Bacteriology at UW-Madison
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.
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.
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 states
The 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
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)