Why colds are more common in winter

It was dismissed by doctors for decades as a myth. But now, scientists have proved that the winter weather really can increase your chances of catching a cold...

Yale University's Akiko Iwasaki has found that when the cells in our noses are infected with viruses, they sound a chemical alarm to help other cells to fend off infection. But at lower temperatures, this doesn't work so well, giving an attacking virus an advantage and partly explaining why colds are much more common in winter, as she explained to Chris Smith.

Akiko - It's been known since the 1960s that the common cold virus or the rhinovirus replicates at a cooler temperature found in the nose. However, it was not known why the virus replicates better in this cooler temperature.

Chris - When you say the virus replicates better in the cooler temperature in the nose, are you saying then that because the nose is colder, that the virus grows better?

Akiko - Yes, the virus grows better in the nose temperature.

Chris - How do we know that?

Akiko - This was done in the 1960s when people used to try to culture the virus from the nose of people with the common cold symptoms. And they couldn't grow the virus well at the typical temperature of 37 degrees. But when they reduced the temperature of the incubator to 33 degrees, the virus could grow.

Chris - 37 degrees being normal body temperature.

Akiko - That's correct.

Chris - Would the inside of the nose normally be about 33 degrees then because it's exposed to cold air?

Akiko - That's correct, yes. Typically, 32 to 33 degrees in the nose.

Chris - And had everyone concluded that the virus wouldn't grow because the virus "prefers" a cooler temperature? Was that their deduction but they just didn't know why?

Akiko - That's correct and many studies have been done in looking at whether there is something about the virus that grows better in the cooler temperature of the nose. For instance, people have taken apart bits and pieces of the virus to see whether each component of the virus works better at the cooler temperature. But none of these studies have revealed the mechanism for this.

Chris - So, they all drew a blank?

Akiko - Right.

Chris - Was your suggestion then that well, if it's not the virus, perhaps it's actually the person?

Akiko - That's right. So, we started to look at this question from a different angle, how the viruses are detected by the immune system in general. So, we've been studying these molecules known as sensors that detect virus RNA inside the cell.

Chris - This is the genetic material of the virus, isn't it?

Akiko - Right. So, the rhinovirus has a RNA genetic material inside of it. The host cell can detect the presence of this RNA inside the cell when the virus grows inside the cell.

Chris - A bit like a sort of tripwire. There's a chemical tripwire. The cells says, "Aha! I've got this funny genetic material in here that shouldn't be here" so it knows that it's been infected.

Akiko - That's correct and these molecules are collectively known as sensors. And so, we have mice from which we've taken away these sensors. And so, we can study the importance of each of these sensors in blocking the virus from growing. When we looked at the ability of the virus to grow, in the absence of these sensors, they could now grow even at the higher temperature of 37 degrees, indicating that it's not the intrinsic property of the virus, but it's the host immune response that dictates which temperature the virus can grow in.

Chris - Why hasn't the virus, given how common these rhinoviruses are (I guarantee everyone listening to this has probably got a cold or they're getting over a cold, probably caused by rhinovirus, right now) why hasn't the virus evolved away around that? It seems so simple because it's literally stopping it doing what it could do. It could grow all over the body if it were to surmount that temperature problem then.

Akiko - That's correct. In fact, that's a very good question. We think that the rhinovirus actually prefers to replicate in the nasal cavity because it is easy to transmit the virus from the nose to the next person by either touching the nose or sneezing or other means.

Chris - Is it not possible to come up with some kind of drug or a nasal spray that could tweak the sensors in our cells in the nose or even give us an artificial sensing system so that tripwire that enables the cells to detect a virus growing in our nose goes off at the lower temperature better.

Akiko - Yes, that would be an innovative way to deal with this problem. But there's another problem with this - is that we found not only the sensors of the virus don't work as well at the lower temperature, but also the interferons that are produced as a result of the sensing don't stimulate the neighbouring cells as well. So, these interferons are molecules that are secreted from infected cells to try to alarm the neighbouring cells from becoming infected by the virus.

Chris - Like a burglar alarm going off when there's been a smash and grab in one cell by a virus.

Akiko - That's correct.

Chris - They're telling the cells next door, "Watch out because there might be a virus in the neighbourhood."

Akiko - That's right. And usually, this alarm works very well. But what we found is that in the nasal cavity temperature, the alarm system also doesn't work very well. So, you would have to overcome both of these issues in order to prevent the virus from growing in the nose.

Chris - It sounds to me like we're stuck with the common cold then. The way you're telling this it sounds like really actually, we're going to have to live with it.

Akiko - Maybe. The other way to deal with it is I guess to warm up your nose to the temperature that's not permissive for the virus to grow.

Chris - What would you recommend then, a large whisky or something to give you a nice warm red nose?

Akiko - That's a possibility. I don't know how much the whisky increases the nasal cavity temperature. I don't think anyone has measured that.

Chris - I'd be very happy to help with that particular experiment!