Epidemiology of hepatitis E: Current status

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1 doi: /j x EPIDEMIOLOGY MINISERIES jgh_ Epidemiology of hepatitis E: Current status Rakesh Aggarwal and Sita Naik Departments of Gastroenterology and Immunology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, India Key words Enterically-transmitted hepatitis, Hepatitis E, Hepatitis E virus. Accepted for publication 11 May Correspondence Dr Rakesh Aggarwal, Department of Gastroenterology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow , India. aggarwal.ra@gmail.com Abstract Hepatitis E, caused by infection with hepatitis E virus (HEV), is a common cause of acute hepatitis in areas with poor sanitation. The virus has four genotypes with one serotype: genotypes 1 and 2 exclusively infect humans, whereas genotypes 3 and 4 also infect other animals, particularly pigs. In endemic areas, both large outbreaks of acute hepatitis as well as sporadic cases occur frequently. These cases are usually due to genotype 1 or 2 HEV and are predominantly caused by fecal oral transmission, usually through contamination of drinking water; contaminated food, materno-fetal (vertical spread) and parenteral routes are less common modes of infection. The acute hepatitis caused by this virus has the highest attack rates in young adults and the disease is particularly severe among pregnant women. HEV superinfection can occur among persons with pre-existing chronic liver disease. In non-endemic regions, locally acquired disease was believed to be extremely uncommon. However, in recent years, an increasing number of cases, due mostly due to genotype 3 or 4 HEV, have been recognized. These are more often elderly men who have other coexisting illnesses, and appear to be related to zoonotic transmission from pigs, wild boars and deer, either food-borne or otherwise. Also, chronic infection with genotype 3 HEV has been reported among immunosuppressed persons in these regions. A subunit vaccine has been shown to be effective in preventing clinical disease, but is not yet commercially available. Our understanding of hepatitis E epidemiology has undergone major changes in recent years, and the future may hold even more surprises. Introduction and history of hepatitis E virus Hepatitis E is caused by infection with hepatitis E virus (HEV), the most recently discovered of the five well-recognized hepatotropic viruses, named A to E. The earliest well-documented report of this disease was a large epidemic of water-borne hepatitis in New Delhi, India during 1955 to Although it was initially believed to be related to hepatitis A, subsequent testing of stored sera from this epidemic and another outbreak during 1978 to 1979 in Kashmir, India failed to demonstrate specific serological markers for hepatitis A and hepatitis B, using tests which had only then become available. 2,3 A new agent for viral hepatitis was implicated and provisionally named as enterically transmitted non-a, non-b hepatitis virus. Shortly thereafter, virus-like particles were identified using immune electron microscopy in the feces of a volunteer who ingested fecal matter from patients with non-a, non-b hepatitis. 4 The virus was subsequently named hepatitis E virus (HEV), based on its enteric route of transmission and propensity to cause epidemics. Its genome was cloned in 1990 and fully sequenced shortly thereafter. 5,6 Hepatitis E is endemic in regions with poor sanitation and hygiene, including large parts of Asia, Africa, the Mediterranean region (Fig. 1), Mexico and South America. 7 In endemic regions, it is responsible for medium- to large-sized waterborne epidemics and a large proportion of cases with sporadic acute hepatitis. In developed countries, the disease was initially found to occur almost exclusively among travelers to disease-endemic regions. However, over the last one decade, an increasing number of autochthonous (locally acquired) cases have been identified in several developed countries and evidence for a zoonotic reservoir has been uncovered. This has led to a global resurgence of interest in this disease. Several hepatitis outbreaks that occurred in Europe during the 19th century and early part of the 20th century, when sanitary conditions in these areas were still poor, had features similar to those of hepatitis E. This suggests that the disease was once common in the areas where it is currently non-endemic. Hepatitis E virus Morphology and genomic organization Hepatitis E virus virions are small, spherical particles of nm diameter in size. The viral genome consists of a polyadenylated, single-stranded RNA, which is approximately 7.2 kb in length. It contains short non-coding regions at both the 5 and the 3 ends, and three discontinuous and partially overlapping open reading 1484 Journal of Gastroenterology and Hepatology 24 (2009)

2 Figure 1 Epidemiology of hepatitis E World map showing hepatitis E endemic regions. frames (ORF).6 ORF1 codes for viral non-structural proteins and contains several conserved domains, including putative methyltransferase, protease, helicase and RNA-dependent RNA polymerase. ORF2 codes for the viral capsid protein, and ORF3 for a small phosphoprotein with uncertain function.8 The virus is placed in genus Hepevirus, which is the only member of the family Hepeviridae. The genus is believed to consist of two species: (i) mammalian HEV, which causes human disease and infects several other mammalian species, in particular pigs; and (ii) avian HEV, which is responsible for big liver and spleen disease in chicken,9,10 and is known to infect other birds such as turkeys.11 Genotypes and genotypic heterogeneity On genomic sequence analysis, human and swine HEV isolates group into four genotypes, namely genotypes 1, 2, 3 and 4 (Fig. 2), each with several subtypes.12 Avian isolates of HEV have a shorter 6.6 kb genome with only approximately 50% sequence homology with mammalian isolates. These thus constitute a genetically distinct group, which was initially proposed to be a fifth HEV genotype, but appears to be a separate genus. Each HEV genotype appears to have a specific geographical distribution (Fig. 3). Genotype 1 HEV has been isolated from human cases of epidemic and sporadic hepatitis E in parts of Asia and Africa, where the disease is known to be endemic.12 It has also been isolated from hepatitis E cases among travelers to these regions from nonendemic areas. These isolates have a nucleotide sequence homology of more than 90% with each other. Genotype 2 sequences, first reported from an outbreak of hepatitis E in Mexico, have subsequently been found in cases reported from western Africa (Nigeria and Chad). These isolates have nucleotide homology of only 75% (amino acid homology 86%) with genotype 1 isolates Genotype 3 HEV was first identified in human cases of locally acquired hepatitis E in the USA (US); these genotype 3 US isolates showed 92% homology to each other but only 74 75% nucleotide homology to isolates from genotypes 1 and Since then, genotype 3 isolates have been reported from human cases in several industrialized countries in Europe (United Kingdom [UK], France, Netherlands, Spain, Austria, Greece, Italy), Japan, Australia and New Zealand; some genotype 3 isolates have also been reported from Korea and Argentina.12,18 Genotype 4 HEV has been found in sporadic cases with acute hepatitis from China, Taiwan, Japan and Vietnam.12,19 All HEV genotypes share at least one major serologically cross-reactive epitope and belong to a single serotype.20 Several mammalian species (pigs, cattle, sheep, goats, horses, macaques, cats, dogs, rats and mice) show serological evidence of HEV infection. The infection has been extensively studied among pigs, in whom it appears to be particularly common. Swine HEV, first identified in Midwestern USA,21 shares approximately 90 92% and 79 83% identity at the amino acid level in the ORF2 and ORF3 regions, respectively, with the Burmese and Mexican isolates of human HEV.16 It has now been found in pigs from nearly all parts of the world, irrespective of the endemicity level of hepatitis E in human populations in the same region. Swine HEV naturally infects pigs, leading to transient viremia and generation of antibodies that cross-react with human HEV strains.21 However, pigs infected with HEV do not show any clinical disease. On phylogenetic analysis, swine HEV isolates belong to genotypes 3 and 4 (genotype 3 in USA, Europe, Australia and Japan, and genotype 4 in Taiwan, China, Japan), depending on which of these genotypes is predominant among human cases in the particular geographical region.12 In addition, swine HEV isolates from Taiwan, Spain and Japan have shown a greater genetic similarity to human HEV isolates from these respective regions than to swine and human HEV isolates from the rest of the world However, in India, where disease is highly endemic, swine HEV isolates have all been genotype 4,25,26 whereas all human cases studied have Journal of Gastroenterology and Hepatology 24 (2009)

3 Epidemiology of hepatitis E Japan 04 Hu China2 Sw Japan 05 Hu China3 Sw India Sw China 02 Hu China1 Sw Japan 01 Hu Japan 07 Hu Japan 01 Sw Canada Sw Japan 02 Hu Japan 06 Hu USA Sw 3 US1 Hu Japan 03 Sw Japan Sw Japan 09 Hu Japan 08 Hu 4 US2 Hu 2 Mexico Hu 1 Morrocco Hu Avian USA (Outgroup) 0.1 Japan 10 Hu Burma 2 Hu Burma Hu India M Hu India Ha Hu India Hy Hu India LF Hu China 3 Hu China 2 Hu Pakistan Hu China 1 Hu Figure 2 Phylogenetic tree based on complete genomic sequences of selected human and swine hepatitis E virus (HEV) isolates, showing their genetic relationship. Human sequences are shown in black and carry a suffix Hu. Swine isolates are shown in red and carry a suffix Sw. Genotype groups are indicated inside circles drawn on each main branch. Genotypes 1 and 2 HEV cause infection only in humans, whereas genotypes 3 and 4 HEV infect both humans and swine. An avian HEV sequence has been used as an outgroup; the line showing distance of this isolate from other HEV isolates has been truncated. The figure was drawn using MEGA software. The horizontal bar represents a scale for relative genetic distances. revealed genotype 1 HEV. Genotype 3 HEV has also been isolated from other animals, such as deer and wild boar. 27,28 No animal HEV isolate belonging to genotype 1 or 2 has been reported to date. Occasional reports of isolation of HEV from horses and rodents appear to represent laboratory contamination, rather than genuine infection. In experimental studies, genotype 3 isolates from humans and swine have been shown to cross species barriers and to infect specific pathogen-free pigs and non-human primates, respectively. 17,29 In addition, genotype 4 HEV from Indian swine has been experimentally transmitted to primates. 30 However, epidemic strains of HEV (genotype 1 and 2) have not yet been successfully transmitted to animals, indicating that these strains may not be capable of crossing the species barrier. 31 HEV isolates belonging to genotypes 3 and 4 appear to be somewhat less pathogenic in humans than those from genotypes 1 and 2. In a study from Japan, genotype 4 virus appeared to be associated with more severe disease than genotype 3 HEV. 32 Epidemiological features Incubation period The incubation period in human volunteers after oral exposure has been 4 to 5 weeks. 33 Slightly more variable incubation periods of 2 10 weeks have been reported during hepatitis E outbreaks in which the time of water contamination was known Journal of Gastroenterology and Hepatology 24 (2009)

4 Epidemiology of hepatitis E Geographic distribution of genotypes Human HEV Swine HEV Genotype 1 Genotype 3 Genotype 3 Genotype 2 Genotype 4 Genotype 4 Figure 3 Map showing geographical distribution of hepatitis E virus genotypes among (a) human isolates, and (b) swine isolates. (Modified from Purcell and Emerson. J. Hepatol. 2008; 48: ) Routes of transmission Four routes of transmission of HEV infection have been reported. These are: (i) fecal oral transmission due to contamination of drinking water supplies; (ii) food-borne transmission; (iii) transfusion of infected blood products; and (iv) vertical (materno-fetal) transmission. Of these, transmission through contaminated water is the most common. In some cases, particularly cases in nonendemic regions and sporadic cases in disease-endemic regions, it is not possible to establish the route of acquisition of infection. Two distinct patterns of epidemiology of hepatitis E have been observed in different geographical regions. These patterns seem to correlate with the disease prevalence. The frequency of various routes of transmission, the population groups affected and several disease characteristics differ sufficiently in the areas where hepatitis E is endemic and where it is not, to merit separate discussion of each. Table 1 areas Epidemiological features of hepatitis E in disease-endemic Large outbreaks involving several hundred to several thousand persons in developing countries Sporadic hepatitis cases frequent Fecal oral transmission (usually through contaminated water) is the predominant route of transmission Insignificant person-to-person transmission Parenteral transmission known but appears to contribute to only a minority of cases Mother-to-newborn (transplacental) transmission known Highest attack rate among young adults aged years, with relative sparing of children High attack rate and mortality among pregnant women, particularly those in second and third trimesters Low overall case fatality rate Chronic infection not reported Superinfection can occur among persons with chronic liver disease Hepatitis E in disease-endemic regions In such regions, epidemics of hepatitis E occur frequently. These epidemics are usually separated by a few years. Such outbreaks have been observed in the Indian subcontinent, China, Southeast and Central Asia, the Middle East, and northern and western parts of Africa. In North America (Mexico), two small outbreaks were reported during 1986 to 1987, but no further outbreaks have since been reported (Fig. 1). The outbreaks are often large, and affect several hundred to several thousand persons (Table 1).1,2,34,35 Most reported outbreaks have been related to consumption of fecally contaminated drinking water. Their time-course varies from unimodal outbreaks, which last a few weeks, to prolonged, multipeaked epidemics which last for over a year.1,34 The latter represent continued water contamination. The outbreaks frequently follow heavy rainfall and floods, which create conditions that favor mixing of human excreta with sources of drinking water.1,2 Some outbreaks have occurred in hot and dry summer months, possibly due to diminished water flow in rivers leading to an increased Journal of Gastroenterology and Hepatology 24 (2009)

5 Epidemiology of hepatitis E concentration of fecal contaminants. 34,36 In Southeast Asia, recurrent epidemics have been shown to be associated with disposal of human excreta into rivers, and use of water from the same river for drinking, cooking and personal hygiene. 36 These practices provide conditions that allow continuous fecal contamination of water. Outbreaks of hepatitis E have occurred in underdeveloped urban areas with leaky water pipes passing through soil that is contaminated with sewage. Intermittent water supply in these areas leads to a negative pressure in pipes during periods of no flow, allowing inward suction of contaminants. 37 Although spread of HEV infection through contamination of food should be possible, few outbreaks related to food-borne transmission have been reported from disease-endemic areas. This may be due to a relatively long incubation period (at least up to 10 weeks), which makes it difficult to establish a relationship between consumption of a particular food and occurrence of disease. Overall attack rates during hepatitis E outbreaks have ranged from 1% to 15%. The rates are highest among young adults (3 30%); the reason for this is not clear. Lower attack rates among children are related probably to a higher proportion of asymptomatic infections than to rarity of infection. Males outnumber females in most outbreaks; this may be due either to their greater risk of exposure to HEV infection or to a greater propensity for clinical disease once infected. Hepatitis E outbreaks have characteristically been associated with a high attack rate and mortality among pregnant women. In a large epidemic in Kashmir, India, 8.8%, 19% and 19% of pregnant women in the first, second, and third trimesters had disease; these rates were significantly higher than those among non-pregnant women (2.1%) and men (2.8%). 38 Furthermore, the rate of development of fulminant liver disease (acute liver failure) among those with symptomatic hepatitis E was also higher among pregnant women. Once fulminant hepatitis appears, the mortality rate may be no different among pregnant women with hepatitis E than in those with other causes of severe liver injury. 39 Because HEV infection during pregnancy is associated with a high attack rate and risk of severe disease, pregnant women are overrepresented among case series of women with liver failure in the endemic areas, and also among fatal cases during hepatitis E outbreaks. Although immunological or hormonal factors have been suspected, the exact cause for this specific predilection among pregnant women remains unknown. Hepatitis E during pregnancy has also been associated with prematurity, low birthweight and an increased risk of perinatal mortality. In disease-endemic areas, HEV infection accounts for a large proportion of acute sporadic hepatitis in all age groups. In India, HEV infection is the most common cause of acute sporadic hepatitis and accounts for up to 70% of such cases among adults. 43 In these regions, patients with sporadic hepatitis E closely resemble those of epidemic hepatitis E in age distribution, severity and duration of illness, propensity for worse prognosis among pregnant women, and absence of chronic sequelae. 44 The route of acquisition of infection in most patients with sporadic hepatitis E is unclear. However, given the high potential for fecal contamination of water and food in these areas, these sources are most likely. We were able to isolate HEV genomic sequences from nearly 40% of sewage specimens obtained through all seasons from a large drain in a large Indian city. 45 This indicates ubiquitous circulation of HEV in the population, even in the absence of a disease outbreak. Unlike several other enterically transmitted infections, personto-person transmission of HEV from either epidemic or sporadic cases is distinctly uncommon. 46,47 The exact reason for this is not known. During outbreaks, secondary attack rates among household contacts of patients with hepatitis E have been as low as %. This is much lower than the 50 75% infection rate among susceptible household contacts of hepatitis A cases. Even in situations with multiple cases in one family, the time interval between cases is short, indicating a shared primary water-borne infection rather than person-to-person spread. 46 Materno-fetal transmission of HEV infection has been reported. In one study, seven of eight babies born to mothers suffering from acute hepatitis E in the third trimester of pregnancy showed evidence of HEV infection in the form of detectable HEV-RNA or immunoglobulin (Ig)M anti-hev antibodies. 48 In regions endemic for hepatitis E, presence of HEV viremia among healthy blood donors and transmission of this infection to transfusion recipients has been documented. However, the contribution of such transmission to the overall disease burden remains unclear. The role of zoonotic transmission of HEV in endemic regions remains unclear. Hepatitis E in non-endemic regions In non-endemic regions, where outbreaks have not been reported, the disease accounts for only a minority of reported cases of acute viral hepatitis. Until a few years ago, most such cases were found to be related to travel to disease-endemic areas. However, in recent years, solitary cases or small series of cases related to autochthonous transmission of hepatitis E in these regions have been recorded in the USA, Europe (including UK, France, the Netherlands, Austria, Spain and Greece), and developed countries of Asia Pacific (Japan, Taiwan, Hong Kong, Australia). Table 2 summarizes the differences in epidemiological features of hepatitis E in non-endemic regions compared to those in endemic regions. In a series of 40 cases with hepatitis E identified in the UK, the disease showed seasonal variations with peaks in spring and summer and no cases during November and December. 49 In the UK, the disease appeared to be more common among residents of coastal and estuarine areas. 50 The mode of transmission in most of these cases could not be identified, although zoonotic spread has been proposed. 49,50 Zoonotic spread of the virus was first suspected when genomic sequences of HEV isolates from two autochthonous cases in the USA were found to be more closely related to swine HEV than to human HEV isolates. 15,17 This was supported by experimental cross-species transmission of human isolates to pigs, and of swine HEV to primates. 17,29 The most direct evidence for zoonotic transmission was, however, provided by a cluster of Japanese cases in those who had consumed inadequately cooked deer meat a few weeks prior to the onset of illness. 27 The genomic sequences of HEV isolated from these cases were identical to those from the leftover frozen meat, establishing foodborne transmission beyond doubt. In another case in Japan, wild boar meat was similarly implicated. 51 Further, genomic sequences from the deer meat had a high (99.7%) sequence 1488 Journal of Gastroenterology and Hepatology 24 (2009)

6 Epidemiology of hepatitis E Table 2 Differences in epidemiological and clinical features associated with hepatitis E in disease-endemic and non-endemic regions Feature Endemic regions Non-endemic regions Geographical locations Underdeveloped countries mostly in Asia and Africa Developed countries in Europe, North America, parts of Asia, Australia Epidemiological patterns Large epidemics, small outbreaks and sporadic cases Only sporadic cases Water-borne transmission Well known, most common route Unknown Animal to human transmission Not reported Shown; a very likely mode of transmission Animal reservoir No Yes Virus genotype Almost entirely genotypes 1 and 2, a few cases of genotype 4 in China Genotype 3; occasional cases of genotype 4 in Taiwan Age group Young men most commonly affected Usually elderly Chronic infection Not known Reported in transplant recipients receiving immunosuppressive drugs Severity Variable severity, including fulminant hepatic failure Severity and poor outcome is related to coexistent disease conditions Relationship with pregnancy Particularly high rates of symptomatic disease and of more severe disease in pregnant women than in men and non-pregnant women No data on pregnant women homology with HEV isolates from a wild boar and another wild deer from the same forest, suggesting transmission between these animal species and from deer to humans. 52 Since then, several additional lines of evidence have supported existence of zoonotic transmission in non-endemic regions. In one study, seven of 363 commercial packets of pig liver sold in Japanese grocery stores were found to contain genotype 3 or 4 HEV. 53 These isolates shared a high degree of identity with HEV isolate from a previous human case. In the USA too, genotype 3 HEV- RNA was isolated from 11 of 127 commercial pig liver packages. 54 Furthermore, a large proportion of Japanese patients with sporadic hepatitis E admitted to have eaten uncooked or undercooked pig livers. A case control study among patients with autochthonous hepatitis E in the UK, however, failed to show any association with consumption of pig meat or contact with pigs. Contaminated shellfish has been implicated in some studies as a mode of transmission. Reservoirs of HEV in endemic regions In disease-endemic areas, the reservoir of HEV responsible for maintaining the disease in a population has not been clearly determined. Protracted viremia and prolonged fecal shedding of HEV have been suggested. However, we found that viral shedding in feces lasts for a short period, making this possibility unlikely. 55 Another potential reservoir of the virus may be a continuously circulating pool of individuals with subclinical HEV infection. In an experimental cynomolgus macaque model, HEV-infected animals that lacked biochemical evidence of liver injury were found to excrete large amounts of viable and infectious HEV. 56 Similar fecal shedding of the virus by persons with subclinical HEV infection could lead to continuous maintenance of a source of infection in a disease-endemic area, somewhat akin to the situation that existed with poliovirus in areas where it was endemic. This pool of infection could, in turn, lead to periodic contamination of drinking water supplies. The issue regarding existence of an animal reservoir in diseaseendemic regions remains unresolved. The zoonotic hypothesis for transmission of HEV is based primarily on: (i) high prevalence of anti-hev antibodies in several animal species; (ii) isolation of HEV genomic sequences from pigs; and (iii) genomic sequence homology between human and animal HEV isolates. As discussed above, most of the supporting genomic data have been from non-endemic regions. In contrast, data from endemic regions are conflicting. Isolates from animals and sporadic human cases have belonged to the same genotype (genotype 4) in China and Vietnam. In India, however, animal isolates have all belonged to genotype 4 and human isolates to genotype 1. 25,26 Genotype 1 HEV, which is responsible for the large majority of cases in all endemic countries, has never been isolated from pigs. Also, in experimental studies, genotype 1 virus is unable to infect pigs. Thus, zoonotic transmission may not be a major mode of spread in these areas, in particular for the widely prevalent genotype 1 HEV. Thus, it appears that in the regions where hepatitis E is endemic, the infection is acquired from either an environmental or a human reservoir through poor general sanitation, contaminated drinking water supplies and lack of attention to personal hygiene. Further data are necessary before zoonotic transmission of HEV can be implicated in these regions. Seroprevalence data The presence of anti-hev IgG antibody has generally been taken as evidence of prior exposure to HEV. The duration of persistence of circulating IgG anti-hev antibodies remains unclear. In one study, nearly half of those who had been affected during a hepatitis E outbreak 14 years previously had no detectable anti-hev. 57 In another study of serial sera from patients with acute hepatitis E, total anti-igg titers showed a rapid decline, although titers remained above the detection threshold 14 months later. 58 Anti-HEV antibodies have been found in healthy subjects living in all geographical areas, although the prevalence varies widely. In general, prevalence rates are higher in developing countries where hepatitis E is common than in countries where clinical cases due to hepatitis E are uncommon. Journal of Gastroenterology and Hepatology 24 (2009)

7 Epidemiology of hepatitis E In India and other endemic countries, despite common occurrence of clinical cases and outbreaks of hepatitis E, the age-specific seroprevalence rates of anti-hev are much lower than those for hepatitis A virus (HAV) and other enterically transmitted infections, such as Helicobacter pylori. 59 These unexpectedly low seroprevalence rates are particularly perplexing when compared with the situation in Egypt, with anti-hev detection rates among adults of above 70%, despite the absence of disease outbreaks. 60 In developed countries, anti-hev antibody prevalence rates ranging from 1% to above 20% have been reported. 49,61,62 These appear to be higher than those expected from the low rate of clinically evident hepatitis E disease in these areas. Some of these variations may be related to differences in the assays used. In a direct comparison using a panel of coded sera, various anti-hev antibody assays showed sensitivity rates varying from 17% to 100%, and concordance rates among reactive sera of 0% to 89% (median 32%). 63 Thus, the relative contributions of subclinical HEV infection, serological cross-reactivity with other agents and false-positive serological tests to seroprevalence rates in nonendemic areas remain unclear. In developed countries, veterinarians and swine farm workers who come in close contact with pigs have higher anti-hev seroprevalence rates than the general population. Clinical presentations and outcome Disease-endemic areas Clinical outcomes associated with HEV infection are quite diverse. Hepatitis E most commonly manifests as self-limiting, acute icteric hepatitis, which is indistinguishable from acute hepatitis caused by other hepatotropic viruses. In a proportion of patients, the illness is particularly severe and presents as fulminant hepatitis (acute liver failure). As discussed above, pregnancy is associated with increased risk of severe disease. In animal studies, the viral inoculum dose determines severity of liver injury, and lower doses are associated with subclinical infection; 56 whether this happens in humans has not been studied. No other factors responsible for increased disease severity have been identified. The illness usually lasts for a few weeks, although some patients have a somewhat prolonged illness with prominent cholestatic manifestations. Frequent detection of anti-hev antibodies among residents of endemic regions in the absence of a history of prior acute hepatitis indicates that asymptomatic or inapparent HEV infection is common. This is supported by the laboratory evidence of anicteric hepatitis E (elevated liver enzymes with normal bilirubin in serum and HEV viremia) among otherwise healthy persons residing in areas affected by hepatitis E outbreaks. In endemic areas, HEV infection occurring in patients with pre-existing chronic liver disease of any etiology may lead to superimposed acute liver injury and clinical presentation with acute on chronic liver disease. We found evidence of recent HEV infection in nearly half of such patients in India. 64 Such patients may be at a higher risk of a poor outcome. In some of these patients, previously silent chronic liver disease may be detected for the first time following HEV infection. In published reports, case-fatality rates have been in the range of 0.5% to 4%. However, these data may suffer from selection bias, as they are derived from hospitalized cases with more severe disease. In population surveys during disease outbreaks, mortality rates of % have been observed. 34,35 Non-endemic areas In non-endemic countries, the disease is most often recognized when serological tests are undertaken in patients with unexplained hepatitis. Clinical illness in these patients is generally similar to that in endemic regions. In a series from the UK, most patients identified to have HEV infection had icterus, whereas a few had anicteric illness with non-specific symptoms or asymptomatic transaminase elevation. 49 Some cases were initially suspected to have drug-induced liver injury until tests for hepatitis E were done. 65 In these regions, most patients have been middle aged or elderly men, who often had another coexistent disease. 66,67 These differences may explain, at least partially, the somewhat higher rate of adverse outcomes in these patients than those in diseaseendemic areas. Chronic hepatitis E Conventionally, HEV infection has been thought to be selflimiting and was not known to cause chronic infection. However, a French group recently described a case series of 14 solid-organ transplant recipients receiving immunosuppressive drugs who had a recent onset of transaminase elevation (seven asymptomatic, seven with non-specific symptoms) and were found to have evidence of HEV infection. 68 Of these 14 patients, eight had persistent HEV viremia and liver enzyme elevation. Liver biopsies in some of the latter patients showed portal hepatitis with dense lymphocytic infiltrate, and variable degrees of piecemeal necrosis and fibrosis. All patients had genotype 3 HEV. The patients with chronic hepatitis E had a significantly shorter time from organ transplantation to the development of HEV infection and, thus, had lower total and CD2, CD3 and CD4 lymphocyte counts than those with resolving HEV infection. Chronic HEV viremia has also been reported in patients receiving anti-cancer chemotherapy. In addition, a few cases of transplant recipients with prolonged transaminase elevation, persistent viremia with genotype 3 HEV, and active chronic hepatitis and cirrhosis on liver biopsy have been reported; 69 this is consistent with the possibility that chronic HEV infection may lead to cirrhosis. It is noteworthy that all reports of chronic hepatitis E have been among immunosuppressed persons and have involved genotype 3 virus. Further data are needed to determine whether genotype 1 HEV, the predominant disease-causing strain worldwide, can cause persistent infection in otherwise healthy persons. Prevention Water and sanitation issues As fecal oral transmission is the predominant mode of transmission of HEV infection, measures aimed at proper treatment and safe disposal of human excreta, provision of safe drinking water supply and improvement in personal hygiene form the keystones for its prevention. In addition, it may be important to place 1490 Journal of Gastroenterology and Hepatology 24 (2009)

8 Epidemiology of hepatitis E emphasis on implementing sanitary food-handling practices, and avoiding consumption of undercooked or uncooked meat and vegetables. The virus has been shown to be heat labile. 70 During an epidemic setting, measures to improve the quality of water, even one as simple as boiling, have been shown to lead to rapid abatement in the number of new cases. Chlorination of water supplies may be useful in neutralizing the virus. In an outbreak in India, a failure of chlorination was associated with an increase in the number of cases. 34 In non-endemic areas where occasional cases of HEV infection appear to have been acquired by a zoonotic route, preventive measures directed against such spread (particularly cooking porcine and deer meat) may be useful. Passive immunoprophylaxis In laboratory studies, anti-hev antibodies have been shown to neutralize the virus and to prevent disease in laboratory animals experimentally inoculated with HEV. However, in human studies, giving serum immunoglobulins has not been shown to protect against hepatitis E. HEV vaccines In animal studies, several recombinant proteins corresponding to HEV capsid protein have been shown to induce specific antibodies and to protect against liver injury following subsequent challenge with the virus. In addition, a HEV-DNA vaccine has also been shown to induce serum anti-hev antibodies in cynomolgus macaques, and protect against rechallenge with a heterologous HEV strain. As a potential human HEV vaccine, a 56-kD truncated HEV ORF2 protein has been produced in Spodoptera frugiperda cells infected with a recombinant baculovirus that forms virus-like particles. This vaccine has undergone safety and efficacy studies in humans. In a phase I trial, three doses of 1, 5, 20 or 40 mg of this recombinant protein, given in an alum-adjuvanted formulation, induced production of anti-hev antibodies among healthy volunteers. 71 The magnitude of antibody response was found to be dose-dependant. 71 In a subsequent phase II III efficacy trial, nearly 2000 volunteers from the Nepalese Army who lacked detectable anti-hev antibodies were randomized to receive either 20 mg of alum-adjuvanted recombinant HEV protein or a matched placebo, each given as three doses at 0, 1 and 6 months, and were followed up for more than 2 years. 72 Overall, 88% of patients completed the study with median follow up of 804 days. Clinically overt acute hepatitis E occurred less frequently among vaccine recipients who completed the three-dose schedule than among placebo recipients, with a vaccine efficacy rate of 95%. A somewhat lower efficacy rate of 86% was observed after administration of two doses of the vaccine. Adverse events were similar except for an increased frequency of injection-site pain among vaccine recipients. This study had a few limitations. The study subjects were overwhelmingly (> 99%) male and mostly young (mean age 25 years). Further data are needed on the safety of this vaccine among pregnant women and children, and in certain groups, such as persons with chronic liver disease. The study focused on clinical disease rate and did not look at the HEV infection rate; it thus remains unclear whether the use of vaccine will reduce transmission of infection in a community. Also, by the end of the follow-up duration, anti-hev antibody titers had declined significantly so that nearly 44% of subjects had antibody titers below the level considered as protective. Thus, further studies are needed to determine the duration of protection afforded by this vaccine. In addition, data on protective efficacy when the vaccine is given postexposure would be essential before its use during an outbreak can be recommended. Unfortunately, this vaccine has not yet reached the market even in resource-rich countries. In disease-endemic regions, where the vaccine is likely to be most useful, high cost is likely to prevent its wide adoption. Recently, another hepatitis E vaccine, named as the HEV 239 vaccine, has been prepared by a Chinese group. It contains a truncated HEV capsid protein (corresponding to amino acids ) expressed in Escherichia coli, which has been purified and adsorbed on aluminum hydroxide suspended in buffered saline. 73 In a human study, all volunteers who lacked anti-hev antibody showed seroconversion 1 month after giving three doses (20 mg each at 0, 1 and 6 months, respectively). 74 After the second dose, new HEV infections occurred less often among vaccine recipients than in control subjects, indicating that the vaccine had a protective effect. A phase III trial is currently in progress. No comparative data on the safety and immunogenicity of the two vaccines tried in humans are yet available. The exact role for the HEV vaccines currently remains unclear. They should be useful for residents of non-endemic regions contemplating travel to HEV-endemic areas. In endemic areas, such vaccines may be useful for persons who are at a particularly high risk of severe disease following HEV infection, such as pregnant women and persons with pre-existing chronic liver disease. Whether HEV vaccines should be used for the general population in endemic regions will depend on cost considerations, the duration of protection afforded by the vaccines and their ability to interrupt the transmission of infection. A population-based vaccination strategy for developed countries too has been proposed, 18 based on an increasing recognition of disease in these areas; however, the disease incidence in these regions appears to be too low to justify such an approach. Also, although animal studies suggest cross-protection between HEV belonging to different genotypes, human data showing protective efficacy of HEV vaccines against genotype 3 virus prevalent in such areas may be required. Conclusion Nearly 10 years ago, we believed that we had a good understanding regarding the distribution and epidemiology of hepatitis E, and that it was a disease predominantly restricted to the developing world. However, since then, increasing numbers of autochthonous cases, zoonotic spread, and chronic infection with possible progression to cirrhosis have been demonstrated in non-endemic areas. This has led to major shifts in our understanding of this field and has generated intense new interest in this disease. It should therefore not be surprising if the additional data that accrue over the next few years lead to new and better understanding of this enigmatic form of viral hepatitis. Journal of Gastroenterology and Hepatology 24 (2009)

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