Polio

Polio is a disease caused by a RNA virus belonging to genus Enterovirus. The most recognizable and devasting disease manifestation is flaccid paralysis. Flaccid paralysis refers to the weak and flabby nature of the affected musculature. Flaccid paralysis, however, occurs in less than 1% of cases. More than 90% of cases are asymptomatic.

Poliovirus primarily infects the gastrointestinal tract, but can migrate to the lymph nodes and central nervous system, where it may cause nerve damage.

Other Names
Infantile paralysis

Poliomyelitis

Signs and Symptoms
Symptoms of polio are flu-like in nature and include fever, malaise (physical discomfort or fatigue), headache, and nausea.

If the disease advances, symptoms include severe muscle pain and stiffness of the neck and back. If paralysis is to occur, it usually manifests 3-4 days post-exposure. The site of paralysis is commenserate with the nerve cells infected. The lower extremities are most often affected, but the arms and diaphragm (the muscle that functions in breathing) may also become paralyzed. Infection of the diaphragm requires medical intervention and can be fatal. Paralysis of the diaphragm carries a 5-10% mortality rate.

Treatment
There is no cure or treatment for polio. Medical intervention is thus directed at facilitating convalescence. Metal braces or crutches, corrective shoes, and physical therapy are used to correct polio-induced muscle damage. An iron lung is used when the diaphragm is affected. An iron lung is a sealed chamber in which all but the head and neck of the patient are enclosed. Iron lungs induce respiration by creating differences in pressure that result in the expansion and depression of the chest, permitting air to enter and exit the lungs.

Transmission
Poliovirus is transmitted by person-to-person contact, primarly via the fecal-oral route. Virus proliferates in both the pharynx (throat) and intenstines. Infection may occur following inhalation of contaminated salivary droplets or ingestion of contaminated food products. It should be made clear that poliovirus is disseminated via droplet spread and is not airborne. Virus may persist in the feces of those with and without symptoms for 3-6 weeks post-infection.

Epidemiology
The virus was at one time universal, but a global effort has been undertaken to eradicate polio. Eradication refers to the elimination of the virus from the wild. A disease can be considered eradicated, but still exist under controlled, laboratory conditions or housed in storage facilities. The only disease that has been successfully eradicated is smallpox, which occurred in 1977. Due to widespread vaccination efforts, the last case of indigenous (wild, non-imported) polio in the United States was reported in 1979 and in Europe in 1998. Despite a proclamation by the World Health Organization (WHO) to eradicate polio by the year 2000, the virus remains endemic in seven countries-- Afghanistan, Pakistan, India, Egypt, Niger, Nigeria, and Somalia. India, Pakistan, and Nigeria account for 95% of the world's remaining polio burden.

Prevention
Oral polio vaccine (OPV) is administered via droplets at birth and at 6, 10. and 14 weeks and involves administration of live, attenuated virus. The virus is attenuated through serial passage in non-human cells in vitro at non-ideal temperatures. OPV stimulates the generation of antibodies in both the pharynx and intestines. OPV has the potential to induce passive immunity, as immunized indviduals shed live, but attentuated virus, that can be transmitted to non-vaccinated individuals and confer protection. Though diarrhea is not contraindicated, it may reduce the efficacy of an administered vaccine dose, as the virus may pass through the gastrointenstinal tract prior to inducing immunity. As a result, more than the recommended four doses may be required to confer immunity. OPV is routinely used in developing countries because of its low cost and ease of administration. Instances of failed seroconversion and reduced efficacy have been reported, however. OPV was introducted in 1961 through the efforts of Albert Sabin.



Inactivated poliovirus vaccine (IPV) is administered subcutaneously and is composed of inactivated poliovirus. In the United States, IPV doses are delivered at 2, 4, 6-18 months and 4-6 years of age. IPV is more expensive than OPV and requires adminstration by trained individuals. IPV incites excellent protection against pharyngeal infection, but intenstinal protection is less robust. IPV premiered in 1955 and was engineered by Jonas Salk.

Both OPV and IPV can confer life-long immunity and are composed of the three poliovirus serotypes.

Controversy
Vaccine-derived poliovirus (VDPV) is associated only with the oral polio vaccine, as the virus is not killed, but only attenuated. Genetically, VDPV more closely resembles wild-type poliovirus than the attenuated viruses found in the OPV. VDPV is as virulent as wild-type poliovirus. VDPV results from the recombination of poliovirus with enteroviruses. VDPV was the cause of an outbreak in the Caribbean in 2000. As VDPV is capable of inciting outbreaks, there is increasing concern over illness due to VDPV than to wild-type poliovirus. Ironically, OPV rather IPV is used to quell an outbreak, increasing the chances of VDPV-induced infection.

There is also concern that wild polio may be released inadvertantly from storage or research facilities or deliberately in an act of bioterrorism. As a consequence, there is debate over whether it is prudent to cease polio vaccination.