however the incidence of sporadic locally acquired hepatitis E in Canada, is believed to be low relative to other non-endemic countries thus posing a relatively low risk in vulnerable Canadian population.64 Anti-HEV antibodies have been detected in 77.6% to 90.6% of serum samples from backyard pigs by indirect enzyme-linked immunosorbent assay in southern Brazil showing a herd seroprevalence of 83.7% to 91.7%.65 HEV RNA has been detected in 0.8% (6/713) of samples falling in three different genotypes 3 subtypes related to human HEV strains indicating backyard pigs as a reservoir of HEV and thus need to control infection and spillover from backyard farms.65
HEV-RNA has been detected in 10 of the total 90 (11.1%) meat products, 7 of 37 (18.9%) liver sausages, and 3 of 53 (5.7%) raw meat sausages in Switzerland.66 Virus loads of up to 5.54 log10 HEV genome copies per gram have been estimated. These belong to HEV genotype 3. This study indicates the presence of HEV on ready-to-eat meat products on the Swiss retail market and hence necessity for developing efficient diagnostic methods for easy detection.66 Boxman et al. 67 have also detected and quantified hepatitis E virus RNA in ready-to-eat raw pork sausages in the Netherlands and have found 46 of 316 samples positive for HEV RNA. The average viral load in positive tested products has been found to be 2.76 to 4.5 log10 genome copies per 5 g. This study provides the basis for risk assessment and risk management with respect to pork and pork product consumption and raw pork sausages as a risk factor for hepatitis E virus infection in the Netherlands.67
Various ingredients related to pigs and piggeries are considered as a potential risk for transmission of HEV. Hepatitis E virus (Genotype 3) has been noted in slurry samples from swine farming activities in Italy.68 Of the 24 samples, 18 samples (75%) have been found positive for HEV RNA and characterized as genotype 3.68 On quantitative Real-Time PCR assay two swine slurry samples that were found positive, contained 600 and 1000 UI per mL of sewage.68 Hence improving swine farming operations safety and increasing operators’ awareness of the zoonotic potential of swine waste can minimize risk for HEV. HEV RNA has been found in fresh meat markets with 0.23 % in the market samples and 3.93 % in the slaughterhouse samples in Bangkok.69 Fecal and bile samples were commonly found positive compared to liver, pork, and intestine samples hence posing a risk of zoonosis. Domestic pigs, wild boars, pig farmworkers, and hunters in Estonia have demonstrated the presence of HEV hence the requirement of attention for direct contact persons or handlers.70 Not only direct contact but also environmental contaminations are responsible for not only the transmission of HEV in pigs but also in humans.71
In one study average latent period has been noted to be 6.9 days (5.8; 7.9) in inoculated animals, an average infectious period of 9.7 days (8.2; 11.2), however the quantity of virus existing in the environment was found to play a vital role in the transmission process.71 It has also been shown that porcine blood which is used as an ingredient in meat productions may serve as a vehicle for HEV transmission.72 HEV RNA has been detected in 33/36 batches of (non-heated) liquid products and in 7/24 spray-dried powder products.72 Contamination levels through variable but were found to be highest in liquid whole blood, plasma, and fibrinogen reaching levels of 2.2×102 to 2.8×102 HEV genome copies per 0.2g respectively.72
A study from Finland, reported 11.9% of domestic pigs to be HEV RNA positive. The occurrence of HEV was found high in fattening pigs (14.8%) and the lowest in weaning pigs (12.5%).73The mean seroprevalence was 87.0% in swine at a slaughterhouse in Italy, 64.6% pigs were positive for HEV RNA in at least one sample, and the HEV genome was detected in bile samples, feces, and liver, 51.1%, 33.3%, and 20.8%, respectively.74 In another study from 4 abattoirs in Italy, 21 (3.6%) out of 585 pigs were tested positive for HEV RNA in either feces or liver by real-time RT-PCR, ELISA tests revealed a high seroprevalence (76.8%) of HEV antibodies pointing out frequent exposure to virus, and circulation of HEV-3c and a novel unclassified subtype was noticed by phylogenetic analyses.75 A study from Denmark reported faecal samples of 49.5% Danish pig population to be positive for HEV RNA.76 In a study published from France, the farm-level HEV seroprevalence was 65%, and 31% in the slaughter-aged pigs. Furthermore, 4% of livers were recorded as HEV RNA positive.77
Studies from other countries also reported a high prevalence of HEV in the pig population.21,51,56 A study published from China reported the overall seroprevalence of anti-HEV antibodies was 64.7% in swine.78 Furthermore, the prevalence of HEV was high in swine farmers than that of the general population. However, peoples living in the urban area showed a high prevalence of HEV infection.78 A systematic review and meta-analysis conducted recently, based on studies published within the last decade (2010 to 2019) indicated that HEV infection is very common among swine population in China, and suggested that reducing the mixed feeding of different stages could decrease HEV infection in pigs and consequently prevent from the risks of transmission of zoonotic HEV infection from pigs to humans.79
A study conducted to analyze the serological prevalence and molecular characterization of HEV in imported pigs in Singapore for the past two decades (2000-2019) from live pig and post-slaughter samples demonstrated the presence of HEV in pigs being used for consumption by humans.80 This study suggested the importance of regular monitoring of the prevalent HEV strains and evaluation of the genetic diversity of HEVs in the imported pigs to confirm the potential association of the role of pigs for transmission of HEV to humans.80
In one study conducted in Japan, of the 160 serum samples collected from pigs and tested for the presence of HEV antibodies (IgG & IgM) showed that 116 (72.5%) were found IgG positive and IgM negative, while 38 (23.8%) were positive for both IgG and IgM.81 A recent study showed that those individuals who had exposure to pigs/pork meat when compared with unexposed individuals had high chances of HEV infection.46 Anti-HEV IgG seroprevalence was 66% in slaughterhouse staff, 51% in pig-farmers, and 38% in pork meat vendors (P =0.00073).46 In a study from Abruzzo, Italy, reportedly hyperendemic region having highest HEV seroprevalence in humans, out of 233 blood samples collected from different local pig slaughterhouses between 87.3% and 100% of serum samples were found positive for anti-HEV antibodies (IgG), such high seroprevalence in pigs indicates that HEV is highly circulating in this region.82 Previously published studies showed that the pigs are successively infected with HEV on farms, and the prevalence rate increases with age.26,83-84