Poliomyelitis, also called polio or infantile paralysis, is a highly
infectious viral disease that may attack the central nervous system and is
characterized by symptoms that range from a mild nonparalytic infection to
total paralysis in a matter of hours.
There are three known types of polioviruses (called 1, 2, and 3), each
causing a different strain of the disease and all being members of the
viral family of enteroviruses (viruses that infect the gastrointestinal
tract). Type 1 is the cause of epidemics, and many cases of paralysis,
which is the most severe manifestation of the infection. The virus is
usually a harmless parasite of human beings. Some statistics quote one in
200 infections as leading to paralysis, while others state that one in
1,000 cases reach the central nervous system (CNS). When it does reach the
CNS, inflammation and destruction of the spinal cord motor cells (anterior
horn cells) occurs, which prevents them from sending out impulses to
muscles. This causes the muscles to become limp or soft, and they cannot
contract, a condition called flaccid paralysis and is the type found in
polio. The extent of the paralysis depends on where the virus strikes and
the number of cells that it destroys. Usually, some of the limb muscles
are paralyzed; the abdominal muscles or muscles of the back may be
paralyzed, affecting posture. The neck muscles may become too weak for the
head to be lifted. Paralysis of the face muscles may cause the mouth to
the eyelids to droop. Life may be threatened if paralysis of the throat
or of the breathing muscles occurs.
Humans are the only natural host for polioviruses, and it most commonly
infects younger children, although older children and adults can be
infected. Crowded living conditions and poor hygiene encourage the spread
of poliovirus. Risk factors for this paralytic illness include older age,
pregnancy, abnormalities of the immune system, and a recent episode of
concurrent with the onset of the CNS phase. As of 2004, the last
naturally occurring polio case in the United States was diagnosed in 1979.
Causes and symptoms
Poliovirus can be spread by direct exposure to an infected individual, and
more rarely, by eating foods contaminated with waste products from the
intestines (feces) and/or droplets of moisture (saliva) from an infected
person. Thus, the major route of transmission is fecal-oral, which occurs
primarily with poor sanitary conditions. The virus is believed to enter
the body through the mouth with primary multiplication occurring in the
lymphoid tissues in the throat, where it can persist for about one week.
During this time, it is absorbed into the blood and lymphatics from the
gastrointestinal tract where it can reside and multiply, sometimes for as
long as 17 weeks. Once absorbed, it is widely distributed throughout the
body until it ultimately reaches the CNS (the brain and spinal cord). The
infection is passed on to others when poor hand washing allows the virus
to remain on the hands after eating or using the bathroom. Transmission
remains possible while the virus is being excreted and it can be
transmitted for as long as the virus remains in the throat or feces. The
incubation period ranges from three to 21 days, but cases are most
infectious from seven to ten days before and after the onset of symptoms.
There are two basic patterns to the virus: the minor illness (abortive
type) and the major illness (which may be paralytic or nonparalytic). The
minor illness accounts for 80 to 90 percent of clinical infections and is
found mostly in young children. It is mild and does not involve the CNS.
Symptoms include a slight
, which generally develop three to five days after exposure. Recovery from
the minor illness occurs within 24 to 72 hours. Symptoms of the major
illness usually appear without a previous minor illness and generally
affect older children and adults.
About 10 percent of people infected with poliovirus develop severe
and stiffness of the neck and back. This is due to an inflammation of the
meninges (tissues which cover the spinal cord and brain). This syndrome is
. The term aseptic is used to differentiate this type of meningitis from
those caused by bacteria. The patient usually recovers completely from
this illness within several days.
About 1 percent of people infected with poliovirus develop the most severe
form. Some of these patients may have two to three symptom-free days
between the minor illness and the major illness, but the symptoms often
appear without any previous minor illness. Symptoms again include headache
and back and neck pain. The major symptoms, however, are due to invasion
of the motor nerves, which are responsible for movement of the muscles.
This viral invasion causes inflammation and then destruction of these
nerves. The muscles, therefore, no longer receive any messages from the
brain or spinal cord. The muscles become weak, floppy, and then totally
paralyzed. All muscle tone is lost in the affected limb and the muscle
becomes soft (flaccid). Within a few days, the muscle begins to decrease
in size (atrophy). The affected muscles may be on both sides of the body
(symmetric paralysis) but are often on unbalanced parts of the body
(asymmetric paralysis). Sensation or the ability to feel is not affected
in these paralyzed limbs.
When poliovirus invades the brainstem (the stalk of brain which connects
the two cerebral hemispheres with the spinal cord, called bulbar polio), a
person may begin to have trouble breathing and swallowing. If the
brainstem is severely affected, the brain's control of such vital
functions as heart rate and blood pressure may be disturbed, a condition
that can lead to death.
The maximum state of paralysis is usually reached within just a few days.
The remaining, unaffected nerves then begin the process of attempting to
grow branches, which can compensate for the destroyed nerves. Fortunately,
the nerve cells are not always completely destroyed. By the end of a
month, the nerve impulses start to return to the apparently paralyzed
muscle and by the end of six months, recovery is almost complete. If the
nerve cells are completely destroyed; however, paralysis is permanent.
Fever and asymmetric flaccid paralysis without sensory loss in a child or
young adult almost always indicate poliomyelitis. Using a long, thin
needle inserted into the lower back to withdraw spinal fluid (lumbar
will reveal increased white blood cells and no bacteria (aseptic
meningitis). Nonparalytic poliomyelitis cannot be distinguished clinically
from aseptic meningitis due to other agents. Virus isolated from a throat
swab and/or feces or blood tests demonstrating the rise in a specific
antibody is required to confirm the diagnosis.
There is no specific treatment for polio except symptomatic. Therapy is
designed to make the patient more comfortable (pain medications and hot
packs to soothe the muscles), and intervention if the muscles responsible
for breathing fail (for instance, a ventilator to take over the work of
breathing). During active infection, rest on a firm bed is indicated.
Physical therapy is the most important part of management of paralytic
polio during recovery.
When poliovirus causes only the minor illness or simple aseptic
meningitis, the patient can be expected to recover completely. Among
patients with the major illness, about 50 percent recover completely.
About 25 percent of such patients have slight disability, and about 25
percent have permanent and serious disability. Approximately 1 percent of
all patients with major illness die. The greatest return of muscle
function occurs in the first six months, but improvements may continue for
Post-polio syndrome (PPS) is a condition that can strike polio survivors
anywhere from 10 to 40 years after their recovery from polio. It is caused
by the death of individual nerve terminals in the motor units that remain
after the initial polio attack. Symptoms include fatigue, slowly
progressive muscle weakness, muscle and joint pain, and muscular atrophy.
The severity of PPS depends upon how seriously the survivors were affected
by the first polio attack.
—Sterile; containing no microorganisms, especially no bacteria.
—Not occurring equally on both sides of the body.
—The progressive wasting and loss of function of any part of the
—The stalk of the brain which connects the two cerebral
hemispheres with the spinal cord. It is involved in controlling vital
functions, movement, sensation, and nerves supplying the head and neck.
—Refers to a situation in which a particular disease rapidly
spreads among many people in the same geographical region in a
relatively short period of time.
—Flabby, limp, weak, or floppy.
—Pertaining to the digestive organs and structures, including the
stomach and intestines.
—Clear, slightly yellow fluid carried by a network of thin tubes
to every part of the body. Cells that fight infection are carried in the
—Loss of the ability to move one or more parts of the body
voluntarily due to muscle or nerve damage.
—Occurring on both sides of the body, in a mirror-image fashion.
Since the year 2000, the Sabin vaccine (also called the oral polio vaccine
or OPV) has been discontinued in the United States, although it is still
being used in other countries. It contains the live, but weakened,
poliovirus and because OPV uses the live virus, it has the potential to
cause infection in individuals with weak immune defenses (both in the
person who receives the vaccine and in close contacts). Approximately nine
cases a year of vaccine related polio was associated with OPV in the
United States. Although this is a rare complication,
occurring in only one in 6.8 million doses administered and one in every
6.4 million doses from having close contact with someone who received the
vaccine, the risk of having polio from OPV was greater than it was of
naturally acquiring it.
Following the launching of the Global Polio Eradication Initiative, the
number of cases fell 99 percent from an estimated 350,000 cases to less
than 3,500 cases worldwide in 2000. At the end of 2000, the number of
polio-infected countries was approximately 20, down from 125. The goal of
the World Health Organization (WHO) is to have polio eliminated from the
planet by the year 2005. The virus has still been identified in Africa and
parts of Asia, so travelers to those areas may want to check with their
physicians concerning booster vaccinations.
Polio: An American Story.
Oxford, UK: Oxford University Press, 2004.
Alexander, L. N., et al. "Vaccine Policy Changes and Epidemiology
of Poliomyelitis in the United States"
Journal of the American Medical Association
292 (2004): 1696–1701.
International Polio Network.
4207 Lindell Blvd., Suite 110, St. Louis, MO 63108–2915. Web site:
March of Dimes Birth Defects Foundation.
National Office, 1275 Mamaroneck Avenue, White Plains, NY 10605. Web
World Health Organization.
Global Polio Eradication Progress 2004.
Available online at http://www.polioeradication.org/.