Environmental considerations in hepatitis E virus transmission: Is there a missing link?

Abstract

Hepatitis E virus (HEV) is a zoonotic infection that varies according to genotype and geographical location. The full environmental and public health impact of hepatitis E remains uncertain. In this review, we aim to synthesise the evidence regarding HEV circulation in the environment. A literature search was conducted regarding HEV transmission and findings in environmental context, and this was synthesised to propose a hypothesis and identify gaps in our current understanding. Environmental spillover occurs due to increased interactions at the human-animal-environmental interface, with multiple causative factors. HEV genotypes 3 and 4 have been documented in pigs, deer, chicken, rat, mongoose, rabbit, dolphin, cats, dogs and horses. Other HEV genotypes 1,2 and 5-7 have been found in bandicoot, musk shrew, mink, moose, fish and camel. HEV seropositivity has also been identified in ruminant species without HEV - related sequences being identified. HEV has been found in fruit, shellfish and crops irrigated with infected water. This leads to environmental contamination. HEV circulates in the environment via the asymptomatic wild animal host with episodes of spillover into domestic animal populations with increasing burden within the everchanging ‘virosphere’. Through domestic animal amplification, the spillover effect from both wild and domestic animal populations leads to human amplification causing disease with high morbidity and mortality in certain patient populations. Further research is required to consider the viral determinants for HEV cross-species infection and how this relates to HEV-contaminated food products and its impact on environment and human health.

Key Words: Hepatitis E; Viral hepatitis; Environmental; Contamination; Zoonosis; Public health

Core Tip: Globally, hepatitis E infection remains a common but unpredictable cause of acute viral hepatitis. Environmental contamination represents the critical missing link in understanding the transmission of hepatitis E virus from animal reservoirs to humans, particularly the pathways enabling interspecies transmission. Whilst many countries screen blood donors for the virus, circulation within the food chain and water supply inhibits our ability to protect those particularly vulnerable to infection.

INTRODUCTION

Hepatitis E virus (HEV) infection occurs globally and can affect both humans and animals. The genotypes of hepatitis E are clinically and epidemiologically distinct. Genotypes 1 and 2 are endemic in many developing countries as a leading cause of acute viral hepatitis[1]. While these genotypes typically cause self-limiting disease, they can result in significant mortality in patients with existing chronic liver disease of up to 70%[2-4]. The occurrence of these genotypes in outbreaks has typically been a priority in research above environmental factors.

Genotypes 3 and 4 are recognised as endemic in the developed world, occurring primarily as a porcine zoonotic infection[5,6]. Genotype 4 is mostly found in Japan and China. The virus has been isolated in pig slurries, water, seawater and pork products[7-9]. Cases are sporadic, with proximity to coastal areas identified as a more important as a risk factor than proximity to pig farms[10,11]. The movement of the virus through the environment appears to be important in the transmission from pigs to humans, but our understanding is incomplete. Transmission is complex with interconnected factors and therefore it is difficult to design focused studies on environmental factors. The methods of measuring seroprevalence are variable and non-standardised, further complicating study of this area.

This complexity may be addressed using a one health approach, a unified strategy that aims to combine understanding of the interaction between human, animal and environmental factors. This has previously been applied to other viruses such as the avian flu[12]. Hepatitis E is an underestimated and emerging threat. Anti-HEV seroprevalance suggests that exposure to the virus is increasing even in hyper endemic areas[13]. Infection may be asymptomatic, or present with an acute hepatitis, with jaundice, abdominal pain, fever, and nausea after a 2-6 weeks incubation[11,14].

Hepatitis E infection is widely regarded as an acute, self-limiting infection. However significant morbidity and mortality can occur in up to 25% of clinically identified cases[15]. Although acute liver failure is seen less commonly, HEV is an important cause of decompensated liver disease in patients with pre-existing chronic liver disease[16]. Immunocompromised patients such as transplant recipients, human immunodeficiency virus-infected patients with low CD4 cell count and patients with haematological malignancy are vulnerable and can develop life-threatening liver failure[17]. A significant minority of patients will develop extra-hepatic manifestations. Neurological illness, including brachial neuritis, Guillain-Barre Syndrome, peripheral neuropathy and neuromyopathy have been observed in 7.5% of patients with HEV. Additionally, 2.4% of patients presenting with acute neurological disorders have evidence of recent HEV infection[18,19]. Haematological disorders such as thrombocytopenia and monoclonal gammopathy of uncertain significance can occur, and other manifestations such as thyroiditis have been observed. Cases of HEV-induced glomerulonephritis have been documented, with regression following viral clearance, whilst other manifestations were only observed with HEV infection[20-22]. Overall, HEV is not always a self-limiting illness, and is an important cause of mortality in vulnerable groups and has the potential to cause morbidity in everyone (Figure 1).

Figure 1 Clinical manifestations of hepatitis E virus infection by organ system, highlighting high-morbidity groups. The figure created in BioRender.

Previous research has focused on the prevalence of HEV in animals, humans and the environment but to date the exact circulation between these has remained elusive. In this review, we summarise the current understanding of the degree of environmental contamination of the HEV and propose a novel hypothesis for the circulation from animal reservoirs to human infection.

LITERATURE REVIEW

A PubMed search was conducted using the search strategy and returned 1206 results. Following automated screening, 190 papers underwent manual review, resulting in the inclusion of 50 studies. Inclusion of high-quality systematic reviews and meta-analyses, as well as smaller studies conducted in other species, to capture the broad scope of HEV. Considerable variability was observed in the detection methods employed, which represents the primary limitation in comparing and synthesizing data[20]. Evidence from these studies was synthesized to consolidate the current understanding of HEV, construct a narrative on its circulation, and identify gaps within the existing literature.

GLOBAL IMMUNOGLOBULIN G SEROPREVALENCE

Screening of blood donor samples for the presence of immunoglobulin G (IgG - past exposure), IgM (acute infection) anti-HEV antibodies and RNA gives us a picture of exposure to HEV in the population. Differences in the performance of available serological assays introduce variability in the estimation of HEV seroprevalence. Earlier IgM assays exhibited sensitivities around 83%, with a reported manufacturer-dependent range of 76%-88%[21]. Contemporary assays demonstrate markedly improved diagnostic performance, with sensitivities exceeding 98% in immunocompetent individuals[23].

Seroprevalence of anti-HEV IgG exhibits substantial heterogeneity across Europe, with notable intra-country variability. The highest national prevalence has been reported in France (22%), with hyperendemic regions such as the South-West reaching rates as high as 52%[19]. By comparison IgG seroprevalence is 6.1% in the United States, 18.02% in China, 60% in India 27.9% in South Africa and 12.7% globally. Overall, 1 in 8 individuals have been exposed to HEV[22,23]. In Scotland, the rate is rising, recently estimated at 6.1%[24]. Seroprevalence is higher in elderly, males and those who eat meat daily[25,26]. This points to the importance of dietary factors, and that contamination in the food chain is an important consideration.

The high IgG seroprevalence of HEV genotype 3 and 4 in these regions indicates a high level of sub-clinical, historic exposure. Therefore, the clinically diagnosed cases of acute hepatitis E are only the ‘tip of the iceberg’ and the virus is much more pervasive within the environment. Understanding the degree of environmental contamination is essential in explaining this phenomenon. Ultimately, acute and chronic hepatitis E infections are diagnosed sporadically, occurring unpredictably within the population. Exposure tracing where a point-source is identified in a cluster or outbreak is logistically difficult and time critical[11]. Everyone is at risk. Given the burden of morbidity and mortality, it is imperative that a deeper understanding is gained regarding how HEV exists in the environment and how human activity spreads infection.

ENVIRONMENTAL CIRCULATION

The circulation of HEV in the environment, and transmission to humans, is complex and it is likely that multiple routes of transmission exist[11]. Genotype 3 is known to be a zoonotic infection in pigs, occurring as a subclinical entity[27]. Prevalence is as high as 85.5% in the United Kingdom pig population[28]. Circulation between pigs within commercial farms mostly occurs by the faecal-oral route, and occurs between pigs of all ages. The seroprevalence in pigs at age of slaughter in the United Kingdom is 93%[29]. Transmission between pigs is common with a reproductive value (R0) of 8.8 - on average one pig will transmit the infection to 8.8 others[30]. The rate varies considerably between farms and is influenced by farming practices and size. Smaller farms and organic farms are at the highest risk[27,31].

In some regions the seroprevalence of anti-HEV antibodies in pig farmers is higher than that of the general population (13% compared to 9% in Sweden) and there have been cases directly linked to occupational exposure[27]. Other studies found that farm workers were not at higher risk[32]. It also appears that living in a coastal area is a greater risk factor than living in proximity to a pig farm[10]. This suggests that contamination of water due to run-off from farms may be part of the process of environmental circulation.

HEV genotype 1 and 2 have been studied widely in water contamination in the developing world and are recognised as a leading cause of water-borne viral hepatitis. Genotype 3 has been shown to be widely present in water courses in developed countries but the importance of this as a route of transmission is less clear. In Europe, 12.2% of water tested has HEV detectable by polymerase chain reaction (PCR). This is highest in untreated wastewater, representing viral shedding in sewage from human sources, but is also detectable in 7.4% of freshwater rivers and lakes[33]. The detection in water does not necessarily represent infectivity, as PCR methods will detect small viral fragments[27]. Increased rainfall, and therefore waste run-off, does not appear to increase the incidence of cases[10]. Contaminated water supplies can explain the transmission of some cases, particularly in water that has not been treated sufficiently[34].

Contamination of coastal waters leads to HEV accumulation in shellfish, which has been linked to outbreaks of disease. The virus, although not heat-stable beyond 70-80 degrees C, can survive in shellfish for years including during freezing processes. Eating raw shellfish may therefore be a risk factor[27]. In Scotland, 92% of mussels tested from the west coast were positive for HEV by PCR testing[35].

Dietary factors are therefore important. Shellfish consumption is associated with exposure to the virus. Meat products containing HEV are a recognised source of transmission to humans, particularly in pork products containing pig liver or if products are undercooked[27,34]. The infection rate in England was shown to be increasing between 2008 and 2016, with the majority of patients reporting an increased consumption of pork products compared to the general population. This pathway of infection is well recognised in the literature, however, modern food processing and industrial practices make tracing the exact source of these infections challenging[29].

There is emerging, evidence of HEV exposure in other animals, which is becoming increasingly recognised as a pathway of transmission. Notably, household animals may be exposed to the virus, although HEV RNA has not been detected. The seroprevalence in dogs and cats has been found to be 10% and 6% respectively, higher than non-domestic animals such as horses. This could represent dietary intake of these animals, perhaps similar to their owners, or a high intake of raw meat[36]. Rocahepevirus has been detected in wild rodent populations, particularly rats, whereas Paslahepevirus has been identified in multiple mammalian hosts including pigs, wild boar, deer, and rabbits. Reports further implicate domesticated ruminants including cattle, sheep, and goats in the epidemiological cycle[27]. This is currently less strongly evidenced than the porcine zoonotic route. Overall, evidence suggests widespread environmental contamination with hepatitis E affecting a wide range of species and several potential transmission routes to humans (Figure 2).

Figure 2 Schematic of hepatitis E virus circulation and transmission in the environment, based on identified routes. The figure created in BioRender.

DISCUSSION

Environmental spillover occurs as a result of increased interactions at the human-animal-environmental interface due to urbanisation, industrialisation, deforestation, migrations, biodiversity loss, pollution, intensive agriculture and animal production. Pathogen spillover from animals to humans occurs via zoonoses, emerging infectious diseases and zooanthroponosis. 75% of global emerging health hazards are zoonotic[37]. Currently the full environmental and public health impact of hepatitis E remains uncertain.

HEV is an RNA virus with a highly diverse genome and appears to have evolved alongside a wide range of hosts[38]. Of the 8 currently identified genotypes, HEV genotype 3 and 4 are well documented zoonotic strains. There are also well described animal hosts across various species. This includes wild and domestic pigs, deer, chicken, rat, mongoose, rabbit, dolphin, cats, dogs and horses. Other HEV genotypes 5-8, plus Avihepevirus and Rocahepevirus and Chirohepevirus (phylogenetically linked, Hepeviridae family), have been found in bandicoot, musk shrew, mink, moose, fish and camel[39-44]. HEV seropositivity has also been identified in ruminant species, for example, goats, sheep and cattle without HEV infection, related strains being identified[36,38-41,43,45-50] (Table 1).

Table 1 Hepatitis E virus genotypes identified by region and animal host[36,38-41,43,45-50].

Genotype	Animal hosts	Countries/regions
HEV 1 and 2	None identified	Africa, Middle-East, Asia
HEV-3	Pig, wild boar, rabbit, deer, mongoose; serology in cattle, goats, sheep, horses, dogs, cats, dolphins	Europe, United States, Asia (China, Japan), Oceania, South (Australia), America, Africa
HEV-4	Pig, wild boar, goat, sheep, cow, deer, bears, zoo birds (crane, pheasant)	China, Japan, France (and some European countries)
HEV-5 and HEV-6	Wild boar	Japan
HEV-7	Camels (plus one human case)	Middle East, China (Xinjiang Uygur Autonomous Region)
HEV-8	Bactrian camels	China (Xinjiang Uygur Autonomous Region)
Avihepevirus	Chicken; infect ducks, geese, rabbits in mixed housing	Australia, United States, Europe, China, Hungary, Taiwan
Rocahepevirus	Rats, ferrets, mink, shrews, bandicoots	Hong Kong
Chirohepevirus	Bats	Various bat-range regions

HEV: Hepatitis E virus.

Virus survival in water is likely to be integral to transmission. HEV has been found in fruit, shellfish and crops irrigated with infected water[35]. Environmental contamination includes slurry and watercourses. The causative relationship between water contamination and human infection is not clear currently and is an area of further research focus. The discrepancy between detection in water and apparent infectivity may be explained in part by immunoglobulin assay methods[51]. Wastewater monitoring may help identify in real time the factors that lead to cases of infection[50]. Further investigation is required to elucidate the role of flooding events in the transmission and spread of HEV.

The virus is unstable when treated with heat. In vitro models for cooking between 70-80 degrees C, for 20 minutes, have been completed. However, it is unclear if this translates to real world cooking or industrial practices such as raw or rare meat, smoked or cured food or highly-processed animal products[52]. HEV has shown significant resistance to drying[44,52].

HEV circulates in the environment via the asymptomatic wild animal host with episodes of spillover into domestic animal populations with increasing burden within the ever changing ‘virosphere’. Through domestic animal amplification, the spillover effect from both wild and domestic animal populations leads to human amplification, causing disease. HEV can infect across species barriers and cause zoonotic infection causing significant burden of human disease with high morbidity and mortality in certain patient populations. Given the pervasive nature of this environmental contamination, reducing the burden of hepatitis E infection in humans is therefore challenging. A human vaccine is currently licensed in China, Pakistan and India[53]. Targeted livestock vaccination is expensive, impractical and may be ineffective given the high degree of cross-over from wild to domestic animal populations, especially considering the high susceptibility of pigs to HEV infection. Blood donor screening is now in place in many countries but is resource intensive and only one of many routes of viral transmission. Water treatment and sewage testing can help track the areas of highest prevalence but does not deal with the virus at the source.

Future research should focus on the routes of transmission occurring within this model of environmental and on further defining the range of animals affected. This will allow a targeted and comprehensive strategy to be designed for reducing spread in accordance with the one health model. A standardised approach to diagnostic assay use would give a more accurate representation of global burden of disease.

CONCLUSION

Globally, hepatitis E remains a common yet unpredictable cause of acute viral hepatitis. Although blood donor screening is implemented in many countries, ongoing circulation of HEV within the food chain and water supply continues to undermine efforts to protect vulnerable populations. Environmental contamination represents the critical missing link in understanding the transmission of HEV from animal reservoirs to humans, particularly the pathways enabling interspecies transmission. Rather than arising solely from a porcine reservoir, HEV circulation reflects a complex cycle of viral exchange between wild and domestic animals across multiple species. Clarifying this environmental loop will be essential to reducing the global burden of disease, as it directly contributes to human morbidity and mortality.

Future research should prioritise identifying environmental sources, characterising transmission routes beyond food-chain and blood donor pathways, and assessing viral persistence through longitudinal studies. Such efforts will enable us to disrupt the transmission cycle, applying a one health framework to design targeted interventions that break the chain of environmental contamination and, ultimately, reduce the incidence of hepatitis E infection in humans.