Eating contaminated seafood, particularly raw or undercooked shellfish, is a well-documented route for contracting hepatitis A and a suspected route for hepatitis E. The risk centers almost entirely on bivalve mollusks like oysters, mussels, and clams, which filter large volumes of water and concentrate viral particles in their digestive tissues. The connection between shellfish and hepatitis has driven outbreaks across multiple countries and decades, and the biology behind it makes the problem stubbornly difficult to eliminate.
Which Types of Hepatitis Are Linked to Seafood
Of the five main hepatitis viruses (A through E), only two are transmitted through food and water: hepatitis A virus (HAV) and hepatitis E virus (HEV). Hepatitis B, C, and D spread through blood and bodily fluids, so eating seafood poses no risk for those forms. When people talk about “getting hepatitis from seafood,” they are almost always talking about hepatitis A, which has the longest and most firmly established track record of shellfish-borne outbreaks. Hepatitis E is a newer concern, with researchers increasingly detecting its genetic material in shellfish, though the evidence that shellfish actually transmit infectious HEV to people remains incomplete.
Hepatitis A typically causes weeks of fatigue, nausea, jaundice, and abdominal pain. Most adults recover fully, but the illness can be severe, and in rare cases it leads to acute liver failure. A safe and effective vaccine exists. Hepatitis E causes similar symptoms in most people but poses a serious danger to pregnant women and to individuals with compromised immune systems, in whom it can become chronic. No widely available vaccine for hepatitis E exists in most Western countries.
Why Shellfish Are the Problem
Bivalve shellfish feed by pumping water through their bodies and filtering out tiny particles, including bacteria, algae, and viruses. When the water contains human sewage carrying hepatitis viruses, the shellfish act like sponges, pulling the virus in and concentrating it in their digestive tissues at levels far higher than the surrounding water.1PubMed. Detection of hepatitis A virus (HAV) in oysters (Crassostrea gigas) Research on both HAV and HEV has confirmed that most of the accumulated virus ends up in the digestive gland and associated tissues, not distributed evenly through the animal’s body.2PubMed Central. Bioaccumulation Efficiency, Tissue Distribution, and Environmental Occurrence of Hepatitis E Virus in Bivalve Shellfish from France
This concentration effect is the core of the problem. A single oyster can harbor enough virus to infect a person even when the water it grew in would test as only lightly contaminated. And because oysters, clams, and mussels are often eaten raw or barely cooked, the virus passes directly to the consumer without ever being destroyed by heat. Studies on a related virus, norovirus, have shown that some viral particles bind to specific sugar structures inside the oyster’s digestive ducts, using the same molecular recognition site the virus uses to latch onto human cells.3PubMed Central. Norwalk virus-specific binding to oyster digestive tissues That binding means the virus is not just passively sitting in the oyster’s gut; it is chemically attached, which makes it much harder to flush out.
Depuration Does Not Solve the Virus Problem
The shellfish industry uses a process called depuration, in which harvested shellfish are placed in tanks of clean, treated water so they can purge contaminants before sale. Depuration works reasonably well for reducing bacterial counts, which is why bacterial testing has long been the regulatory standard. But viruses are a different story. Research has shown that conventional depuration does not eliminate human enteric viruses from oyster tissue, precisely because the viruses bind to structures deep in the digestive system rather than floating freely in the gut.3PubMed Central. Norwalk virus-specific binding to oyster digestive tissues Laboratory work comparing two oyster species found that one species retained HAV for a full month at all salinity levels tested, showing that even extended time in clean water may not clear the virus.4PubMed Central. Detection of hepatitis A virus RNA in oyster meat
This gap between bacterial safety standards and viral reality is important. An oyster can pass regulatory bacterial testing with flying colors and still carry hepatitis A. The industry and regulators know this, but viral testing of every batch remains technically challenging and expensive.
Major Outbreaks Traced to Oysters
The United States has a documented history of hepatitis A outbreaks caused by shellfish stretching back to the 1960s. One well-studied multistate outbreak identified 61 cases of hepatitis A across five states, all tied to raw oysters. The risk for people who ate raw oysters was dramatically elevated, and investigators found HAV antigen and genetic material in oysters from both approved and unapproved harvesting beds, as well as in confiscated illegally harvested oysters.5PubMed Central. A multistate outbreak of hepatitis A caused by the consumption of raw oysters That finding was striking because it meant even legally harvested, supposedly safe oysters carried the virus.
After a relative quiet period from 1989 to 2004, a new multistate outbreak in 2005 sickened at least 39 people in four states. Molecular analysis of the virus confirmed the oysters as the source, and traceback identified specific Gulf Coast harvesting areas.6PubMed. Molecular confirmation of oysters as the vector for hepatitis A in a 2005 multistate outbreak These outbreaks illustrate a recurring pattern: contamination at the harvest site, sometimes from illegal harvesting in polluted waters and sometimes from sewage runoff into approved areas, leads to clusters of illness far downstream in restaurants and homes.
How Sewage Gets Into Shellfish Waters
The virus has to get into the water before shellfish can concentrate it, and the source is almost always human sewage. Most outbreaks trace back to waters affected by untreated or insufficiently treated sewage. Storm overflows are a major culprit: heavy rainfall overwhelms sewer systems, causing raw or minimally treated sewage to spill into rivers and coastal waters near shellfish beds.7PubMed Central. Environmental transmission of human noroviruses in shellfish waters The amount of virus that reaches shellfish depends on the level of sewage treatment, how close the beds are to discharge points, rainfall patterns, river flow, and water temperature.
Another source, particularly relevant in coastal areas, is illegal discharge from boats. Even in regions with modern sewage infrastructure, a single sewage event near a productive shellfish bed can contaminate a harvest. Research tracking the fate of viruses in an oyster harvesting area repeatedly hit by stormwater discharges confirmed that virus levels in shellfish spiked after sewage overflow events.8PubMed. Fate of Human Noroviruses in Shellfish and Water Impacted by Frequent Sewage Pollution Events This means the risk is not constant. Shellfish harvested after a period of dry weather from a well-regulated area are far less likely to be contaminated than shellfish pulled from waters near a recent overflow event.
The Hepatitis E Question
Hepatitis E from seafood is a newer and less settled area of research. HEV genotype 3, which circulates in pigs and other animals and causes most human HEV cases in industrialized countries, has been detected in shellfish across multiple regions. A study of mussels harvested in Scotland found HEV RNA in over 90% of west coast samples, and genetic analysis showed the mussel-derived virus sequences clustered closely with HEV from both humans and swine.9PubMed Central. Hepatitis E Virus Genotype 3 in Shellfish, United Kingdom That is a worrying signal, because it suggests animal waste running into coastal waters can seed shellfish with a virus capable of infecting people.
However, there is a crucial caveat. Detecting viral RNA in shellfish tells you the genetic material is there, but not whether the virus is still alive and capable of causing infection. To date, no study has recovered infectious HEV from shellfish, and no investigation has established a clear molecular link between a specific shellfish-derived HEV strain and a human infection.10PubMed Central. Shellfish as a Potential Source of Hepatitis E Virus: Epidemiological Evidence, Biological Plausibility, and Research Gaps Epidemiological studies have found associations between shellfish consumption and HEV exposure, but many of those studies did not even document whether the shellfish was eaten raw or cooked. So while the biological plausibility is strong and the epidemiological signals are suggestive, the smoking gun for shellfish-transmitted hepatitis E has not been found. The dominant proven food source for HEV in industrialized countries remains undercooked pork and game meat, not shellfish.
How Common Is Contamination in Shellfish You Might Actually Buy
Surveillance studies that test shellfish at retail or at harvest give a useful snapshot of real-world contamination levels, and the picture varies enormously by region. A study of mussels from retail stores in Italy’s Campania region found high rates of norovirus and rotavirus contamination, but every sample tested negative for both hepatitis A and hepatitis E.11PubMed Central. Presence of Potentially Infectious Human Enteric Viruses and Antibiotic Resistance Genes in Mussels from the Campania Region, Italy: Implications for Consumer’s Safety A separate surveillance program covering shellfish production and distribution in Sicily found HAV in less than 1% of samples and HEV in a similarly small fraction.12PubMed Central. Occurrence of Human Enteric Viruses in Shellfish along the Production and Distribution Chain in Sicily, Italy
Contrast that with Morocco, where testing of 156 shellfish samples from three coastal areas found about 15% positive for HAV by molecular testing, with rates varying sharply between sites classified as different water quality grades.13Letters in Applied Microbiology. Hepatitis A virus detection by RT-qPCR in shellfish samples from three Moroccan Atlantic coastal areas: Dakhla, Oualidia, and Moulay Bousselham The Philippines, another country where shellfish is widely consumed, has also documented HAV in local oysters and mussels harvested from contaminated waters.14PubMed Central. Occurrence of hepatitis A virus in Philippine cupped oysters (Magallana bilineata) and green mussels (Perna viridis) collected in the Philippines The pattern is consistent: areas with better sewage infrastructure and stricter harvest-area classification tend to have lower hepatitis contamination rates, but nowhere is the risk zero for raw consumption.
Can Cooking Make Shellfish Safe
Heat is the most reliable way to destroy hepatitis viruses in shellfish, but the details matter. HAV is tougher than many foodborne pathogens. Laboratory inactivation studies have shown that at 80°C (176°F), it takes about two minutes to eliminate infectious HAV, while at 72°C (roughly 162°F), it takes about five minutes.15PubMed Central. An RTCA-based assay as an innovative approach for thermal inactivation studies of hepatitis A virus That means a quick steam that barely opens the shell is not enough. You need sustained, thorough cooking so that the internal temperature of the shellfish stays high long enough to inactivate the virus. A common mistake is pulling steamed mussels or clams the moment the shells pop open, which may leave the center below the critical temperature for the necessary duration.
For hepatitis E, heat inactivation follows similar principles, though HEV has shown some resilience to certain non-thermal preservation methods. High-pressure processing (HPP), a technology used commercially to extend the shelf life of foods like juices and deli meats, can reduce HAV and norovirus levels in shellfish effectively.16PubMed Central. Inactivation of Foodborne Viruses by High-Pressure Processing (HPP) But HEV appears to be relatively resistant to HPP. Experiments applying pressures up to 600 megapascals showed that while most of the virus was inactivated, small amounts of infectious HEV sometimes survived even at the highest pressure tested.17PubMed. Stability of hepatitis E virus at high hydrostatic pressure processing So HPP is a useful extra layer of safety for HAV but not a reliable standalone fix for HEV.
Electron beam irradiation is another technology that has been explored. Risk modeling suggests that for a highly contaminated serving of 12 raw oysters, a moderate irradiation dose would reduce HAV infection risk by about 16%, and for lower contamination levels the reduction can reach over 90%.18PubMed Central. Susceptibility of murine norovirus and hepatitis A virus to electron beam irradiation in oysters and quantifying the reduction in potential infection risks These technologies are promising but are not yet widely adopted for commercial shellfish. For the foreseeable future, thorough cooking remains the individual consumer’s best defense.
What About Fish, Shrimp, and Other Non-Bivalve Seafood
The risk is overwhelmingly concentrated in bivalve mollusks. Finfish (salmon, tuna, cod, and so on), shrimp, crab, and lobster do not filter-feed the way oysters and mussels do, so they do not bioaccumulate viruses from the water column in the same way. That does not make them immune to contamination: any food can transmit hepatitis A if an infected person handles it without proper hygiene, and raw fish preparations carry their own parasitic and bacterial risks. But the specific shellfish-as-concentrator mechanism that makes bivalves a hepatitis vector does not apply to other types of seafood.
Within bivalves, oysters get the most attention because they are most often eaten raw, but mussels and clams carry similar risks when eaten raw or lightly cooked. Some studies have even detected HAV in scallops from approved harvesting areas.5PubMed Central. A multistate outbreak of hepatitis A caused by the consumption of raw oysters The safest assumption is that any filter-feeding bivalve harvested from waters with any sewage exposure can carry the virus.
Practical Steps to Reduce Your Risk
If you eat raw shellfish regularly, the single most effective protection is the hepatitis A vaccine, which is safe, widely available, and provides long-lasting immunity. It will not protect against hepatitis E or other viruses in shellfish, but hepatitis A is the form with the strongest evidence of shellfish transmission and the one most likely to make you seriously ill.
Beyond vaccination, a few practical choices make a real difference:
- Cook thoroughly: Steam shellfish until the internal temperature stays above 85°C (185°F) for at least a minute or two, not just until the shells crack open.
- Know the source: Shellfish from well-regulated waters with strong sewage infrastructure carry lower viral risk than shellfish from poorly monitored areas. If you are traveling, the local sanitation situation matters for the local shellfish.
- Be cautious after storms: Heavy rainfall increases the chance of sewage overflow into shellfish waters. Some regions issue advisories after major storms; pay attention to those.
- Recognize that raw bars are inherently risky: No amount of depuration, lemon juice, or hot sauce destroys hepatitis viruses. If you eat raw oysters, you are accepting a small but real risk.
Why Bacterial Testing Gives a False Sense of Security
Most countries regulate shellfish safety primarily through bacterial indicators, particularly E. coli counts in the growing waters and in the shellfish meat. If bacterial levels are below a threshold, the shellfish is cleared for sale. The logic is that E. coli indicates fecal contamination, and fecal contamination is the route for viruses too, so controlling bacteria should control viruses.
In practice, this correlation breaks down. Bacteria die off more quickly in seawater than viruses do, so waters can test clean for E. coli while still carrying infectious HAV or norovirus. Depuration clears bacteria effectively but leaves viruses bound in the tissue. The European Union has moved toward requiring direct viral testing (specifically for norovirus and hepatitis A) in certain shellfish production areas, but globally this approach is still the exception. Most shellfish you buy has been cleared based on bacterial standards that were never designed to catch viruses.
Researchers have developed sensitive molecular methods capable of detecting very small quantities of HAV RNA in oyster meat, with detection limits as low as a few infectious units per gram.4PubMed Central. Detection of hepatitis A virus RNA in oyster meat These methods work well in the lab, but scaling them to routine testing of commercial shellfish batches remains a logistical and economic challenge. Until viral testing becomes standard, the regulatory system will continue to have a blind spot for hepatitis.
Climate Change and the Trajectory of Risk
Heavier and more frequent storms, rising sea temperatures, and shifting rainfall patterns all feed into the shellfish-hepatitis equation. More storm events mean more sewage overflows into coastal waters. Warmer waters can change the distribution and behavior of both pathogens and shellfish species, and alter the efficiency with which viruses survive in the marine environment. Researchers have flagged that disease outbreaks affecting marine organisms and the food systems that depend on them are expected to become more frequent and intense as the climate changes. For shellfish consumers, this means the already-imperfect protections offered by harvest-area classification and depuration may come under increasing pressure. Areas that are currently low-risk may not stay that way as precipitation patterns shift and coastal infrastructure is tested by more extreme weather. Surveillance systems designed for historical conditions will need to adapt, and the case for direct viral testing of shellfish will only grow stronger.