Poliovirus : Pathogenesis,Clinical Features,Laboratory Testing and Epidemiology
Poliovirus is a member of the enterovirus subgroup, family Picornaviridae. Enteroviruses are transient inhabitants of the gastrointestinal tract, and are stable at acid pH. Picornaviruses are small, ether-insensitive viruses with an RNA genome.
There are three poliovirus serotypes (P1, P2, and P3). There is minimal heterotypic immunity between the three serotypes. That is, immunity to one serotype does not produce significant immunity to the other serotypes.
The poliovirus is rapidly inactivated by heat, formaldehyde, chlorine, and ultraviolet light.
Pathogenesis
The virus enters through the mouth, and primary multiplication of the virus occurs at the site of implantation in the pharynx and gastrointestinal tract. The virus is usually present in the throat and in the stool before the onset of illness. One week after onset there is less virus in the throat, but virus continues to be excreted in the stool for several weeks. The virus invades local lymphoid tissue, enters the bloodstream, and then may infect cells of the central nervous system. Replication of poliovirus in motor neurons of the anterior horn and brain stem results in cell destruction and causes the typical manifestations of poliomyelitis.
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Clinical Features
The incubation period for nonparalytic poliomyelitis is 3-6 days. For the onset of paralysis in paralytic poliomyelitis, the incubation period usually is 7 to 21 days.
The response to poliovirus infection is highly variable and has been categorized on the basis of the severity of clinical presentation.
Up to 72% of all polio infections in children are asymptomatic. Infected persons without symptoms shed virus in the stool and are able to transmit the virus to others.
Approximately 24% of polio infections in children consist of a minor, nonspecific illness without clinical or laboratory evidence of central nervous system invasion. This clinical presentation is known as abortive poliomyelitis, and is characterized by complete recovery in less than a week. This is characterized by a low grade fever and sore throat.
Nonparalytic aseptic meningitis (symptoms of stiffness of the neck, back, and/or legs), usually following several days after a prodrome similar to that of minor illness, occurs in 1%–5% of polio infections in children. Increased or abnormal sensations can also occur. Typically these symptoms will last from 2 to 10 days, followed by complete recovery.
Fewer than 1% of all polio infections in children result in flaccid paralysis. Paralytic symptoms generally begin 1 to 18 days after prodromal symptoms and progress for 2 to 3 days. Generally, no further paralysis occurs after the temperature returns to normal. The prodrome may be biphasic, especially in children, with initial minor symptoms separated by a 1- to 7-day period from more major symptoms.
Additional prodromal signs and symptoms can include a loss of superficial reflexes, initially increased deep tendon reflexes and severe muscle aches and spasms in the limbs or back. The illness progresses to flaccid paralysis with diminished deep tendon reflexes, reaches a plateau without change for days to weeks, and is usually asymmetrical. Strength then begins to return. Patients do not experience sensory losses or changes in cognition.
Many persons with paralytic poliomyelitis recover completely and, in most, muscle function returns to some degree. Weakness or paralysis still present 12 months after onset is usually permanent.
Paralytic polio is classified into three types, depending on the level of involvement. Spinal polio is most common, and during 1969–1979, accounted for 79% of paralytic cases. It is characterized by asymmetric paralysis that most often involves the legs. Bulbar polio leads to weakness of muscles innervated by cranial nerves and accounted for 2% of cases during this period. Bulbospinal polio, a combination of bulbar and spinal paralysis, accounted for 19% of cases.
The death-to-case ratio for paralytic polio is generally 2%–5% among children and up to 15%–30% for adults (depending on age). It increases to 25%–75% with bulbar involvement.
Laboratory Testing
Viral Isolation
Poliovirus may be recovered from the stool, is less likely recovered from the pharynx, and only rarely recovered from cerebrospinal fluid (CSF) or blood. If poliovirus is isolated from a person with acute flaccid paralysis, it must be tested further, using reverse transcriptase - polymerase chain reaction (RT-PCR) or genomic sequencing, to determine if the virus is “wild type” (that is, the virus that causes polio disease) or vaccine type (virus that could derive from a vaccine strain).
Serology
Serology may be helpful in establishing a diagnosis of disease if obtained early in the course of disease. Two specimens are needed, one early in the course of the illness and another three weeks later. A four-fold rise in the titer suggests poliovirus infection. Two specimens in which no antibody is detected may rule out poliovirus infection. There are limitations to antibody titers. Patients who are immunocompromised may have two titers with no antibody detected and still be infected with poliovirus.
For any patient, neutralizing antibodies appear early and may be at high levels by the time the patient is hospitalized; therefore, a four-fold rise in antibody titer may not be demonstrated. Someone who has been vaccinated and does not have poliovirus infection may have a specimen with detectable antibody from the vaccine.
Cerebrospinal Fluid (CSF)
In poliovirus infection, the CSF usually contains an increased number of white blood cells (10–200 cells/mm3, primarily lymphocytes) and a mildly elevated protein (40–50 mg/100 mL).
Epidemiology
Occurrence
At one time poliovirus infection occurred throughout the world. Transmission of wild poliovirus was interrupted in the United States in 1979 or possibly earlier. A polio eradication program conducted by the Pan American Health Organization led to elimination of polio in the Western Hemisphere in 1991. The Global Polio Eradication Program has dramatically reduced poliovirus transmission throughout the world. In 2012, only 223 confirmed cases of polio were reported globally and polio was endemic only in three countries.
Reservoir
Humans are the only known reservoir of poliovirus, which is transmitted most frequently by persons with inapparent infections. There is no asymptomatic carrier state except in immune deficient persons.
Transmission
Person-to-person spread of poliovirus via the fecal-oral route is the most important route of transmission, although the oral-oral route is possible.
Temporal Pattern
Poliovirus infection typically peaks in the summer months in temperate climates. There is no seasonal pattern in tropical climates.
Communicability
Poliovirus is highly infectious, with seroconversion rates among susceptible household contacts of children nearly 100%, and greater than 90% among susceptible household contacts of adults. Persons infected with poliovirus are most infectious from 7 to 10 days before and after the onset of symptoms, but poliovirus may be present in the stool from 3 to 6 weeks.
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