Updated: Noviembre 2, 2005
Poliomyelitis (“polio”) is caused by intestinal viruses that spread from person to person in stool and saliva. International campaigns to eliminate polio have shrunk dramatically the areas of the world where this crippling disease is seen. April 2005 was the 50th anniversary of the first vaccine to control polio.
When polioviruses are present in a community, many may become infected but most have few or no symptoms. Less than 1% of polioviruses infections (about 1 of every 100-1,000) result in paralysis. In some cases that develop paralysis, the muscles used to breathe may be weakened, leaving the victim unable to breathe on his or her own. Some who develop paralytic polio die, some recover completely, but many are left with some lingering paralysis often involving one or more arms or legs.
Those who had paralytic polio when younger—whether or not they recovered from paralysis—may develop muscle pain and progressive weakness years later. This condition is known as post-polio syndrome (PPS).
Before invention of the polio vaccine, 13,000 to 20,000 people were paralyzed by polio, and about 1,000 people died from it each year in the United States. Two main types of polio vaccination have been used: injection with killed polioviruses (IPV) or swallowing weakened (attenuated) polioviruses (OPV). Widespread vaccination has eliminated polio from the U.S. and the entire Western Hemisphere1, although it remains a threat in a few remaining countries in Africa and South East Asia.
Most people vaccinated against polio have no adverse reactions. But the oral vaccine containing live viruses that are weakened can cause vaccine-associated paralytic polio (VAPP) in a very small percentage of those immunized or their close contacts since vaccinated persons can spread the vaccine virus to others. Immunocompromised people can shed vaccine virus in their feces for many years, with the virus becoming neurovirulent (capable of causing paralysis). Oral vaccine polioviruses can also spread from person to person in communities with low immunization levels. People who have had VAPP, whether related to vaccine virus or not, can also develop PPS.
There are two types of polio vaccine, OPV, oral polio vaccine, and IPV, inactivated polio vaccine. Initially, OPV was preferred because by spreading in the community it helped to increase community immunity to polio. The rapid reduction of cases of polio worldwide has largely resulted from using OPV, because it is easy to use in mass campaigns, provides long-term immunity, and rapidly reduces the spread of the polioviruses when many in the community are immunized.2
However, about 1 out of 2.4 million doses of OPV distributed in the US, caused vaccine-associated paralytic polio (VAPP).3 This is because OPV contains live-attenuated viruses that sometimes become neurovirulent—cause paralysis—again.4 Immune-compromised people are at increased risk for VAPP and may shed neurovirulent viruses for years. Continuous circulation of OPV-derived viruses in populations with poor hygiene and low immunity levels can lead to rare outbreaks of polio.2
In the United States, a new polio vaccine schedule was recommended in 1997 using both IPV and OPV3. Then, in 2000, an all-IPV schedule was recommended.5 OPV is no longer recommended and is not available for routine use in the US, although it is widely used elsewhere.
OPV continues to be used in countries where polio infections still occur and may be used if there were to be an outbreak in the United States—as OPV is felt by many to be the preferred vaccine to contain an outbreak. The World Health Organization estimates between 250 and 500 cases of VAPP occur each year worldwide.6
Rarely, as vaccine virus has spread in some populations it has mutated to become more like wild polio viruses by being able to damage nerve tissue (this is called ‘circulating vaccine derived polio viruses’ (cVDPV)). Mass vaccination with OPV has eliminated cVDPV.
Recently an unimmunized child was reported from Minnesota with poliovirus infection. So far, three other unimmunized children in the same community have also been identified as being infected.7 The virus isolates have been determined to have been derived from an OPV strain, probably acquired from a traveler to a region where the OPV is still being used in an effort to eliminate poliomyelitis worldwide. Because the community where these infections were detected is under immunized, it remains to be determined how widespread the current outbreak is. Although none of these children have had paralytic illness, prior experience would suggest that these types of viruses have the potential for spread in the community and to cause paralytic illness.
Despite geopolitical difficulties encountered in regions of the world still experiencing polio, it is probable that the world is approaching the elimination of polio. International health officials are working on procedures for how best to assure the disease is completely wiped out, a process made harder in the countries still using OPV. Much can be learned from the smallpox experience but polioviruses and their infections pose enormous challenges:6
About 25% of paralytic polio survivors may experience post-polio syndrome (PPS) between 10 and 40 years after their recovery from the disease. Paralytic polio is caused by the death of individual nerves going to muscles. Because there are extra nerves present there may be some recovery. As people age, however, these extra nerves are no longer available and paralysis may gradually return as PPS.8 PPS is a weakening of muscles that were previously affected by paralytic polio. Symptoms of PPS include:
Currently, there is no treatment for PPS. It is recommended, however, that polio survivors follow a healthy diet, exercise in moderation, and visit a physician regularly.9 Except in people with severe respiratory problems, PPS is not usually life-threatening.8