In this article, we're going to dive deep into the world of noroviruses. We'll explore their basic characteristics, such as their remarkable resilience in various environments and their non-replicating nature. We'll also discuss why noroviruses are frequently transmitted through numerous food sources, the incubation period from infection to onset of symptoms, the typical symptoms like acute gastroenteritis, and how healthy individuals can inadvertently spread noroviruses. Additionally, we'll examine the survival capabilities of noroviruses under dry conditions and provide practical measures to prevent norovirus food poisoning.
Understanding Characteristics Through the Domino Effect of Their Habitat
To fully grasp the nature of specific foodborne pathogens like noroviruses, it's crucial to start with their habitats. By understanding their habitats, we can sequentially comprehend their other characteristics, much like a domino effect.
１．Norovirus's Exclusive Habitat: The only place on Earth where noroviruses can multiply is inside the human body. This is a key point in food hygiene management.
２．Gram Staining? Not for Viruses: As noroviruses are viruses, they don't have Gram staining properties.
３．Classification: Noroviruses are categorized as infectious microbes.
４．Optimal Temperature for Multiplication? Not Applicable: The concept doesn't exist for noroviruses as they don't multiply outside the human body, including in food or environmental surfaces. In the environment, noroviruses exist more like particles, exhibiting remarkable resilience to physical stress such as dryness.
５．The Need for Oxygen in Multiplication? Also, Not Applicable: As noroviruses don’t multiply in food or environmental conditions, the concept of requiring oxygen for multiplication is irrelevant.
６．Resistant to Acid: Noroviruses show strong resistance to acid, meaning they are not easily killed by our stomach acid. This allows even a small number of noroviruses to pass through the stomach and reach the small intestine, making even minimal environmental contamination a risk for infection.
７．The Concept of Using Antimicrobials in Food: This concept does not exist for noroviruses.
８．Selective Media for Norovirus? Not a Thing: There is no concept of selective media for noroviruses as they cannot be cultivated. Though there have been reports of successful laboratory-level cultivation, practical cultivation techniques are not yet a reality.
Understanding these aspects sequentially, just like toppling dominoes, can help us get a clearer picture of noroviruses.
Humans - The Spreaders of Norovirus
Noroviruses behave more like inert material rather than living organisms outside human host cells. Once they infect human intestinal cells, however, they begin to multiply vigorously and burst out of the cells. The fact that humans are the only hosts for noroviruses is crucial to understand. This means that when it comes to various food contaminations, humans are the sole culprits. For instance, oysters are often associated with food poisoning, but that doesn't mean they're natural habitats or breeding grounds for noroviruses. When toilet wastewater flows to treatment plants, if noroviruses aren't completely killed or removed, they can make their way to the sea through rivers. Bivalves like oysters filter and consume plankton from seawater, inadvertently accumulating noroviruses in the process. However, it's important to note that noroviruses do not multiply within oysters.
Another contamination route is direct contamination from the hands of food factory workers to bread and other food products. Generally, dry bread is unlikely to cause bacterial food poisoning, but it's a different story with noroviruses. Here, the crucial point is that noroviruses get transferred from human hands.
Furthermore, another route of contamination is direct transmission from person to person. As mentioned earlier, noroviruses are quite resistant to acid. They can survive for up to three hours at a pH level of 3, found in stomach acid. This means that even a tiny number of virus particles can infect us. Noroviruses can enter our bodies not only through food but also through air or objects like towels. This fact raises the question: should noroviruses even be classified under food poisoning microbes?
Symptoms of Norovirus Infection
The hallmark symptom of a norovirus infection is vomiting. The infection is usually confined to the upper part of the small intestine. So, as soon as noroviruses enter our body and pass through the stomach, the infection begins.
Our body detects the infection and tries to expel the causative agent. Since the infection is felt closer to the mouth than the anus, the body's reaction is vomiting, not diarrhea.
Typically, bacterial food poisoning first infects the small intestine and eventually spreads to the colon, resulting in diarrhea. This is a fundamental difference between bacterial infections and norovirus infections.
Healthy Individuals Can Also Spread Norovirus
Even after recovering from norovirus food poisoning, people continue to shed the virus in their stool for about a week. In other words, even after recovery, there's a high chance that their fingers carry the virus. Therefore, if someone who has recovered from norovirus works in a food factory or food retail, there's an extremely high chance they could contaminate the food with the virus. Employees diagnosed with norovirus food poisoning are often advised to stay away from food production for at least a week.
Another critical point is that both symptomatic and asymptomatic individuals (those infected but not showing symptoms) excrete the same amount of norovirus. The difference is that symptomatic patients vomit more often, resulting in higher virus spreading efficiency than asymptomatic carriers. This fact highlights the difficulty in controlling the spread of noroviruses in food production environments.
Norovirus Doesn't Multiply in Food
Since humans are the only hosts for noroviruses, these cheeky viruses do not multiply at all in food. Therefore, the concept of preventing norovirus food poisoning by storing food in the fridge or under other low-temperature conditions doesn't apply here.
Similarly, the idea of developing preservatives to inhibit norovirus multiplication in food or designing food quality to counteract norovirus is also not relevant. Previously, I playfully inquired to a new senior in my university laboratory if they would like to research preservatives that prevent the growth of Norovirus in food. The student showed great enthusiasm and interest.
However, he was soon to be disappointed. The reason? The concept of researching Norovirus preservatives doesn't exist. Why? Because Norovirus does not proliferate in food in the first place.
The only real strategy to prevent norovirus risk is to ensure that anyone who might be infected with norovirus does not come into contact with food. This approach is quite different from the management points for other foodborne pathogens we've discussed so far.
Surviving Dry Conditions and Infecting Many From Just One Person
Another significant characteristic of noroviruses is their ability to infect a large number of people from a single source. Take, for example, the norovirus outbreak in Tokyo in February 2017 involving chopped seaweed. This incident resulted in 1,193 students and staff at an elementary school getting infected with norovirus. The chopped seaweed used in the school meals was believed to have been contaminated two months earlier, in December 2016, by an elderly subcontractor in Osaka who was handling the seaweed with his bare hands. During an interview, he indeed recalled feeling under the weather, like having a cold, back in that December.
What this outbreak teaches us is twofold:
- Noroviruses can retain their infectiousness for at least two months in a dry state
- They have the alarming capacity to infect over a thousand people from just one contaminated source, like seaweed
