Causes and mechanisms of colds

Magnified graphic image of mucus being produced inside the nose

What causes colds?

Colds are usually caused by viral infections. One of the major symptoms of a cold is nasal congestion, which can also be the result of several other causes.¹ Here we take a closer look at the causes of colds and their mechanisms.

What are the functions of a healthy nose?

A healthy nose:^2,3

Warms (just under normal body temperature 31-33^oC), moistens (close to 98%), and filters inspired air before it enters the lungs, to ensure that the delicate lung tissue doesn’t become damaged by cold or dry air, or irritated by small particles (e.g. dust)
Reduces infections caused by viruses
Detects and destroys foreign substances before they enter the rest of the body (nasal membranes bring air in contact with immunoglobin, a protein that detects foreign substances such as viruses, bacteria, and allergens, triggering an immune response to destroy them before they enter the rest of the body)
Allows us to smell the air as it flows through the nose, which contributes towards our sense of taste
Allows the air resonating in the nose to help give our voice its particular sound

Causes of nasal congestion^4,5
Nasal congestion has a variety of causes

Nasal congestion is caused by swelling of the mucus membrane:⁶

It is not caused by too much mucus, as is often thought, although excessive mucus can exacerbate nasal congestion⁶

It is one of the oldest and most common human complaints⁷- a ‘stuffy’ nose is among the most troublesome symptoms of viral infections of the nose⁶

It leads to difficulties in breathing, hearing and sleeping; and can also cause voice changes that may be socially embarrassing⁸

It could also lead to impairment in the ability to smell
1- Viral infection

Common cold and influenza

2- Allergies

Pollens, animal dander, certain foods, insect venom, latex and various medicines.

3. Inhaled irritants/pollutants

Gases, metals, organic chemicals and biological material surrounding a carbon core.

4. Structural abnormalities

Septal deviation, concha bullosa and adenoid hypertrophy
Nasal congestion: Viral infections
Colds and influenza are major causes of nasal congestion

The common cold and influenza are both classed as upper respiratory tract viral infections. They share similar symptoms, but there are some notable differences:

The common cold is the most prevalent acute illness in the world:¹

Referred to as a single illness, but caused by numerous viruses from different families¹

Rhinoviruses are the most common cause, accounting for >80% of cases in the peak season¹

Usually self-limiting and confined to the upper respiratory tract¹

Nasal congestion and rhinorrhoea (runny nose) are the main clinical symptoms, with sore throat and cough also common^1,9

The average adult experiences 2 to 4 colds annually¹⁰

Children experience 6 to 8 episodes per year¹⁰

Influenza:

Primarily caused by either of two categories of virus: Influenza A or B¹¹

Symptoms vary from person to person

Additional symptoms could be non-nasal, including fever, myalgia and fatigue, and can lead to serious complications^12,13
Mechanisms of viral nasal congestion
How colds and flu cause nasal symptoms^14-16

Colds and flu cause nasal congestion in a similar way, by triggering the immune response and stimulating the inflammatory cascade to release pro-inflammatory mediators such as cytokines, bradykinins and prostaglandins. These dilate the blood vessels in the nasal epithelium, leading to congestion in the nasal passages.

The body’s natural defences against viral infection cause the familiar symptoms of the common cold and flu:^17-19

Rhinovirus infection triggers the release of pro-inflammatory and vasoactive mediators (cytokines, bradykinins, prostaglandin) from nasal epithelial cells^14-16

Vasoactive mediators cause dilation of large capacitance veins resulting in increased vascular permeability and leakage of plasma exudate into the submucosal space^14-16

Excess fluid and inflammatory cells in the submucosal space cause swelling of the nasal mucosa, blocking the nasal passageway.^14-16

A young brunette woman sits at home and blows her nose, looking as if she is in pain.

Climate change and rhinoviral infections

As well as the methods by which rhinoviral infections are transmitted, looking at the wider picture is also necessary. Climate change is underpinning many environmental factors which could cause us to rethink how we look at colds.

For example, studies have shown links between climate change and the risk of respiratory infections. They also have discovered links between the severity of these infections (which include the common cold) due to climate change.^20-22

There’s evidence that, in cold climates, a decrease in temperature and humidity often occurs followed by an increase in rhinovirus infections.²¹

Temperatures around the world are increasing year-on-year due to global warming. However, despite warmer winters due to global warming, the common cold remains an important risk factor to public health.²²

As a pharmacist, knowing when cold and flu season is approaching means you can prepare for a rise in patients looking for cold and flu treatments. You are also well placed as a first point of contact for many to provide advice on treating colds.