Poliomyelitis, often called polio or infantile paralysis, is an acute viral infectious disease spread from person to person, primarily via the fecal-oral route.
Although approximately 90% of polio infections cause no visible symptoms, it has been shown by autopsy that more than 50% of motor neurons must be damaged before there is visable weakness. Affected individuals can exhibit a range of symptoms if the virus enters the blood stream.
In about 1% of cases the virus enters the central nervous system, preferentially infecting and destroying motor neurons, leading to muscle weakness and acute flaccid paralysis. Different types of paralysis may occur, depending on the nerves involved. Spinal polio is the most common form, characterized by asymmetric paralysis that most often involves the legs. Bulbar polio leads to weakness of muscles innervated by cranial nerves. Bulbospinal polio is a combination of bulbar and spinal paralysis.
Poliomyelitis was first recognized as a distinct condition by Jakob Heine in 1840. Its causative agent, poliovirus, was identified in 1908 by Karl Landsteiner. Although major polio epidemics were unknown before the late 19th century, polio was one of the most dreaded childhood diseases of the 20th century.
Polio epidemics have crippled thousands of people, mostly young children; the disease has caused paralysis and death for much of human history. Polio had existed for thousands of years quietly as an endemic pathogen until the 1880s, when major epidemics began to occur in Europe; soon after, widespread epidemics appeared in the United States.
By 1910, much of the world experienced a dramatic increase in polio cases and frequent epidemics became regular events, primarily in cities during the summer months. These epidemics — which left thousands of children and adults paralyzed — provided the impetus for a "Great Race" towards the development of a vaccine.
Developed in the 1950s, polio vaccines are credited with reducing the global number of polio cases per year from many hundreds of thousands to around a thousand. Enhanced vaccination efforts led by the World Health Organization, UNICEF, and Rotary International could result in global eradication of the disease.
The signs and symptoms of polio differ depending on the extent of the infection. Signs and symptoms can be divided into paralytic and non-paralytic polio.
In non-paralytic polio which accounts for most individuals infected with polio, patients remain asymptomatic or develop only mild flu-like symptoms, including fatigue, malaise, fever, headache, sore throat, and vomiting. The symptoms, if present, may only last 48-72 hours, though usually they last for one to two weeks.
Paralytic polio occurs in about 2% of people infected with the polio virus and is a much more serious disease. Symptoms occur as a result of nervous system and spinal cord infection and inflammation.
Symptoms can include:
- abnormal sensation,
- breathing difficulty,
- difficulty swallowing,
- urinary retention,
- mood swings,
- muscle pain and spasms,
Approximately 5%-10% of patients who develop paralytic polio often die from respiratory failure, since they are unable to breathe on their own. That is why it is imperative that patients receive appropriate medical evaluation and treatment.
Prior to the vaccine era and the use of modern ventilators, patients would be placed in an "iron lung" (a negative pressure ventilator, which was used to support breathing in patients suffering from paralytic polio).
Many people ask, "What causes polio?". Polio has only one cause, which is an infection with the poliovirus.
Poliovirus is a very contagious virus that can spread easily from person to person. In fact, when a person is infected with poliovirus, it is expected that polio transmission among susceptible household contacts will occur in nearly 100 percent of children and over 90 percent of adults.
Poliovirus is a single-stranded RNA virus from the family Picornaviridae and genusenterovirus.
Poliovirus only infects humans, and it is more common during summer months in temperate climates. In tropical climates, there is no seasonal pattern. The poliovirus is rapidly inactivated by heat, formaldehyde, chlorine, and ultraviolet light.
When a person is infected with poliovirus, the virus resides in the intestinal tract and mucus in the nose and throat. Poliovirus is usually spread through contact with stool of the infected person (known as fecal-oral transmission). Less frequently, poliovirus is spread through contact with infected respiratory secretions or saliva (oral-oral transmission).
Following polio transmission, a person does not become immediately sick. A person who is infected with polio can spread polio about 7-10 days before symptoms begin. A person can continue to spread polio for about three to six weeks after the beginning of polio symptoms. However, a person is most contagious for the 7-10 days after symptoms of poliohave begun.
There is no cure for polio, so prevention is very important. Patients with non-paralytic polio need to be monitored for progression to paralytic polio. Patients with paralytic polio need to be monitored for signs and symptoms of respiratory failure, which may require lifesaving therapies such as respiratory support.
In addition, a number of treatments are available to decrease some of the less severe symptoms. There are medications to treat urinary infections and urinary retention and pain management plans for muscle spasms.
Unfortunately, there are only supportive measures available to treat the symptoms of paralytic polio. Patients who recover from polio may require physical therapy, leg braces, or even orthopedic surgery to improve physical function.
Is there a vaccine that prevents polio?
The story of polio vaccine is a true medical success story. It is not over yet since polio still causes significant illness in less developed areas of the world such as in India and Africa.
During the last half of the 19th century and into the first half of the 20th century, polio was a global epidemic. Even the future U.S. president, Franklin D. Roosevelt, contracted paralytic polio in 1921. President Franklin D. Roosevelt was quite influential in increasing both public awareness and scientific research dedicated to eradicating the disease.
In 1938, after the founding of the National Foundation for Infantile Paralysis (March of Dimes), there was a significant effort to develop a vaccine to prevent polio. This came to fruition in 1955 when Dr. Jonas Salk developed an injectable inactivated polio vaccine (IVP) which was soon distributed and administered to children all over the United States and Canada. The current inactivated polio vaccine has evolved over time, but since 1999, it has been the recommended form of the polio vaccine in developed nations.
In 1961, an oral live virus vaccine against polio (OVP) was developed byAlbert Sabin which became available and widely used from 1963 to 1999 in developed countries and to present day in underdeveloped countries. This oral virus vaccine is still recommended to control polio pandemics all over the world due to its ease of administration (no needles needed).
Both vaccines were developed for children since they are the group that generally seemed to be at highest risk. However, the oral vaccine (OVP) should not be given to children who are immunodepressed as they can develop vaccine-associated paralytic poliomyelitis (VAPP).
The newest injected vaccine is an enhanced inactivated polio vaccine that is more immunogenic (produces a strong immune system response) than the previous IVP and is used in the U.S.; it does not cause VAPP. The original OVP (also termed tOVP) was a trivalent oral vaccine (polio viruses types 1-3) but caused a measurable immune response in only about 40%-50% of people who obtained it.
Unfortunately, this trivalent oral vaccine was often not immunogenic fast enough to withstand dilution or removal from the gastrointestinal tract by chronic diarrhea that existed in many patients.
OVP was modified in 2005 to a monovalent (type 1 polio virus only) termed mOVP1. This change caused the vaccine to be three times more immunogenic than the original trivalent OVP and generated an immune response in over 80% of people who obtained this oral vaccine.
This newer oral vaccine is used in many developing countries where no needles or trained personnel are available and where chronic diarrhea further reduces the effectiveness of the original trivalent OVP. Other monovalent OVP (for example, mOVP3, used for the infrequent polio type 3 outbreaks) are occasionally used.
Currently, four doses of inactivated polio vaccine (IPV) are recommended for children when they are 2 months old, 4 months old, 6-18 months old, and finally at 4-6 years of age.
Due to vaccination programs, there have been very few cases of polio in the western hemisphere since the 1970s, and although current worldwide eradication programs continue to be successful, there is still work to be done to eliminate polio in developing countries.