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Bacteriology at UW-Madison |
Poliovirus and
Poliomyelitis
Polio (poliomyelitis) is caused by one of three antigenic types of the
human poliovirus. Poliovirus is an Enterovirus. Entroviruses are one of
the genera in the picornavirus family. Other picornaviruses include
human rhinoviruses and human hepatitis B virus. Picornaviruses are very
small, icosahedral, nonenveloped ss (+)RNA viruses. Humans are the only
natural
hosts for polioviruses.
Purified
Poliovirus particles. CDC.
Poliovirus
Type 1 as solved by X-ray crystallography.
There are 3 serotypes of poliovirus with no common poliovirus
antigen. They have identical physical properties and their genomic base
sequences share 36 - 52% homology. Antigenic variants of types 1 and 2
have been reported, but these antigenic differences do not
affect the capacity of antibodies induced by one strain to protect
against other strains of the same type. Despite these minor
differences, polioviruses show marked antigenic
stability.
Epidemiology
Polioviruses are disseminated globally. In densely-populated
developing countries, almost 100% of the population have antibodies to
all 3 types of the virus before 5 years of age. Epidemics do not
occur and paralytic disease is rare as the incidence of paralytic
poliomyelitis increases with age, especially after 15 years of
age. In countries with improved sanitation, infection is often
delayed until adulthood with a consequent increase in the number
of cases of paralytic poliomyelitis.
Poliomyelitis occurs primarily in the summer,
like the common diarrheal diseases. The patient is maximally
contagious during the first week of illness, when the virus is
excreted both in the pharynx and feces, but the virus continues
to be excreted in the feces for to 5 to 17 weeks after the onset
of illness.
With the advent of
immunization, poliomyelitis is on the verge of eradication in
many countries.
Pathogenesis
As an enterovirus, poliovirus replicates in cells of the human
gastrointestinal tract and is excreted in the feces. Symptoms of
infection are an "intestinal flu", usually mild and transient. Fecal
contamination of food and water spreads the virus to other individuals.
In rare instances, poliovirus invades the CNS and causes the the
paralytic disease poliomyelitis.
The incubation period is usually 7 - 14 days. Following ingestion, the
virus multiplies in the
oropharyngeal and intestinal mucosa. The lymphatic system, in
particular the tonsils and the Peyer's patches of the ileum are
invaded, and the virus enters the blood resulting in a transient
viremia. There are three possible outcomes
following poliovirus infection:
Subclinical infection (90-95%) - inapparent subclinical infection
account for the vast majority of poliovirus infections.
Abortive infection (4-8%) - minor illness characterized by
influenza-like symptoms such as fever, malaise, drowsiness, headache,
nausea, vomiting, constipation and sore throat. Recovery occurs within
a few days. The minor illness may be accompanied by aseptic meningitis
which is similar to the meningitis caused by other enteroviruses and
usually resolves without complications within 2 - 10 days.
Major illness (1-2%) - the major illness may present 2 - 3 days
following the minor illness or it may occur without evidence of any
preceding minor illness. Signs of aseptic meningitis are common.
Involvement of the anterior horn cells leads to flaccid paralysis.
Painful muscle spasms and incoordination of non-paralysed muscles may
occur. Involvement of the medulla may lead to respiratory paralysis and
death. The paralysis usually develops over several days and some
recovery may take place. Any effects persisting for more than 6 months
are usually permanent.
Treatment and Prevention
No specific treatment is available except supportive measures
in paralytic poliomyelitis. However, it is possible to prevent
the disease through active immunization. There are two vaccines
available, the inactivated Salk vaccine, and the attenuated Sabin
vaccine.
The Inactivated Salk Vaccine, or formalin inactivated Intramuscular Polio
Vaccine (IPV), was developed by Jonas
Salk and licensed for use in 1955. It consists of an
injected dose of three antigenic strains of killed poliovirus. IPV is
of high potency and
purity. It is both safe and effective. The use of IPV in Sweden
and Finland virtually eliminated paralytic poliomyelitis in
these countries. However, IPV has the disadvantage that it does not
induce local IgA mediated immunity
to polioviruses in the gut.
The attenuated Sabin Vaccine, or Oral Polio Vaccine (OPV), was
produced by Albert Sabin and
licensed for use in 1962. OPV has several advantages over IPV: (1) it
induces long lasting immunity, similar to that seen after natural
infection; (2) it induces IgA formation and thus local immunity
against reinfection in the pharynx and gut; and (3) it is inexpensive
to produce and allows mass immunization without the need for expensive
sterile
equipment.
When properly administered, OPV is extremely effective, as shown by the dramatic decrease in poliomyelitis since its introduction in Europe and North America. However, the vaccine strains, in particular the type 3 strains, can revert to virulence and cause disease in those who have just been vaccinated. It is estimated that vaccine induced poliomyelitis is seen at a rate of 1 in 2.4 million vaccinations. The majority of cases of paralytic polio seen in many developed countries which use OPV are associated with the vaccine rather than wild type virus.
When the live-virus Sabin vaccine, OPV, was licensed in 1962, it spread in popularity for several reasons. First, it can "infect" other, non-vaccinated individuals with whom the vaccinated person has close contact and confer some immunity to them. Second, because the oral vaccine acts in the gut, it confers immunity there and reduces the spread of the wild virus. The injected vaccine, acting through the bloodstream, immunizes the individual but does not reduce his ability to spread the wild virus. Third, the live-virus vaccine is cheaper than the killed-virus vaccine. Finally, the oral vaccine is easier to administer than the injected vaccine, so patients are more likely to complete the vaccination series and attain full immunity. Though Salk's vaccine, IVP, had reduced the incidence of polio to a tiny fraction of what it was in the early 1950s, Sabin's vaccine was considered superior for these reasons and became the standard treatment. The killed-virus vaccine immunized people against the effects of the virus, but the virus could still spread from person to person. It was the live-virus vaccine that enabled the complete elimination of the wild polio virus in the United statesThe major disadvantage of the OPV is that it can
itself cause polio, and does so in about one in 2.4 million recipients.
There is no such risk from IPV. In the United
States, once polio was eradicated within its population, the CDC
decided that the slight advantages of the OPV were not
worth several vaccine-induced cases of polio each year. Hence the use
of the OPV was discontinued after 2000, but it is still used
elsewhere where polio remains a common threat.
On the whole, the two vaccines have eliminated polio from most of the
countries in
the world and reduced early cases from hundreds of thousands per year
to only 1000 worldwide in 2001.
Current Vaccination Regimen in the
United States
In the U.S., inactivated polio vaccine (IPV) is recommended.
Children should receive doses of IPV or IPV combined with
the DTaP and hepatitis B vaccines at two months,
four months, and 6 to 18 months. A booster dose of
IPV (not a combination vaccine) is given at four to six years.
Young adults who have never been vaccinated against polio may
receive the first dose of IPV (not a combination vaccine) at any time.
The second dose should be given one to two months later, and third dose
6 to 12 months after the second. Adults who have never
been immunized and who are traveling to
areas where polio outbreaks occur can receive three doses of IPV, each
given four weeks apart.
WHO Poliovirus Eradication Campaign
Poliovirus was targeted for eradication by the WHO by the end of year 2000 (now 2005). To this end, an extensive monitoring network had been set up. Poliovirus has been eradicated from most regions of the world except the Indian subcontinent and sub-Saharan Africa. It is possible that the WHO target may be achieved.
Current status of Poliovirus transmission as of 1999 (WHO)
