Etiology

The most common pathogens associated with the common cold are the rhinoviruses (Chapter 255), but the syndrome can be caused by many different viruses (Table 371-1). The role of bocavirus as a cause of colds is uncertain because the virus is often isolated from patients who are co-infected with other recognized pathogens.

Table 371-1 PATHOGENS ASSOCIATED WITH THE COMMON COLD

* Relative frequency of colds caused by the agent.

Epidemiology

Colds occur year-round, but the incidence is greatest from the early fall until the late spring, reflecting the seasonal prevalence of the viral pathogens associated with cold symptoms. The highest incidence of rhinovirus infection occurs in the early fall (August-October) and in the late spring (April-May). The seasonal incidence for parainfluenza viruses (Chapter 251) usually peaks in the late fall and late spring and is highest between December and April for respiratory syncytial virus (RSV; Chapter 252) and influenza viruses (Chapter 250).

Young children have an average of 6-8 colds per year, but 10-15% of children have at least 12 infections per year. The incidence of illness decreases with age, with 2 to 3 illnesses per year by adulthood. The incidence of infection is primarily a function of exposure to the virus. Children in out-of-home daycare centers during the 1st year of life have 50% more colds than children cared for only at home. The difference in the incidence of illness between these groups of children decreases as the length of time spent in daycare increases, although the incidence of illness remains higher in the daycare group through at least the 1st 3 yr of life. Mannose-binding lectin deficiency with impaired innate immunity may be associated with an increased incidence of colds in children.

Pathogenesis

Viruses that cause the common cold are spread by small-particle aerosols, large-particle aerosols, and direct contact. Although the different common cold pathogens can presumably be spread by any of these mechanisms, some routes of transmission appear to be more efficient than others for particular viruses. Studies of rhinoviruses and RSV suggest that direct contact is an efficient mechanism of transmission of these viruses, although transmission by large-particle aerosols can also occur. In contrast to rhinoviruses and RSV, influenza viruses appear to be most efficiently spread by small-particle aerosols.

The respiratory viruses have evolved different mechanisms to avoid host defenses. Infections with rhinoviruses and adenoviruses result in the development of serotype-specific protective immunity. Repeated infections with these pathogens occur because there are a large number of distinct serotypes of each virus. Influenza viruses have the ability to change the antigens presented on the surface of the virus and thus behave as though there were multiple viral serotypes. The interaction of coronaviruses (Chapter 256) with host immunity is not well defined, but it appears that multiple distinct strains of coronaviruses are capable of inducing at least short-term protective immunity. The parainfluenza viruses and RSV each have a small number of distinct serotypes. Reinfection with these viruses occurs because protective immunity to these pathogens does not develop after an infection. Although reinfection is not prevented by the adaptive host response to these viruses, the severity of subsequent illness is moderated by pre-existing immunity.

Viral infection of the nasal epithelium can be associated with destruction of the epithelial lining, as with influenza viruses and adenoviruses, or there can be no apparent histologic damage, as with rhinoviruses and RSV. Regardless of the histopathologic findings, infection of the nasal epithelium is associated with an acute inflammatory response characterized by release of a variety of inflammatory cytokines and infiltration of the mucosa by inflammatory cells. This acute inflammatory response appears to be responsible, at least in part, for many of the symptoms associated with the common cold. Inflammation can obstruct the sinus ostium or eustachian tube and predispose to bacterial sinusitis or otitis media.

Clinical Manifestations

The onset of common cold symptoms typically occurs 1-3 days after viral infection. The 1st symptom noted is often sore or scratchy throat, followed closely by nasal obstruction and rhinorrhea. The sore throat usually resolves quickly and, by the 2nd and 3rd day of illness, nasal symptoms predominate. Cough is associated with ∼30% of colds and usually begins after the onset of nasal symptoms. Influenza viruses, RSV, and adenoviruses are more likely than rhinoviruses or coronaviruses to be associated with fever and other constitutional symptoms. The usual cold persists for about 1 wk, although 10% last for 2 wk.

The physical findings of the common cold are limited to the upper respiratory tract. Increased nasal secretion is usually obvious; a change in the color or consistency of the secretions is common during the course of the illness and does not indicate sinusitis or bacterial superinfection. Examination of the nasal cavity might reveal swollen, erythematous nasal turbinates, although this finding is nonspecific and of limited diagnostic value.

Diagnosis

The most important task of the physician caring for a patient with a cold is to exclude other conditions that are potentially more serious or treatable. The differential diagnosis of the common cold includes noninfectious disorders as well as other upper respiratory tract infections (Table 371-2).

Table 371-2 CONDITIONS THAT CAN MIMIC THE COMMON COLD