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The word
Poliomyelitis derives from the Greek words polio (grey) and myelon (marrow,
indicating the spinal cord). Paralysis occurs as a result of the viral
infection in the spinal cord.
Poliovirus is a member of the Enterovirus
subgroup of the family Picornaviridae (small RNA virus). Enteroviruses
are transient inhabitants of the gastrointestinal tract, and are stable
at acid pH. Picornaviruses are small, ether-insensitive (non-enveloped)
viruses with an positive sense single stranded RNA genome about 8,000 nucleotides
long. Polioviruses have icosahederal capsids consisting of one copy of
the RNA genome and 60 copies of each of the viral capsid proteins VP1,
VP2, VP3 and VP4.
In 1949, three different poliovirus serotypes
were identified: Type 1 is known as Brunhilde (named after a female chimpanzee
from which this virus type was first isolated.) Type 2 is known as Lansing
(named after the city in Michigan where the patient lived who was host
to this virus type.) Type 3 is known as Leon (named after the patient from
California from whom this virus was isolated.) The diseases they cause
can not be distinguished by symptoms. The fact that they are different
serotypes means that antibodies produced against each virus serotype cannot
cross react. As a result, there is minimal heterotypic immunity between
the three serotypes. Iimmunity to one strain does not provide protection
against the other two. Therefore, it is possible to have paralytic poliomyelitis
two or three times from infections by different serotypes. For this reason,
all three serotypes are included in both IPV and OPV vaccines. The poliovirus
is rapidly inactivated by heat, formaldehyde, chlorine, and ultraviolet
light.
The
virus enters through the mouth and primary multiplication of the virus
occurs at the site of implantation in the pharynx and gastrointestinal
tract (alimentary phase). The incubation period lasts between 3 and 35
days. The virus is usually present in the throat and in the stools before
the onset of illness. One week after onset there is little virus in the
throat, but virus continues to be excreted in the stools for several weeks.
In most cases, this results in a transient, self-limiting diarrhea, or
it may be completely asymptomatic. In some people, the virus invades the
bloodstream and gets no further, causing a vague flulike illness called
the minor illness of poliomyelitis.
It has been shown that poliovirus preferentially
binds to M cells, Replication in the Organized Mucosa-Associated
Lymphoid Tissue (O-MALT) During the alimentary stage, viruses can be neutralized
by secretory IgA from the mucosal immune response or flushed out of the
GI tract through peristalsis.
The infection then enters a lymphatic
stage by infiltrating M cells in Peyerís patches. The tonsils, deep cervical
lymph nodes and mesenteric lymph nodes are typical sites of secondary replication.
Transient viremia can either lead to minor illness in which antibody appears,
viremia ceases, and viral content diminishes. Subsequently, the virus is
carried by the bloodstream to various internal organs and regional lymph
nodes (the viremic phase). In most cases, no further virus spread
occurs, and there is asymptomatic or mild undifferentiated illness such
as fever, malaise, headache, nausea, gastrointestinal disturbances, and
sore throat, or combinations of these. In a minority of infected individuals,
poliovirus can infect the central nervous system. Replication of poliovirus
in motorneurons of the anterior horn and brain stem causes inflammation
and destruction of the anterior horn cells (motor cells) of the spinal
cord. The impulses that constantly move down the motor nerve from the spinal
cord to muscles are cut off in poliomyelitis when the nerve cells are destroyed.
The result is paralysis of muscles, the extent of which depends on where
the virus strikes and the number of cells that it destroys. As a result,
muscles become limp and cannot contract. This is known as flaccid paralysis.
The limb muscles are most often paralyzed. The abdominal muscles or the
muscles of the back may also become paralyzed. The neck muscles may become
weak, so that the head cannot be raised. Paralysis of the face muscles
may cause twisting of the mouth or drooping eyelids. Paralysis of the throat
or of the muscles of breathing is life threatening. The poliovirus does
not always completely destroy nerve cells. By the end of a month power
returns to apparently paralyzed muscle, and by the end of six months recovery
can be complete. If the nerve cells are destroyed completely, however,
paralysis is permanent.