Understanding the symptoms of the common cold and influenza

Abstract

The common cold and influenza (flu) are the most common syndromes of infection in human beings. These diseases are diagnosed on symptomatology, and treatments are mainly symptomatic, yet our understanding of the mechanisms that generate the familiar symptoms is poor compared with the amount of knowledge available on the molecular biology of the viruses involved. New knowledge of the effects of cytokines in human beings now helps to explain some of the symptoms of colds and flu that were previously in the realm of folklore rather than medicine-eg, fever, anorexia, malaise, chilliness, headache, and muscle aches and pains. The mechanisms of symptoms of sore throat, rhinorrhoea, sneezing, nasal congestion, cough, watery eyes, and sinus pain are discussed, since these mechanisms are not dealt with in any detail in standard medical textbooks.

Figure 1

Sneezing is mainly triggered by stimulation of trigeminal sensory nerves in the nasal epithelium The trigeminal nerves provide a sensory input to the “sneeze centre” in the medulla that triggers reflex activation of nasal and lacrimal glands to cause a rhinorrhoea, facial muscles to cause closure of the eyes and grimace, and respiratory muscles to cause inspiration followed by an explosive expiration.

Figure 2

Spontaneous changes in unilateral nasal airway resistance recorded in one subject with symptoms of common cold (A), and 6–8 weeks later when healthy (B) Blue symbols represent left nasal airway resistance and red symbols represent right nasal airway resistance. Redrawn from Eccles et al.

Figure 3

Cough is normally caused by aspiration of food or fluid in the airway and this stimulates sensory receptors supplied by the vagus nerve to trigger cough Cough associated with URTIs is caused by a hyper-reactivity of this response, and cough occurs spontaneously. Cough can also be initiated and inhibited by voluntary control, indicating some control of cough from higher centres such as the cerebral cortex.

Figure 4

Fever is caused by cytokines released from macrophages and other immune cells The cytokines may act on vagal nerve endings or enter the brain to cause a resetting of the temperature control centre in the hypothalamus. The hypothalamus causes shivering and constriction of skin blood vessels and also initiates a sensation of chilliness that is perceived at the level of the cerebral cortex. IL=interleukin; TNF=tumour necrosis factor.

Figure 5

Time course of symptoms of sneezing and cough using challenge with infected nasal secretions to infect human volunteers Results redrawn from the study by Jackson et al.