Common cold

Signs and symptoms

The typical symptoms of a cold include a cough, a runny nose, nasal congestion and a sore throat, sometimes accompanied by muscle ache, fatigue, headache, and loss of appetite. A sore throat is present in about 40% of the cases and a cough in about 50%, while muscle ache occurs in about half. In adults, a fever is generally not present but it is common in infants and young children. The cough is usually mild compared to that accompanying influenza. While a cough and a fever indicate a higher likelihood of influenza in adults, a great deal of similarity exists between these two conditions. A number of the viruses that cause the common cold may also result in asymptomatic infections.

The color of the sputum or nasal secretion may vary from clear to yellow to green and does not indicate the class of agent causing the infection.

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. Symptoms may begin within sixteen hours of exposure and typically peak two to four days after onset. They usually resolve in seven to ten days, but some can last for up to three weeks. The average duration of cough is eighteen days and in some cases people develop a post-viral cough which can linger after the infection is gone. 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%.

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. Other commonly implicated viruses include human coronavirus (≈15%), influenza viruses (10%–15%), adenoviruses (5%), human respiratory syncytial virus, enteroviruses other than rhinoviruses, human parainfluenza viruses, and metapneumovirus. Frequently more than one virus is present. In total over 200 different viral types are associated with colds.

Transmission

The common cold virus is typically transmitted via airborne droplets (aerosols), direct contact with infected nasal secretions, or fomites (contaminated objects). Which of these routes is of primary importance has not been determined; however, hand-to-hand and hand-to-surface-to-hand contact seems of more importance than transmission via aerosols. The viruses may survive for prolonged periods in the environment (over 18 hours for rhinoviruses) and can be picked up by people's hands and subsequently carried to their eyes or nose where infection occurs. Transmission is common in daycare and at school due to the proximity of many children with little immunity and frequently poor hygiene. These infections are then brought home to other members of the family. There is no evidence that recirculated air during commercial flight is a method of transmission. People sitting in close proximity appear to be at greater risk of infection.

Rhinovirus-caused colds are most infectious during the first three days of symptoms; they are much less infectious afterwards.

Weather

The traditional 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. Some of the viruses that cause the common colds are seasonal, occurring more frequently during cold or wet weather. The reason for the seasonality has not been conclusively determined. Possible explanations may include cold temperature-induced changes in the respiratory system, decreased immune response, and low humidity causing an increase in viral transmission rates, perhaps due to dry air allowing small viral droplets to disperse farther and stay in the air longer.

The apparent seasonality may also be due to social factors, such as people spending more time indoors, near infected people, and specifically children at school. There is some controversy over the role of low body temperature as a risk factor for the common cold; the majority of the evidence suggests that it may result in greater susceptibility to infection.

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. Poor immune function is a risk factor for disease. 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. Breast feeding decreases the risk of acute otitis media and lower respiratory tract infections among other diseases, and it is recommended that breast feeding be continued when an infant has a cold. In the developed world breast feeding may not be protective against the common cold in and of itself.

Pathophysiology

The symptoms of the common cold are believed to be primarily related to the immune response to the virus. 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. These inflammatory mediators then produce the symptoms. It does not generally cause damage to the nasal epithelium. The respiratory syncytial virus (RSV), on the other hand, is contracted by direct contact and airborne droplets. It then replicates in the nose and throat before frequently spreading to the lower respiratory tract. RSV does cause epithelium damage. Human parainfluenza virus typically results in inflammation of the nose, throat, and bronchi. In young children when it affects the trachea it may produce the symptoms of croup due to the small size of their airways.

Diagnosis

The distinction between 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. However, there can be significant overlap and multiple areas can be affected. The common cold is frequently defined as nasal inflammation with varying amount of throat inflammation. Self-diagnosis is frequent. Isolation of the viral agent involved is rarely performed, and it is generally not possible to identify the virus type through symptoms.

Prevention

The only useful ways to reduce the spread of cold viruses are physical measures such as hand washing and face masks; in the healthcare environment, gowns and disposable gloves are also used. Isolation or quarantine is not used as the disease is so widespread and symptoms are non-specific. Vaccination has proved difficult as there are many viruses involved and they mutate rapidly. Creation of a broadly effective vaccine is, thus, highly improbable.

Regular hand washing appears to be effective in reducing the transmission of cold viruses, especially among children. Whether the addition of antivirals or antibacterials to normal hand washing provides greater benefit is unknown. Wearing face masks when around people who are infected may be beneficial; however, there is insufficient evidence for maintaining a greater social distance.

Zinc supplements may help to reduce the prevalence of colds. Routine vitamin C supplements do not reduce the risk or severity of the common cold, though they may reduce its duration. Gargling with water was found useful in one small trial.

Management

No medications or herbal remedies have been conclusively demonstrated to shorten the duration of infection. Treatment thus comprises symptomatic relief. Getting plenty of rest, drinking fluids to maintain hydration, and gargling with warm salt water are reasonable conservative measures. Much of the benefit from treatment is, however, attributed to the placebo effect.

Symptomatic

Treatments that help with symptoms include simple pain medication and medications for fevers such as ibuprofen and acetaminophen/paracetamol. There is not good evidence for cough medicines. Cough medicine are not recommended for use in children due to a lack of evidence supporting effectiveness and the potential for harm. In 2009, Canada restricted the use of over-the-counter cough and cold medication in children six years and under due to concerns regarding risks and unproven benefits. The misuse of dextromethorphan (an over-the-counter cough medicine) has led to its ban in a number of countries.

In adults short term use of nasal decongestants may have a small benefit. Antihistamines may improve symptoms in the first day or two; however, there is no longer-term benefit and they have adverse effects such as drowsiness. Other decongestants such as pseudoephedrine appear effective in adults. Ipratropium nasal spray may reduce the symptoms of a runny nose but has little effect on stuffiness. The safety and effectiveness of nasal decongestant use in children is unclear.

Due to lack of studies, it is not known whether increased fluid intake improves symptoms or shortens respiratory illness, and there is a similar lack of data for the use of heated humidified air. One study has found chest vapor rub to provide some relief of nocturnal cough, congestion, and sleep difficulty.

Antibiotics and antivirals

Antibiotics have no effect against viral infections or against the viruses that cause the common cold. Due to their side effects, antibiotics cause overall harm but are still frequently prescribed. Some of the reasons that antibiotics are so commonly prescribed include people's expectations for them, physicians' desire to help, and the difficulty in excluding complications that may be amenable to antibiotics. There are no effective antiviral drugs for the common cold even though some preliminary research has shown benefits.

Alternative medicine

While there are many alternative treatments used for the common cold, there is insufficient scientific evidence to support the use of most. As of 2014 there is insufficient evidence to recommend for or against honey. As of 2015 there is tentative evidence to support nasal irrigation. Zinc has been used to treat symptoms, with studies suggesting that zinc, if taken within 24 hours of the onset of symptoms, reduces the duration and severity of the common cold in otherwise healthy people. Due to wide differences between the studies, further research may be needed to determine how and when zinc may be effective. Whereas zinc lozenges may produce side effects, there is only a weak rationale for physicians to recommend zinc for the treatment of the common cold. Some zinc remedies directly applied to the inside of the nose have led to the loss of the sense of smell.

Vitamin C's effect on the common cold, while extensively researched, is disappointing, except in limited circumstances: specifically, individuals exercising vigorously in cold environments. There is no firm evidence that Echinacea products provide any meaningful benefit in treating or preventing colds. It is unknown if garlic is effective. A single trial of vitamin D did not find benefit.

Prognosis

The common cold is generally mild and self-limiting with most symptoms generally improving in a week. Half of cases go away in 10 days and 90% in 15 days. Severe complications, if they occur, are usually in the very old, the very young, or those who are immunosuppressed. Secondary bacterial infections may occur resulting in sinusitis, pharyngitis, or an ear infection. It is estimated that sinusitis occurs in 8% and ear infection in 30% of cases.

Epidemiology

The common cold is the most common human disease and affects people all over the globe. Adults typically have two to five infections annually, and children may have six to ten colds a year (and up to twelve colds a year for school children). Rates of symptomatic infections increase in the elderly due to declining immunity.

Native Americans and Inuit are more likely to be infected with colds and develop complications such as otitis media than Caucasians. This may be explained by issues such as poverty and overcrowding rather than by ethnicity.

History

While the cause of the common cold has only been identified since the 1950s, the disease has been with humanity since ancient times. Its symptoms and treatment are described in the Egyptian Ebers papyrus, the oldest existing medical text, written before the 16th century BCE. The name "cold" came into use in the 16th century, due to the similarity between its symptoms and those of exposure to cold weather.

In the United Kingdom, the Common Cold Unit was set up by the Medical Research Council in 1946 and it was where the rhinovirus was discovered in 1956. In the 1970s, the CCU demonstrated that treatment with interferon during the incubation phase of rhinovirus infection protects somewhat against the disease, 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.

Society and culture

The economic impact of the common cold is not well understood in much of the world. 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 symptom relief. More than one-third of people who saw a doctor received an antibiotic prescription, which has implications for antibiotic resistance. 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. This accounts for 40% of time lost from work in the United States.

Research directions

Antivirals have been tested for effectiveness in the common cold; as of 2009, none had been both found effective and licensed for use. There are ongoing trials of the anti-viral drug pleconaril which shows promise against picornaviruses as well as trials of BTA-798. The oral form of pleconaril had safety issues and an aerosol form is being studied. DRACO, a broad-spectrum antiviral therapy, has shown preliminary effectiveness in treating rhinovirus, as well as other infectious viruses.

The genomes for all known human rhinovirus strains have been sequenced.