The Winter Vomiting Virus
Norovirus, or the winter vomiting bug as most people call it is the most common stomach bug in the UK, affecting up to 1 million people a year and it's particularly prevalent at the moment.
Chris Smith caught up with Ian Goodfellow, from the Department of Pathology at the University of Cambridge, to find out more about what we know about the virus.
Ian - Noroviruses are small highly infectious viruses that cause gastroenteritis. They're typically referred to in the public press as the causative agent of a winter vomiting disease. But outbreaks occur all year round. The reason they're referred to as winter vomiting disease is simply because of the peak incidence appears to occur in and around the winter months.
Chris - Why is that?
Ian - It's probable that they tend to survive longer in the environment in the cold and wet. But also, people tend to spend a bit more time together during the winter. The reality is, we still don't know. It's one of the important questions that need to be addressed for noroviruses.
Chris - When you say the agent lurks in the environment, give us a sort of a snapshot view. What do these viruses look like and how do they get into the environment?
Ian - So, these are tiny, tiny little viruses, one of the smallest viruses around. The virus culture, the shell is made up entirely of protein. So, this makes them particularly resistant to inactivation by detergents and things like that. They get into the environment primarily through the vomiting or the diarrhoea episodes. People would have to get into the sewer systems and they contaminate the water courses. They often will also contaminate shell fish and people will get norovirus infection from eating contaminated uncooked shell fish.
Chris - When you say that it can stay in the environment, if a person is symptomatic in an area, how long will that area retain virus that's capable of infecting someone?
Ian - Probably in the region between 7 to 10 days. If not, longer. If it's untreated, it is probably going to remain enough infectious material that infects somebody for at least a week. This is partly because you need to be exposed. Very few particles are required to become infected so less than 20 particles are thought to be enough to cause an infection.
Chris - When a person is symptomatic, to put that into perspective, how much virus are they shedding?
Ian - An individual would normally shed somewhere in the region of a million or more virus particles per millilitre of faeces or vomit, and you need only 20 to infect an individual. So, each infected individual can infect many million others.
Chris - Wow! It's amazing to think there's millions of infectious doses in every single person, isn't it?
Ian - Absolutely.
Chris - So, when I ingest norovirus, I've got to pick it up from the environment. It presumably gets either onto the food I'm eating onto my fingers and gets into my mouth and I then swallow the particles. What happens next?
Ian; - That's a very good question. In fact, we don't really know much about the life cycle of noroviruses particularly in the gut. So for example, we know it grows in the intestine, but the reality is, we don't know precisely what cells in the intestine it infects. What we do know is it basically mimics the body's response to eating something that's poisonous. It stimulates this response known as the emetic response where effectively, your body will eject all the contents of the stomach and this is the vomiting episodes that you get. And then you will flood the intestine with fluid to try and wash out any poison. We think what the viruses do in stimulating this process either directly or indirectly by interacting with certain cells of your nervous system.
Chris - What about other animals because people often ask me, "Can my dog give me diarrhoea?"? Do we exchange these infections with animals or is this purely a human infection?
Ian - This is a very good question. Norovirus has been identified in a whole range of animals and there appears to be very specific noroviruses for cats, for dogs, for cows, for pigs for example. But there is limited evidence now that human norovirus can be found in the pig population. Whether or not there's this [genetic] transfer from pigs to humans is not entirely known. And in fact, there's limited data as well to say that in some cases, human noroviruses can be found in dogs and this is something that we're working on my lab to try and see if this is a common occurrence. At present, we think that human noroviruses are not a [genetic] disease so they primarily infect humans.
Chris - When I catch it, am I immune to it almost immediately that I've caught it. Is that why the symptoms go away because the symptoms do go away pretty quickly? Within 48 hours, you're feeling all right again usually, aren't you?
Ian - That's right, but it's likely you don't generate a very good immune response and we think one of the reasons these are such effective pathogens is simply because you don't generate long lasting antibody responses to this virus. It's more a process known as you're innate in your response. It clears the virus so the virus is not there for very long. Therefore, we don't generate very good antibodies. So, we can be re-infected probably once every year by the same virus.
Chris - Got you. Are there also lots of other strains circulating? So, although I've had one type previously, I may in fact pickup another next week to which I have no immunity and I can go down with this again?
Ian - Yes, again so one of the features of these viruses is that every time they multiply or copy the genetic material, they make mistakes. This means the virus can evolve very rapidly. Typically, in any one year, you'll find that there are at least one dominant strain or isolate of norovirus, but that will evolve. There'll always be minor species within that. In fact, within any individual who's infected, whilst maybe one strain of virus, each individual virus particle will contain a slightly different sequence. This gives the virus the ability to evolve away from any immune response.
Chris - Does that frustrate efforts to make a vaccine then?
Ian - This causes a lot of problems to the vaccine production. However, there's very encouraging data coming from a company known as Takeda. Their data suggests that you can protect from infection if you immunise individuals with the same type of norovirus strain and that will protect primarily from symptoms. So, less than 50% of individuals who develop symptoms and this can prevent spread. It's likely though if norovirus vaccine becomes viable. It's likely it would need to be changed every year, very similar to the influenza virus vaccine. However, us and a number of labs around the world are trying to develop new vaccines that give cross protective immunity. Probably in the next 5 to 10 years, they'd be looking at larger scale trials of human norovirus vaccines.
Chris - Thank you very much to Ian Goodfellow, from the Department of Pathology at the University of Cambridge.