Why common cold may thrive at low temperatures

The “common cold 'prefers cold noses',” reports BBC News today, while The Independent recommends that you “heed your mother’s warning: cover up or you’ll catch a cold”.

While these headlines might make you think this study is proof of a link between colder temperatures outside and catching a cold, this isn’t quite what the researchers looked at.

Our nasal passages are naturally a few degrees colder than the core of our body. It has long been known that rhinovirus – the most common cause of the human cold – grows much better at these lower temperatures.

The current study has looked at why this might be. It found that mouse airway cells were less able to mount immune defences against the cold virus at the lower temperature seen in the human nose than at the higher temperature seen at the core of the body.

While this study may suggest a possible explanation for the known effect of temperature on cold viruses, it is very early stage research, testing just one strain of rhinovirus in mouse cells. The experiments will need to be repeated with different strains and ideally with human airway cells.

Also, while the authors speculate about whether this could explain beliefs around the impact of cold environmental temperatures on catching a cold, and wrapping up warm to prevent a cold, this study didn’t actually assess this.

Where did the story come from?

This study was carried out by researchers at Yale University. It was funded by the US National Institutes of Health, National Institute of Allergy and Infectious Diseases, and National Science Foundation.

It was published in the peer-reviewed journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).

The media has focused on the potential impact of cold outdoor temperatures on our risk of catching a cold, when this is not what the study assessed. The common cold-causing rhinovirus was already known to grow better at the naturally cooler temperatures in the nose than at higher temperatures found in the centre of the body. This study looked at why this might be the case.

What kind of research was this?

This was a laboratory study looking at whether temperature affects how the cells in the airways are able to respond to the cold virus.

The insides of our noses are naturally a few degrees cooler than our core body temperature: 33C – 35C, compared to 37C. The cold virus is already known to be able to reproduce itself better in cells at these cooler temperatures. However, it is not known why this is. Researchers wanted to test whether it could be because the cells in the airways are less able to mount defences against the cold virus at cooler temperatures.

Laboratory research is often the first step to understanding what happens in our bodies. As cells in isolation in the lab may behave differently to when they are in the body, these early experiments usually need to be followed up by studies in animals or humans to confirm their findings.

What did the research involve?

The researchers took samples of the cells lining mouse airways and grew them in the lab at either 33C or 37C. They exposed these cells to the rhinovirus – the most common cause of the cold in humans. The virus was selected and grown in a way that allowed it to better infect the mouse cells. They then compared what responses the cells were having to the virus at the different temperatures. In particular, they looked at how well the cells were switching on the production of proteins to help them fight the virus.

What were the basic results?

The researchers found that the mouse airway cells were better at switching on the production of proteins to help them fight the cold virus at the warmer (core body) temperature than the cooler (nasal cavity) temperature. The researchers went on to identify some of the proteins involved in prompting this response. They found that if these proteins were not present, then the virus was better able to replicate itself in cells at the warmer temperature.

How did the researchers interpret the results?

The researchers concluded that airway cells are less able to mount defences against the cold virus at the cooler temperatures of the nasal passages than in warmer temperatures at the core of the body. This at least partly explains why the cold virus is able to grow better in the cooler nasal passages than in the warmer lungs. They say that this could be a possible explanation for the “popular but controversial idea that exposure to cool weather conditions can increase susceptibility to common colds”.

Conclusion

This laboratory study looked into why the cold virus is able to grow better in the cooler temperatures found in the nasal passages, than in the warmer core body temperature found, for example, in the lungs. The authors note that while this difference has been known since the 1960s, the reasons are still not clear.

Their findings, using cells from mouse airways grown in the laboratory, suggest that at the cooler temperatures these cells are less able to switch on the production of proteins that fight the virus. However, it is important to bear in mind some of the limitations of this early stage research. One limitation was that it tested just one strain of the most common human cold virus (rhinovirus) in mouse cells. The experiments will need to be repeated with different strains of rhinovirus and other cold-causing viruses, and with human airway cells. The authors also note that this might not be the only reason why cold viruses grow better in the nose.

Also, while the authors speculate that this could explain the impact of cold environmental temperatures on colds, this study only really looked at cells at the normal temperature of the human nose, and didn’t assess the impact on nose temperature of it being colder outside.

Regardless of this, it is important to protect your body against the potentially harmful effects of very cold weather. Older people, those who cannot afford heating, and those with long-term health conditions or who are disabled are particularly vulnerable to cold-related illnesses.