Write about Poliomyelitis here.
- A Group 1 enterovirus of the Picornaviridae family (single stranded RNA), now not routinely seen in practice due to the routine vaccination using Live Attenuated Vaccine for Polio.
- In temperate countries is seasonal with infections peaking in the summer months, with vaccination aiming to build up a highly immune population in the winter to prevent wild-type transmission.
- Polio free regions include: the Americas, Western Pacific region, Western Europe
- Regions that still have endemic infections include: India, with surrounding countries, countries of West and Central Africa and the Eastern Mediterranean.
- The appearance of poliomyelitis coincided with improvement in standards of public hygiene and is explained by the consequent exposure of infants to infection at a later age. Maternal antibody is capable of confining infection to the gut, where the virus can persist until the immune response develops to eliminate it. In contrast, when maternal antibody has declined in older infants, the virus can spread to sites outside the intestine, causing paralysis.
- Even under modern conditions of hygiene, infection with all three poliovirus types is normally inapparent, but illness with neurological symptoms results in about 1 per cent of infections.
- This can present as aseptic meningitis with neck stiffness, usually recovering after 10 days (abortive or non-paralytic poliomyelitis).
- Meningitis is also caused by several other enteroviruses (see below).
- The more serious presentation is paralytic poliomyelitis, appearing 5 to 10 days after a mild upper respiratory tract infection (‘minor illness’) and progressing to flaccid paralysis resulting from motor neurone destruction (‘major illness’). This may be accompanied by spasms and inco-ordination of non-paralysed muscles. Various forms of the ‘major illness’ reflect infection of different parts of the CNS. Paralysis of limbs results from destruction of motor neurones in the lower part of the spinal cord (‘spinal form’), while the more life threatening bulbar poliomyelitis (‘bulbar form’) involves infections of the medulla oblongata or bulb. Respiratory functions can be affected in both the spinal and bulbar forms of the disease. Encephalitis is rare.
- In children under 5 years old, paralysis of one leg is most common; in children 5 to 15 years of age, weakness of one limb or paraplegia are frequent; quadriplegia is most common in adults, often accompanied by urinary bladder and respiratory muscle dysfunction. Muscular function in limbs may return slowly but there is residual paralysis in 90 per cent of survivors. Ten to 25 per cent of paralytic cases have bulbar symptoms with hypertension, shock, and dysphonia.
- Complications are nosocomial pneumonias (by staphylococci or Gram-negative bacteria), urinary tract infections, and emotional problems.
- The mortality from paralytic polio is 2 to 5 per cent among children and 15 to 30 per cent among adults. Muscle weakness may develop many years after the initial polio disease (postpolio syndrome or postpolio neuromuscular atrophy). A persistent poliovirus infection as cause of this has been assumed, based on the presence of viral RNA in cerebrospinal fluid and neural tissue. However, such RNA has also been found in patients with other neurological and non-neurological diseases and is therefore less likely to be related to the postpolio syndrome. The alternative view is that the postpolio syndrome is anatomical in origin, such that the initial attack of polio destroys motor neurones and reduces the backup available as the patient ages.
- Live attenuated poliovirus vaccine:
- This vaccine has a number of advantages compared to the inactivated vaccine:
- It parallels the natural infection
- stimulates both local secretory IgA in the pharynx and alimentary tract, and systemic circulating virus-specific IgG antibody
- is easy to administer as an oral vaccine
- is more cost effective.
- The disadvantage is that in a few cases the attenuated vaccine strains have reverted to virulence. Since the early 1980s, all cases of polio in the United States and Europe were found to be caused by vaccine-related, that is reverted poliovirus, or were imported from endemic countries and were not indigenous, original wild-type strains. The risk of vaccine-associated poliomyelitis is between 0.5 and 3.4 cases per million of susceptible children immunized. Vaccine-related polio is mostly caused by type 2 or type 3 viruses, probably due to the fact that the number of point mutations in type 1 vaccine virus compared to wild type is much higher than in type 2 and type 3 vaccine viruses. However, this finding raises the question of whether oral vaccination should be continued. In the United States, guidelines have been developed recently which replace the oral vaccination programme by a mixed procedure, initially using inactivated vaccine, followed by booster doses of oral vaccine. For a variety of reasons many countries (United States, France, Germany) have either subscribed to the exclusive use of IPV or are likely to do so in the near future. The decision was influenced by the good progress made towards the eradication of polio due to wild-type virus.
- OPV is the vaccine of choice for people travelling into poliovirus endemic areas if their immune status is unknown or in doubt. The vaccine should be given at least 2 weeks before departure.