It was the largest outbreak of norovirus Germany had ever recorded: almost 11,000 cases across the country, in the autumn of 2012. The culprit? A batch of imported frozen strawberries that were contaminated. They’d been delivered to commercial kitchens and used as ingredients in various dishes – and served to thousands of people who were none the wiser. Now, over ten years later, norovirus remains high on the public health agenda. It’s the leading cause of foodborne illness in the US and is on the rise in the UK and Europe. Worldwide, there are around 685 million total cases of acute gastroenteritis caused by norovirus annually.
Found in human vomit or faecal material, norovirus spreads when infected particles enter the mouth. That could be via contaminated food or water, or through airborne virus particles that are released when someone who is sick vomits. And it’s very contagious. “When someone is infected with norovirus, they are excreting literally billions of virus particles,” says Donald W Schaffner, distinguished professor and department chair in food science at Rutgers University. While food and water can act as carriers for norovirus, it’s transmitted via the actions of people – from upstream operations such as sourcing and manufacturing through to food service settings.
The good news is that there’s plenty that workers and companies can do to prevent the spread.
Clear from contamination
If contamination occurs during primary production – the cultivation stage – it’s probably due to water that’s been contaminated with sewage that contains norovirus, says Schaffner. This water could be used for irrigation or to dilute pesticides.
Here, there are various ways contamination could occur. For instance, there might be inadvertent spillage of untreated sewage into a water supply due to flooding or from a leaking septic tank, adds Lee-Ann Jaykus, distinguished professor emerita at North Carolina State University’s Department of Food, Bioprocessing and Nutrition Sciences. Individual actions could play a role here, too, she adds. “You can get contamination in produce fields from people defecating in the fields.”
Yet, Jaykus notes, “gross contamination events of water, certainly for produce production, are pretty rare”. That’s likely due to the standards and controls in place, such as good agricultural practices. “We have lots of systems in place,” says Schaffner. “Things that we know are our best practices: the water needs to be of good quality, the workers who are harvesting the crop need to be educated with respect to sanitation.”
He gives an example: “We know that with overhead irrigation, where we’re spraying the water directly down on the crop, that is more likely to lead to contamination. It’s also not very efficient; we end up wasting a lot of water.” Instead, producers could turn to drip irrigation, where water is dripped around the roots of the plants. The idea is to apply just the right amount of water to the soil, rather than the edible portion of the crop.
For those who will buy and process the produce, vetting suppliers and ensuring they meet the appropriate standards is key. After all, you sometimes can’t tell if products are contaminated just by looking at them. In any case, foods contaminated with norovirus may look, smell or even taste normal.
Here, it’s worth noting that freezing won’t kill the virus. “The virus is extraordinarily stable under freezing conditions,” says Jaykus. In fact, she still has the virus from the original 1960s outbreak in the US, frozen at -80°C in her lab. “And it’s still infectious,” she adds. This poses an additional problem: because frozen produce has a longer shelf life, it’s more likely that it will stay in the supply chain for longer. So, if it’s contaminated, this prolongs the risk of outbreak.
Norovirus is also relatively heat resistant. It can survive temperatures of up to around 63°C (145°F). So, while steaming may not do the trick in killing it, other processes like boiling might. During the 2012 incident in Germany, the kitchens that used the contaminated strawberries but boiled them before serving weren’t associated with the outbreak.
Safe sourcing
There are two key food categories associated with norovirus: shellfish, and fruits and vegetables. On both counts, the issue is largely water contamination.
With shellfish, infected faecal matter can get into the waters where they’re growing, explains Jaykus. “These animals are filter feeders, so they bring in solids from their environment and the viruses kind of stick to the oyster tissue.” Again, there are systems in place to identify the risk of contamination and take action to prevent it. “We have shellfish sanitation programmes all over the world,” she adds. “And the shellfish beds are closed when it appears that there’s been a sewage contamination event.” Before the beds are reopened, they’re tested for indicator bacteria, which are used as a surrogate measure for pathogens.
“When someone is infected with norovirus, they are excreting literally billions of virus particles.” Donald W Schaffner
It’s crucial that we’re only harvesting shellfish from approved waters, adds Schaffner. Water quality can be assessed through testing or predicted via computational models, he explains.
“We also have a technique called depuration, where you basically take the shellfish out of the water that they’ve been growing in and you transfer them to another water source that’s of known quality. And you keep them in that water source for a certain period of time.” The idea is that this will eventually purge the contaminants.
What about fruits and vegetables? Here, the main driver of norovirus outbreaks is berries, says Jaykus. Specifically: frozen berries. There are a few reasons for this. One is that they have a complicated supply chain – to meet consumer demand, berries are typically sourced from suppliers all over the world. That means that adherence to safety practices and standards is harder to control, particularly when dealing with small farms.
To boot, frozen berries are being consumed more often and have a shelf life of up to two years. Recall that norovirus can survive in frozen foods for long periods of time. Berries are also often hand-picked, says Jaykus. “They’re very fragile, so they’re much more likely to be contaminated by a food handler than a larger product, like an apple.” Manufacturers can help reduce their risk of accepting contaminated goods through robust inspection of produce and auditing of suppliers.
Handling food
“Food handlers, usually at the retail or restaurant end of the chain, are big, big drivers of norovirus outbreaks,” says Jaykus. “And that is all about poor personal hygiene of infected food workers.
” Hygiene practices are important across the whole chain: an ill worker who handles produce is a risk both while they’re harvesting and when preparing meals for service. Most outbreaks occur in food service settings like restaurants, says CDC public affairs specialist in the office of communications, Gabe Alvarado. “Infected food workers are frequently the source of outbreaks in food service settings, often by touching ready-toeat foods (such as raw fruits and vegetables) with their bare hands before serving them. However, any food served raw or handled after being cooked can get contaminated with norovirus.”
“Food handlers, usually at the retail or restaurant end of the chain, are big, big drivers of norovirus outbreaks.” Lee-Ann Jaykus
Schaffner and Jaykus agree that if a worker is infected, the best thing to do is have them stay home. A US FDA assessment showed that there’s a risk of transmission even if that person is given a different job where they’re not handling food, such as front-ofhouse, Schaffner says. “What they discovered was, yes, [giving them a non-food-handling job] had a small effect on risk, but the best way to avoid that sick person giving anyone norovirus is just to get them the heck out of the restaurant.”
That’s because of cross-contamination. If a person has norovirus and it’s on their hands, and they touch different surfaces in a workspace, there’s a chance it will spread. “Just a few virus particles have a reasonable chance of making someone sick,” says Schaffner. This is also why other measures – such as hand washing and regularly disinfecting surfaces – while extremely important, are sometimes not enough on their own to prevent an outbreak.
Schaffner, who has studied hand washing in the lab, gives an example: when you wash your hands properly, you get around a two-fold reduction in the amount of virus particles present. That’s about a 99% reduction. But if there are a million virus particles on your hand, that means you’re left with about 10,000 particles. Which is more than enough to cause illness.
With surfaces, however, we can use harsher chemicals than we can on human skin, which are more effective than soap at killing pathogens. Though this depends on the surface: it will be easier to sanitise a hard, stainless-steel surface than a carpet.
Before preparing food, the CDC recommends that all food handlers wash their hands with soap and water for at least 20 seconds, wash fruits and vegetables well, routinely clean and sanitise kitchen utensils and surfaces, cook shellfish to at least 145°F, and keep raw oysters away from other ready-to-eat foods. Are we making progress in preventing the spread of norovirus? “Based on the most recent information I’ve seen from the CDC,” Schaffner says, “We’re kind of staying in place.”
Though this could perhaps be seen as a kind of progress, he explains, as we get better at studying and tracking disease, we also get better at finding problems. “So, we have to be better to be able to stay in the same place.”
