Polio

 Polio in short

• a highly infectious viral disease
• global incidence has decreased 99% since 1988
• endemic only in parts of India, Nigeria, Afghanistan and Pakistan
• after the eradication of smallpox in 1980, poliomyelitis is the next infectious disease to be disappearing from the world

Polio in detail

Pathogenic agent
Polio is caused by a highly infectious virus within the family of Picornaviridae, genus Enterovirus. There are three types of polioviruses, named 1 to 3, that differ in the molecular structure of certain capsid proteins. The viral particles are very small, about 30 nm in diameter. Picornaviridae possess an icosahedric nucleocapsid with a single-stranded (SS) RNA genome and four capsid proteins (VP1–VP4).

Transmission/pathogenesis
The poliovirus enters the human body through the mouth, where it begins to proliferate within the pharyngeal epithelium. Next it colonizes the intestines, where it rapidly multiplies. It is shed in large amounts with the faeces; thus, inadequate hygienic conditions facilitate spread of the virus. Humans constitute the only reservoir for polioviruses. Polio affects mainly children under 5 years of age.

Symptoms
The incubation period for poliovirus is 3 to 35 days. Up to 95% of infected individuals show no symptoms. Those that do suffer from fever, fatigue, headache, vomiting, stiffness of the neck and back or limb pain. Paralysis only occurs when the virus enters cells of the nervous system. Less than 1% of infected individuals develop permanent paralysis of the limbs, usually in the legs; 5–10% of these patients die due to paralysis of the breathing muscles.

Treatment
There is no treatment for the virus itself, only symptoms can be treated.

Vaccination
There are two polio vaccines available: the oral polio vaccine (OPV), which contains a live attenuated virus, and the inactivated polio vaccine (IPV), which has to be injected. Both are highly effective against all three poliovirus types.

The OPV is the vaccine of choice for global polio eradication because it’s inexpensive and – as it is not injected – no trained health personnel are needed for its administration. In addition, recently immunized individuals shed the vaccine virus in their stools; in areas with poor sanitation (where polio incidence is highest), immunization with OPV leads to passive immunization of persons within close contact.

Another advantage of the oral vaccine lies in its capacity to produce a local immune response in the intestines, the primary site of poliovirus proliferation; the antibodies prevent wild poliovirus from multiplying, which not only protects the immunized individual but also prevents spread via excretion in the faeces. The disadvantage of the OPV is that in a limited number of cases (~1 in 2.5 million), it can cause paralysis in the immunized individual or a close contact.

With the injected IPV, there is no risk of vaccine-associated paralyses. It is mainly used in countries that are considered polio-free.

The Global Polio Eradication Initiative
In 1988, the Global Polio Eradication Initiative was launched; since then, the number of polio cases has dropped by 99% globally. Today, there are only four countries in which poliovirus is endemic: India, Nigeria, Afghanistan and Pakistan.

Nigeria poses the greatest risk to international health. In 2003 immunization programs were stopped as rumours about negative consequences of the vaccination spread; this resulted in a polio epidemic in 12 African countries. In 2008 the new Minister of Health established a task force to improve immunization activities. But according to the Global Polio Eradication Initiative, there are still significant gaps in immunization coverage, and more than 60% of children are not fully vaccinated.

The WHO Region of the Americas was declared polio-free 1994, the Western Pacific Region followed in 2000 and the European Region in 2002.

Incidence and mortality
In 2008 a total of 1652 cases of polio were reported in 18 countries worldwide; 91% of these were reported from the four polio-endemic countries. According to the WHO, 25 non-endemic countries were “re-infected” due to poliovirus imports between 2003 and 2005. As long as there is a single case of polio in the world, all countries have to maintain their national vaccination programs.

To know or not to know...
Thanks to the Global Polio Eradication Initiative, five million people that would have been paralysed by the poliovirus are still walking.

More information
- WHO on Poliomyelitis: www.who.int/topics/poliomyelitis/en/
- The Polio eradication initiative: www.polioeradication.org

Literature
- WHO fact sheet on Poliomyelitis: www.who.int/mediacentre/factsheets/fs114/en/index.html
- The Polio eradication initiative: www.polioeradication.org
- Robert Koch Institute (in German only): http://www.rki.de/cln_100/nn_494686/DE/Content/Infekt/EpidBull/Merkblaetter/Ratgeber__Mbl__Poliomyelitis.html#doc200738bodyText9
- Centers for Disease Control and Prevention: www.cdc.gov/polio/
- Molekulare Virologie, Susanne Modrow, Dietrich Falke, Spektrum Akademischer Verlag GmbH Heidelberg, 1997
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