The Common ColdSneezing, scratchy throat, runny nose –
everyone knows the first signs of a cold, probably the most common illness
known. Although the common cold is usually mild, with symptoms lasting one
to two weeks, it is a leading cause of doctor visits and of school and job
absenteeism.
The ProblemIn the course of a year, individuals in the United
States suffer 1 billion colds, according to some estimates.
Colds
are most prevalent among children, and seem to be related to youngsters'
relative lack of resistance to infection and to contacts with other
children in day-care centers and schools. Children have about six to ten
colds a year. In families with children in school, the number of colds per
child can be as high as 12 a year. Adults average about two to four colds
a year, although the range varies widely. Women, especially those aged 20
to 30 years, have more colds than men, possibly because of their closer
contact with children. On average, individuals older than 60 have fewer
than one cold a year.
The economic impact of the common cold is
enormous. The National Center for Health Statistics (NCHS) estimates that,
in 1996, 62 million cases of the common cold in the United States required
medical attention or resulted in restricted activity. In 1996, colds
caused 45 million days of restricted activity and 22 million days lost
from school, according to NCHS.
The CausesThe Viruses. More than 200 different viruses
are known to cause the symptoms of the common cold. Some, such as the
rhinoviruses, seldom produce serious illnesses. Others, such as
parainfluenza and respiratory syncytial virus, produce mild infections in
adults but can precipitate severe lower respiratory infections in young
children.
Rhinoviruses (from the Greek rhin, meaning
"nose") cause an estimated 30 to 35 percent of all adult colds, and are
most active in early fall, spring and summer. More than 110 distinct
rhinovirus types have been identified. These agents grow best at
temperatures of 33 degrees Celsius [about 91 degrees Fahrenheit (F)], the
temperature of the human nasal mucosa.
Coronaviruses are believed
to cause a large percentage of all adult colds. They induce colds
primarily in the winter and early spring. Of the more than 30 isolated
strains, three or four infect humans. The importance of coronaviruses as
causative agents is hard to assess because, unlike rhinoviruses, they are
difficult to grow in the laboratory.
Approximately 10 to 15 percent
of adult colds are caused by viruses also responsible for other, more
severe illnesses: adenoviruses, coxsackieviruses, echoviruses,
orthomyxoviruses (including influenza A and B viruses), paramyxoviruses
(including several parainfluenza viruses), respiratory syncytial virus and
enteroviruses.
The causes of 30 to 50 percent of adult colds,
presumed to be viral, remain unidentified. The same viruses that produce
colds in adults appear to cause colds in children. The relative importance
of various viruses in pediatric colds, however, is unclear because of the
difficulty in isolating the precise cause of symptoms in studies of
children with colds.
Does cold weather cause a cold?
Although many people are convinced that a cold results from exposure to
cold weather, or from getting chilled or overheated, NIAID grantees have
found that these conditions have little or no effect on the development or
severity of a cold. Nor is susceptibility apparently related to factors
such as exercise, diet, or enlarged tonsils or adenoids. On the other
hand, research suggests that psychological stress, allergic disorders
affecting the nasal passages or pharynx (throat), and menstrual cycles may
have an impact on a person's susceptibility to colds.
The Cold SeasonIn the United States, most colds occur during the
fall and winter. Beginning in late August or early September, the
incidence of colds increases slowly for a few weeks and remains high until
March or April, when it declines. The seasonal variation may relate to the
opening of schools and to cold weather, which prompt people to spend more
time indoors and increase the chances that viruses will spread from person
to person.
Seasonal changes in relative humidity also may affect
the prevalence of colds. The most common cold-causing viruses survive
better when humidity is low—the colder months of the year. Cold weather
also may make the nasal passages' lining drier and more vulnerable to
viral infection.
Cold SymptomsSymptoms of the common cold usually begin two to
three days after infection and often include nasal discharge, obstruction
of nasal breathing, swelling of the sinus membranes, sneezing, sore
throat, cough, and headache. Fever is usually slight but can climb to
102o F in infants and young children. Cold symptoms can last
from two to 14 days, but two-thirds of people recover in a week. If
symptoms occur often or last much longer than two weeks, they may be the
result of an allergy rather than a cold.
Colds occasionally can
lead to secondary bacterial infections of the middle ear or sinuses,
requiring treatment with antibiotics. High fever, significantly swollen
glands, severe facial pain in the sinuses, and a cough that produces
mucus, may indicate a complication or more serious illness requiring a
doctor's attention.
How Cold Viruses Cause DiseaseViruses cause infection by
overcoming the body's complex defense system. The body's first line of
defense is mucus, produced by the membranes in the nose and throat. Mucus
traps the material we inhale: pollen, dust, bacteria and viruses. When a
virus penetrates the mucus and enters a cell, it commandeers the
protein-making machinery to manufacture new viruses which, in turn, attack
surrounding cells.
Cold symptoms: the body fights back.
Cold symptoms are probably the result of the body's immune response to the
viral invasion. Virus-infected cells in the nose send out signals that
recruit specialized white blood cells to the site of the infection. In
turn, these cells emit a range of immune system chemicals such as kinins.
These chemicals probably lead to the symptoms of the common cold by
causing swelling and inflammation of the nasal membranes, leakage of
proteins and fluid from capillaries and lymph vessels, and the increased
production of mucus.
Kinins and other chemicals released by immune
system cells in the nasal membranes are the subject of intensive research.
Researchers are examining whether drugs to block them, or the receptors on
cells to which they bind, might benefit people with colds.
How Colds are SpreadDepending on the virus type, any or all of
the following routes of transmission may be common:
- Touching infectious respiratory secretions on skin and on
environmental surfaces and then touching the eyes or nose.
- Inhaling relatively large particles of respiratory secretions
transported briefly in the air.
- Inhaling droplet nuclei: smaller infectious particles suspended in
the air for long periods of time.
Research on rhinovirus
transmission. Much of the research on the transmission of the common
cold has been done with rhinoviruses, which are shed in the highest
concentration in nasal secretions. Studies suggest a person is most likely
to transmit rhinoviruses in the second to fourth day of infection, when
the amount of virus in nasal secretions is highest. Researchers also have
shown that using aspirin to treat colds increases the amount of virus shed
in nasal secretions, possibly making the cold sufferer more of a hazard to
others.
PreventionHandwashing is the simplest and most effective way to
keep from getting rhinovirus colds. Not touching the nose or eyes is
another. Individuals with colds should always sneeze or cough into a
facial tissue, and promptly throw it away. If possible, one should avoid
close, prolonged exposure to persons who have colds.
Because
rhinoviruses can survive up to three hours outside the nasal passages on
inanimate objects and skin, cleaning environmental surfaces with a
virus-killing disinfectant might help prevent spread of
infection.
A cold vaccine? The development of a vaccine
that could prevent the common cold has reached an impasse because of the
discovery of many different cold viruses. Each virus carries its own
specific antigens, substances that induce the formation of specific
protective proteins (antibodies) produced by the body. Until ways are
found to combine many viral antigens in one vaccine, or take advantage of
the antigenic cross-relationships that exist, prospects for a vaccine are
dim. Evidence that changes occur in common-cold virus antigens further
complicate development of a vaccine. Such changes occur in some influenza
virus antigens and make it necessary to alter the influenza vaccine each
year.
TreatmentOnly symptomatic treatment is available for
uncomplicated cases of the common cold: bed rest, plenty of fluids,
gargling with warm salt water, petroleum jelly for a raw nose, and aspirin
or acetaminophen to relieve headache or fever.
A word of
caution: several studies have linked the use of aspirin to the
development of Reye's syndrome in children recovering
from influenza or chickenpox. Reye's syndrome is a rare but serious
illness that usually occurs in children between the ages of three and 12
years. It can affect all organs of the body, but most often injures the
brain and liver. While most children who survive an episode of Reye's
syndrome do not suffer any lasting consequences, the illness can lead to
permanent brain damage or death. The American Academy of Pediatrics
recommends children and teenagers not be given aspirin or any medications
containing aspirin when they have any viral illness, particularly
chickenpox or influenza. Many doctors recommend these medications be used
for colds in adults only when headache or fever is present. Researchers,
however, have found that aspirin and acetaminophen can suppress certain
immune responses and increase nasal stuffiness in
adults.
Nonprescription cold remedies, including decongestants and
cough suppressants, may relieve some cold symptoms but will not prevent,
cure, or even shorten the duration of illness. Moreover, most have some
side effects, such as drowsiness, dizziness, insomnia, or upset stomach,
and should be taken with care.
Nonprescription antihistamines may
have some effect in relieving inflammatory responses such as runny nose
and watery eyes that are commonly associated with
colds.
Antibiotics do not kill viruses. These prescription drugs
should be used only for rare bacterial complications, such as sinusitis or
ear infections, that can develop as secondary infections. The use of
antibiotics "just in case" will not prevent secondary bacterial
infections.
Does vitamin C have a role? Many people are
convinced that taking large quantities of vitamin C will prevent colds or
relieve symptoms. To test this theory, several large-scale, controlled
studies involving children and adults have been conducted. To date, no
conclusive data has shown that large doses of vitamin C prevent colds. The
vitamin may reduce the severity or duration of symptoms, but there is no
definitive evidence.
Taking vitamin C over long periods of time in
large amounts may be harmful. Too much vitamin C can cause severe
diarrhea, a particular danger for elderly people and small children. In
addition, too much vitamin C distorts results of tests commonly used to
measure the amount of glucose in urine and blood. Combining oral
anticoagulant drugs and excessive amounts of vitamin C can produce
abnormal results in blood-clotting tests.
Inhaling steam also has
been proposed as a treatment of colds on the assumption that increasing
the temperature inside the nose inhibits rhinovirus replication. Recent
studies found that this approach had no effect on the symptoms or amount
of viral shedding in individuals with rhinovirus colds. But steam may
temporarily relieve symptoms of congestion associated with
colds.
Interferon-alpha has been studied extensively for the
treatment of the common cold. Investigators have shown interferon, given
in daily doses by nasal spray, can prevent infection and illness.
Interferon, however, causes unacceptable side effects such as nosebleeds
and does not appear useful in treating established colds. Most cold
researchers are concentrating on other approaches to combatting cold
viruses.
The OutlookThanks to basic research, scientists know more about
the rhinovirus than almost any other virus, and have powerful new tools
for developing antiviral drugs. Although the common cold may never be
uncommon, further investigations offer the hope of reducing the huge
burden of this universal problem.
Keeping on Top of Your Condition
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