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Allergen Causes

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The Common Cold

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How viruses cause the disease

What is the Common Cold?
The Common Cold and Flu are umbrella terms for acute upper respiratory tract infections, usually of viral origin. Colds and Flus are seasonal. They are most common during the colder months and at the change of the seasons.

What are the main causes of Common Cold
Common Colds are spread from person to person by expelled respiratory secretions containing a virus. The infectious nature of colds has caught the imagination of many investigators since the early 1900's. However, progress in identifying the causative agents was not made until the 1950's after the first recognised rhinovirus was isolated. It was soon demonstrated that a number of strains of rhinovirus existed and that they were a significant cause of the common cold. Other respiratory viruses were identified during this period which also cause symptoms of the common cold included the adenoviruses, the coronaviruses and others.

Common Cold Symptoms
The signs (what you can see, eg. a runny nose) and symptoms (what you can only feel, eg. an itch) of the common cold are generally characterized by the symptoms of coughing, sneezing and a nasal discharge. The cardinal symptoms are:

Cough
Coughing is an important adjunct to mucociliary clearance and is thus in principle a protective or cleansing reflex. A single cough involves several organs. The lungs, peripheral nervous pathways and the central nervous system form a unit which is responsible for the stimulation, regulation and coordination of cough. Coughing is regulated by a reflex known as the cough reflex. The reflex arc consists of the following chain of structures:

receptor » afferent neuron » central synapse » efferent neuron » target organ

Runny Nose
A runny nose or rhinorroeah is a typical symptom of a common cold and the sneeze and watery nasal secretion often heralds the start of a common cold infection. The nasal secretions associated with the common cold can be divided into two categories:

• The watery secretions at the early phase of an infection, usually within the first two days.

• The thick viscous catarrh like secretions which may occur together with nasal congestion later in the course of infection and which may persist for several weeks following resolution of other symptoms.

The watery nasal secretions at the start of a common cold may be explained by bradykinin formation and sensory nerve stimulation. The later phase of thick, viscous secretions may be related to the formation of other mediators such as prostaglandins and to mediators such as leukotrienes released from leukocytes attracted into the nasal mucosa by the inflammatory response.

The watery and mucus secretions are essential components of the process of mucociliary clearance which continually cleanses the nose of filtered particulate matter. Viral infection of the nose causes an increase in nasal secretions which aid in controlling the infection. Nasal secretion provides a barrier between the hostile environment of the respired air and the delicate membrane of the respiratory epithelium. Nasal secretion has three main functions; respiratory defense, air conditioning and mucociliary transport.

Nasal Congestion
Infection of the upper airway causes an inflammatory response of the airway epithelium. One of the main components of inflammation is vasodilation with a great increase in blood flow to the inflamed tissue. The classical signs of inflammation are pain, swelling and redness. The congestion of the nose that occurs with the common cold is probably as a result of the nasal inflammation causing increased nasal blood flow and increased filling of the venous sinusoids. Obstruction of the nasal passage may be beneficial as this will raise the temperature of the nasal mucosa. Rhinoviruses replicate best at temperatures several degrees below the normal body temperature and these conditions are normally found within the nasal passages which are cooled by the inspired air. Obstruction of the nasal passage with nasal congestion will raise the temperature of the nasal passage and thus inhibit rhinovirus replication.

Sore Throat
A dry, 'scratchy' or sore throat is often the first sign of a common cold and if this symptom is accompanied by sneezing one can almost confidently predict a common cold infection. The mild sore throat at the start of a common cold is sometimes followed by a more severe sore throat several days later and this may represent a secondary bacterial infection of the throat.

The early sore throat may be caused by viral infection of the back of the nose, the nasopharynx, where the nose enters the throat region or pharynx. Studies on the spread of rhinovirus infection have shown that the virus first infects the nasopharynx region even when drops of infected fluid are placed in the nose or eye. Viral damage to the mucosa causes the production of inflammatory mediators such as bradykinin which initiate a sensation of pain and irritation when they contact sensory nerve endings.

The sore throat associated with a common cold may be caused by at least three mechanisms: viral infection of the nasopharynx and throat; clearance of bradykinin from the infected nasal region towards the nasopharynx; and bacterial infection of the nasopharynx and throat.

Fever
Mild fever and chilliness is found in around 20% of rhinovirus common cold infections in adults and the incidence is up to 60% in children. Fever is caused by the infectious agent stimulating the release of endogenous substances (e.g. interleukin 1, interleukin 6, tumour necrosis factor) from white cells, which can act as pyrogens.

In common cold infections, it is not clear how the virus triggers the white cells to release endogenous pyrogens. Inflammatory mediators such as bradykinin may be the trigger for pyrogen release as bradykinin is a major mediator of common cold symptoms and is also known to interact with white cells.

Headache
A headache is a common symptom associated with the common cold and its incidence in rhinovirus infections is quoted as ranging from around 20% to 60% of infections. A headache has various causes, but with the common cold there are probably three major causes:

1. Psychogenic headache:
   The most commonly encountered headache is the ?psychogenic? or ?muscle contraction headache? and over 80% of headache patients seen by the general practitioner suffer from this type of headache. Psychogenic headache results from stress, depression, anxiety and fatigue. The symptoms of a common cold may cause stress, depression, anxiety and fatigue by restricting normal activities or if the symptoms are perceived to affect the quality of life.

2. Vascular headache:
   Vascular headache is due to the dilation of blood vessels in and around the brain and it often accompanies fever. An increase in body temperature causes dilation of brain blood vessels and this results in a throbbing headache.

3. Headache of nasal origin:
   Congestion of the nasal turbinates and occlusion of the ostia of paranasal sinuses are a source of headaches with the common cold. A severe frontal headache is associated with inflammation of the frontal sinus mucosa. This frontal headache is located directly over the sinus and may radiate to behind the eyes, and is caused by a build up of inflammatory exudate in the frontal sinus. The nasal congestion and inflammation of the common cold infection causes swelling of the mucosa around the ostia of the frontal sinuses and obstruction of the drainage from the sinuses.