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Sneezing, 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 a week or less, it is a
leading cause of doctor visits and of school and
job absenteeism.
The Problem
In 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 1994, 66 million cases of
the common cold in the United States required
medical attention or resulted in restricted
activity. In 1994, colds caused 24 million
days of restricted activity and 20 million
days lost from school, according to NCHS.
The Causes
The
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 Season
In 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 Symptoms
Symptoms 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 Disease
Viruses 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
Spread
Depending 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.
Prevention
Handwashing 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.
Treatment
Only
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 Outlook
Thanks 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.
Credits
U.S. Department Of Health And Human Services
For more information visit them at
http://www.nih.gov
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