Common cold
Upper respiratory tract infections are loosely divided by the areas they affect, with the common cold primarily affecting the nose, pharyngitis the throat, and sinusitis the sinuses. Symptoms are mostly due to the body's immune response to the infection rather than to tissue destruction by the viruses themselves. The primary method of prevention is by hand washing with some evidence to support the effectiveness of wearing face masks.
There is no cure for the common cold but the symptoms can be treated. It is the most frequent infectious disease in humans with the average adult contracting two to three colds a year and the average child contracting between six and twelve. These infections have been with humanity since antiquity.
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Signs and symptoms
The typical symptoms of a cold include cough, runny nose, nasal congestion and a sore throat, sometimes accompanied by muscle ache, fatigue, headache, and loss of appetite.[1] A sore throat is present in about 40% of the cases and a cough in about 50%,[2] while muscle ache occurs in about half.[3] In adults, a fever is generally not present but it is common in infants and young children.[3] The cough is usually mild compared to that accompanying influenza.[3] While a cough and a fever indicate a higher likelihood of influenza in adults, there is a great deal of similarity between these two conditions.[4] A number of the viruses that cause the common cold may also result in asymptomatic infections.[5][6] The color of the sputum or nasal secretion may vary from clear to yellow to green and does not predict the class of agent causing the infection.[7]
Progression
A cold usually begins with fatigue, a feeling of being chilled, sneezing and a headache, followed in a couple of days by a runny nose and cough.[1] Symptoms typically peak two to three days after infection onset,[3] and usually resolve in seven to ten days but some can last for up to three weeks.[8] In children, the cough lasts for more than ten days in 35–40% of the cases and continues for more than 25 days in 10%.[9]
Cause
Viruses
The common cold is a viral infection of the upper respiratory tract. The most commonly implicated virus is a rhinovirus (30–80%), a type of picornavirus with 99 known serotypes.[10][11] Others include: coronavirus (10–15%), influenza (5–15%),[3] human parainfluenza viruses, human respiratory syncytial virus, adenoviruses, enteroviruses, and metapneumovirus.[12] Frequently more than one virus is present.[13] In total over 200 different viral types are associated with colds.[3]
Transmission
The common cold virus is typically transmitted either via airborne droplets or contact with infected nasal secretions or other contaminated objects.[2][14] Which of these routes is of primary importance has not been determined.[15] The viruses may survive for prolonged periods in the environment and can be picked up by people's hands and subsequently carried to their eyes or nose where infection occurs.[14] Transmission is common in daycare and at school due to the close proximity of many children with little immunity and frequently poor hygiene.[16] These infections are then brought home to other members of the family.[16] There is no evidence that recirculated air during commercial flight is a method of transmission.[14] However, people sitting in close proximity appear at greater risk.[15] Rhinovirus-caused colds are most infectious during the first three days of symptoms; they are much less infectious afterwards.[17]
Weather
The traditional folk theory is that a cold can be "caught" by prolonged exposure to cold weather such as rain or winter conditions, which is how the disease got its name.[18] The role of body cooling as a risk factor for the common cold is controversial.[19] Some of the viruses that cause the common colds are seasonal, occurring more frequently during cold or wet weather.[20] Some believe this to be due primarily to increased time spent indoors in close proximity;[21] specifically children returning to school.[16] However, it may also be related to changes in the respiratory system that result in greater susceptibility.[22] Low humidity increases viral transmission rates potentially due to dry air allowing small viral droplets to disperse farther and stay in the air longer.[23]
Other
Herd immunity, generated from previous exposure to cold viruses, plays an important role in limiting viral spread, as seen with younger populations that have greater rates of respiratory infections.[24] Poor immune function is also a risk factor for disease.[24][25] Insufficient sleep and malnutrition have been associated with a greater risk of developing infection following rhinovirus exposure; this is believed to be due to their effects on immune function.[26][27]
Pathophysiology
The symptoms of the common cold are believed to be primarily related to the immune response to the virus.[28] The mechanism of this immune response is virus specific. For example, the rhinovirus is typically acquired by direct contact; it binds to human ICAM-1 receptors through unknown mechanisms to trigger the release of inflammatory mediators.[28] These inflammatory mediators then produce the symptoms.[28] It does not generally cause damage to the nasal epithelium.[3] The respiratory syncytial virus (RSV) on the other hand is contacted by both direct contact and air born droplets. It then replicates in the nose and throat before frequently spreading to the lower respiratory tract.[29] RSV does cause epithelium damage.[29] Human parainfluenza virus typically results in inflammation of the nose, throat, and bronchi.[30] In young children when it affects the trachea it may produce the symptoms of croup due to the small size of their airway.[30]
Diagnosis
The distinction between different viral upper respiratory tract infections is loosely based on the location of symptoms with the common cold affecting primarily the nose, pharyngitis the throat, and bronchitis the lungs.[2] There however can be significant overlap and multiple areas can be affected.[2] The common cold is frequently defined as nasal inflammation with varying amount of throat inflammation.[31] Self diagnosis is frequent.[3] Isolation of the actual viral agent involved is rarely performed,[31] and it is generally not possible to identify the virus type through symptoms.[3]
Prevention
Physical measure to prevent the spread of cold viruses has been deemed the only potentially effective measure for prevention.[32] These measures include primarily hand washing and face masks; in the health care environment, gowns and disposable gloves are also used.[32] Efforts such as quarantine are not possible as the disease is so widespread and symptoms are non-specific. Vaccination has proved difficult as there are so many viruses involved and they change rapidly.[32] Creation of a broadly effective vaccine is thus highly improbable.[33]
Regular hand washing appears to be effective at reducing the transmission of cold viruses especially among children.[34] Whether the addition of antivirals or antibacterials to normal hand washing provides greater benefit is unknown.[34] Wearing face masks when around people who are infected may be beneficial; however, there is insufficient evidence for maintaining a greater social distance.[34] Zinc supplementation may be effective at decreasing the rate of colds.[35] Routine vitamin C supplementation does not reduce the risk or severity of the common cold, though it may reduce its duration.[36]
Management
There are currently no medications or herbal remedies which have been conclusively demonstrated to shorten the duration of infection.[37] Treatment thus comprises symptomatic relief.[38] Getting plenty of rest, drinking fluids to maintain hydration, and gargling with warm salt water, are reasonable conservative measures.[12] Much of the benefit from treatment is however attributed to the placebo effect.[39]
Symptomatic
Treatments that help alleviate symptoms include simple analgesics and antipyretics such as ibuprofen[40] and acetaminophen/paracetamol.[41] Evidence does not show that cough medicines are any more effective than simple analgesics[42] and they are not recommended for use in children due to a lack of evidence supporting effectiveness and the potential for harm.[43][44] Canada in 2009 restricted the use of over-the-counter cough and cold medication in children 6 years and under due to concerns regarding risks and unproven benefits.[43] The misuse of dextromethorphan (an over-the-counter cough medicine) has lead to its ban in a number of countries.[45]
In adults the symptoms of a runny nose can be reduced by first generation antihistamines; however, they are associated with adverse effects such as drowsiness.[38] Other decongestants such as pseudoephedrine are also effective in this population.[46] Ipratropium nasal spray may reduce the symptoms of a runny nose but there is little effect on stuffiness.[47] Second-generation antihistamines however do not appear to be effective.[48]
Due to lack of studies, it is not known whether increased fluid intake improves symptoms or shortens respiratory illness[49] and a similar lack of data exists for the use of heated humidified air.[50] One study has found chest vapor rub to be effective at providing some symptomatic relief of nocturnal cough, congestion, and sleep difficulty.[51]
Antibiotics have no effect against viral infections and thus have no effect against the viruses that cause the common cold.[52] Due to their side effects they cause overall harm; however, they are still frequently prescribed.[52][53] Some of the reasons that antibiotics are so commonly prescribed include: people's expectations for them, physicians' desire to do something, and the difficulty in excluding complications that may be amenable to antibiotics.[54] There are no effective antiviral drugs for the common cold even though some preliminary research has shown benefit.[38][55]
Alternative treatments
While there are many alternative treatments used for the common cold, there is insufficient scientific evidence to support the use of most.[38] As of 2010 there is insufficient evidence to recommend for or against either honey or nasal irrigation.[56][57] Studies suggested that Zinc, if taken within 24 hours of the onset of symptoms, reduces the duration and severity of the common cold in healthy people.[35] Due to wide differences between the studies, further research may be needed to determine how and when zinc may be effective.[58] Vitamin C's effect on the common cold while extensively researched is disappointing, except in limited circumstances, specifically, individuals exercising vigorously in cold environments.[36][59] Evidence about the usefulness of echinacea is inconsistent.[60][61] Different types of echinacea supplements may vary in their effectiveness.[60]
Prognosis
The common cold is generally mild and self-limiting with most symptoms generally improving in a week.[2] Severe complications, if they occur, are usually in the very old, the very young or those who are immunosuppressed.[62] Secondary bacterial infections may occur resulting in sinusitis, pharyngitis, or an ear infection.[63] It is estimated that sinusitis occurs in 8% and an ear infection in 30% of cases.[64]
Epidemiology
The common cold is the most common human disease[62] and all peoples globally are affected.[16] Adults typically have two to five infections annually[2][3] and children may have six to ten colds a year (and up to twelve colds a year for school children).[38] Rates of symptomatic infections increase in the elderly due to a worsening immune system.[24]
History
While the cause of the common cold has only been identified since the 1950s the disease has been with humanity since antiquity.[65] Its symptoms and treatment are described in the Egyptian Ebers papyrus, the oldest existing medical text, written before the 16th century BCE.[66] The name "common cold" came into use in the 16th century, due to the similarity between its symptoms and those of exposure to cold weather.[67]
In the United Kingdom, the Common Cold Unit was set up by the Medical Research Council in 1946 and it was here that the rhinovirus was discovered in 1956.[68] In the 1970s, the CCU demonstrated that treatment with interferon during the incubation phase of rhinovirus infection protects somewhat against the disease,[69] but no practical treatment could be developed. The unit was closed in 1989, two years after it completed research of zinc gluconate lozenges in the prophylaxis and treatment of rhinovirus colds, the only successful treatment in the history of the unit.[70]
Economic impact
The economic impact of the common cold is poorly understood in much of the world.[72] In the United States, the common cold leads to 75–100 million physician visits annually at a conservative cost estimate of $7.7 billion per year. Americans spend $2.9 billion on over-the-counter drugs and another $400 million on prescription medicines for symptomatic relief.[73] More than one-third of people who saw a doctor received an antibiotic prescription, which has implications for antibiotic resistance.[73] An estimated 22–189 million school days are missed annually due to a cold. As a result, parents missed 126 million workdays to stay home to care for their children. When added to the 150 million workdays missed by employees suffering from a cold, the total economic impact of cold-related work loss exceeds $20 billion per year.[12][73] This accounts for 40% of time lost from work in the United States.[74]
Research
A number of antivirals have been tested for effectiveness in the common cold; however as of 2009 none have been both found effective and licensed for use.[75] There are ongoing trials of the anti-viral drug pleconaril which shows promise against picornaviruses as well as trials of BTA-798.[76] The oral form of pleconaril had safety issues and an aerosol form is being studied.[76]
Researchers from University of Maryland, College Park and University of Wisconsin–Madison have mapped the genome for all known virus strains that cause the common cold.[77]
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