<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